Thunderstorm

Thunderstorms are relatively small, but intense, storm systems. An average thunderstorm is only 15 miles (24 kilometers) in diameter and lasts about thirty minutes. During that time, it produces strong winds, heavy rain, and lightning. Lightning is a short-lived, bright flash of light that is produced by a 100 million-volt electrical discharge in the atmosphere. Only 10 percent of thunderstorms are considered "severe," meaning they produce some combination of high winds, hail, flash floods, and tornadoes. Only about 1 percent of all thunderstorms are the source of tornadoes, whose violently rotating winds reach the ground and can cause great damage.

Severe thunderstorms and their related phenomena produce significant human injuries and fatalities, as well as property damage, each year. Hailstorms, for instance, storms which bring frozen precipitation called hailstones that range in size from peas to softballs, are responsible for nearly $1 billion a year in crop damage. Tornadoes also cause about eighty deaths and fifteen hundred injuries annually. While straight-line winds called derechos occur less frequently and result in fewer fatalities, a single derecho can cause millions of dollars in damage.

Lightning, which occurs with all thunderstorms, sets off about ten thousand forest fires each year in the United States alone, causing several hundred million dollars in property damage. Furthermore, in the United States lightning causes between 75 and 100 deaths and about 550 injuries annually. It is the second biggest weather killer in the country, topped only by flash floods. Flash floods, which are sudden, intense, localized floods caused by heavy rainfall, kill an average of 140 people yearly.

Thunderstorms are produced by cumulonimbus clouds, which are tall, dark, and ominous—and they are giant storehouses of energy. A typical thunderstorm unleashes 125 million gallons (568 liters) of water and enough electricity to provide power to the entire United States for twenty minutes.

At any given time, there are about two thousand thunderstorms underway around the world. About forty thousand thunderstorms occur worldwide each day, and fourteen million thunderstorms happen each year. Earth is struck by lightning from these storms one hundred times every second.

Watching a thunderstorm approach can be quite an exhilarating experience. Imagine standing on your front porch on a hot, humid afternoon. The morning's haze, when the sky appeared milky, has given way to a line of tall cotton-ball-like cumulus clouds. On the horizon there are enormous thunderstorm clouds, with their whitish tops and dark undersides. As these clouds approach, the sky darkens, almost blocking out the sunlight. Then comes a calm period in which the air feels still, hot, and very muggy. Next the wind picks up and the rain begins to fall in large drops. Soon the rain intensifies to a downpour and the wind turns cold and blows wildly. Lightning brightens the sky here and there. Then it strikes nearby and is followed by a thunderclap so loud it jolts you to your feet. Half an hour later the storm is over. The storm clouds move away, the sun comes out, and the air is cooler and less humid. Before long, however, the heat returns.

Evolution of thunderstorms

Two atmospheric conditions are required for the development of a thunderstorm. The first is that the surface air must be warm and humid. The other is that the atmosphere must be unstable. "Unstable" means that the surrounding air is colder than a rising air parcel. As long as the atmosphere remains unstable, an air parcel will continue to rise. When the air parcel reaches a height at which the atmosphere is stable, meaning that the surrounding air is warmer than the air parcel, it will rise no further.

Thunderstorms occur when warm, moist air rises quickly through an unstable atmosphere. On reaching the dew point, the temperature at which the air is saturated, the moisture within the air condenses, forming a cloud. If the atmospheric instability is great enough, this air will rise to great heights, and the cloud will develop vertically (upwardly) into a towering cumulus cloud. In conditions of great instability, the cloud will develop into a full-fledged cumulonimbus, or thunderstorm, cloud.

A number of factors may trigger the uplift of warm air. For example: an air mass (a large quantity of air throughout which temperature and moisture is constant) rides up, along a mountainside. Or air is forced upward by an advancing cold front, the forward boundary of a cold air mass and/or thermal (a pocket of rising warm air). At the same time, there must be a divergence (movement outward) of winds aloft. This divergence causes surface winds to converge beneath, and rise to the point of divergence.

WORDS TO KNOW

air mass:

a large quantity of air throughout which temperature and moisture content is fairly constant.

anvil:

the flattened formation at the top of a mature cumulonimbus cloud.

cirrus:

clouds at high levels of the troposphere, created by wind-blown ice crystals, that are so thin as to be nearly transparent.

cold front:

the line behind which a cold air mass is advancing, and in front of which a warm air mass is retreating.

condensation:

the process by which water changes from a gas to a liquid.

convective cell:

a unit within a thunderstorm cloud that contains updrafts and downdrafts.

convergence:

the movement of air inward, toward a central point.

cumulonimbus:

tall, dark, ominous-looking clouds that produce thunderstorms. Also called thunderstorm clouds.

cumulus:

clouds that look like white or light-gray cotton balls of various shapes.

dart leaders:

the series of dim lightning strokes that occur immediately after the original lightning stroke, that serve to discharge the remaining buildup of electrons near the base of the cloud.

derecho:

a destructive, straight-line wind, which travels faster than 58 mph (93 kph) and has a path of damage at least 280 miles (450 kilometers) long. Also called plow wind.

dew point:

the temperature at which a given parcel of air reaches its saturation point and can no longer hold water in the vapor state.

divergence:

the movement of air outward, away from a central point.

downburst:

an extremely strong, localized downdraft beneath a thunderstorm that spreads horizontally when it hits the ground, destroying objects in its path.

downdraft:

a downward blast of air from a thunderstorm cloud, felt at the surface as a cool wind gust.

dry adiabatic lapse rate:

the constant rate at which the temperature of an unsaturated air parcel changes as it ascends or descends through the atmosphere. Specifically, air cools by 5.5°F for every 1,000 feet (1.0°C for every 100 meters) it ascends and warms by 5.5°F for every 1,000 feet (1.0°C for every 100 meters) it descends.

The life cycle of a thunderstorm can be broken down into three stages that trace its development from inception to dispersion. These stages are called: the cumulus stage, the mature stage, and the dissipating stage.

Cumulus stage

A thunderstorm begins its development in the cumulus stage, also known as the developing stage. A thunderstorm usually begins forming late in the afternoon or early in the evening. It follows a period in which cumulus clouds have been forming, then evaporating into the dry air, only to form again at higher altitudes. Each time the clouds evaporate, they raise the humidity of the air. This fact is important because as long as the air is dry, any moisture that condenses within rising warm air will quickly evaporate. Only when the air is humid will moisture condense into a cloud that remains in the air. As the air is humidified at increasingly higher levels, the conditions are right for the development of towering cumulus clouds.

In the cumulus stage, which takes only fifteen minutes or so, cumulus clouds undergo dramatic vertical growth. The cloud tops rise to a height of about 30,000 feet (9,000 meters). At the same time, the clouds spread horizontally and merge into a line up to about 9 miles (15 kilometers) across. When air rises it cools by the dry adiabatic lapse rate (5.5°F per 1,000 feet, or 9.8°C for every 1,000 meters). Once the air has cooled to the dew point, it becomes saturated and the moisture within it condenses (becomes liquid). Latent heat, which is the heat that must be removed from water vapor to cause it to turn into a liquid, or that must be added to a liquid water to cause it to turn into a vapor, is released into the cloud through condensation. This heat increases the temperature contrast between the cloud and the surrounding air, fueling the upward growth of the cloud as long as warm air continues rising into it.

Once air enters a cloud and becomes saturated, it cools by the moist adiabatic lapse rate (2.7°F per 1,000 feet, or 4.9°C for every 1,000 meters). Thus, it cools more slowly than it did at the dry adiabatic lapse rate, when it was unsaturated (had less than 100 percent relative humidity). This change enables the air to rise to even greater heights before reaching a stable layer of atmosphere. Air will continue rising as long as it is warmer, and less dense, than the surrounding air. The greater the instability of the air (the more rapidly the air cools with height, the higher the air parcel will ascend. As air rises beyond the top of the cloud and into the dry air, the cycle is repeated. The moisture evaporates and increases the humidity of the dry air, enabling condensation to take place at ever-greater altitudes. In this way, a cumulus cloud develops upward, into a cumulonimbus cloud.

Experiment: Lightning in your mouth

Experimenting with lightning or electricity is too dangerous for ordinary people. But you can experiment with mild electric charges that operate on the same principle as lightning. Here's one fun way to do it: get a wintergreen-flavored Life Savers candy and go into a completely dark room that has a mirror. Let your eyes adjust for a few minutes. Then, crunch down on the candy with your teeth, keeping your mouth open. As it breaks, you should see little sparks or flashes of light. This happens because the breaking of the sugar inside the candy releases electrical charges into the air which attract oppositely charged nitrogen in the air. When the opposite charges meet, they produce "lightning.r"

Precipitation rarely occurs during the cumulus stage, because water droplets or ice crystals are blown upward by rising air, into the tops of the clouds. Within the cloud, the speed of updrafts, columns of air blowing upward, may exceed 30 feet per second (10 meters per second). Lightning and thunder are produced only occasionally during the cumulus stage.

WORDS TO KNOW

entrainment:

the process by which cool, unsaturated air next to a thunderstorm cloud gets pulled into the cloud during the mature stage of a thunderstorm.

evaporation:

the process by which water changes from a liquid to a gas.

flash flood:

a sudden, intense, localized flooding caused by persistent heavy rainfall or the failure of a levee or dam.

front:

the boundary between two air masses.

frontal system:

a weather pattern that accompanies an advancing front.

gust front:

the dividing line between cold downdrafts and warm air at the surface, characterized by strong, cold, shifting winds.

hail:

precipitation comprised of hailstones.

hailstone:

frozen precipitation that is either round or has a jagged surface, is either totally or partially transparent, and ranges in size from that of a pea to that of a softball.

haze:

the uniform, milky-white appearance of the sky that results when humidity is high and there are a large number of particles in the air.

hurricane:

the most intense form of tropical cyclone. A hurricane is a storm made up of a series of tightly coiled bands of thunderstorm clouds, with a well-defined pattern of rotating winds and maximum sustained winds greater than 74 mph (119 kph).

induction:

the process by which excess electrical charges in one object causes the accumulation by displacement of electrical charges with opposite charge in another nearby object.

insulator:

a substance through which electricity does not readily flow.

inversion:

a stable reversal of the normal pattern of atmospheric temperature, formed when a warm air mass sits over a cold air mass near the surface.

ion:

an atom that has lost or gained an electron, thereby acquiring a positive or negative electrical charge.

jet stream:

a fast-flowing, relatively narrow air stream found at an altitude of approximately 36,000 feet (11,000 meters).

latent heat:

the heat that must be removed from a quantity of water vapor to cause it to turn into a liquid, or that must be added to a quantity of liquid water to cause it to turn into a vapor; called latent because the temperature of the quantity of water or water vapor does not change.

lightning:

a short-lived, bright flash of light during a thunderstorm that is produced by a 100 million-volt electrical discharge in the atmosphere.

Mature stage

The mature stage of a thunderstorm begins when the first drops of rain reach the ground. It is during the mature stage that one sees heavy rain, strong winds, lightning, and sometimes hail and tornadoes. If the thunderstorm is severe, the sky may appear black or dark green. A thunderstorm generally remains in the mature stage for ten to thirty minutes, occasionally longer.

Throughout the mature stage, the cumulonimbus cloud continues building. Eventually it builds to the tropopause, the boundary between the troposphere and the stratosphere which is between 5 and 7 miles (8 to 11 kilometers) above Earth's surface. The cumulonimbus cloud may even overshoot the tropopause. Above the tropopause, the temperature of the atmosphere increases with altitude. Thus the rising air becomes cooler than the surrounding air, so it ceases to rise and starts to spread out laterally in the anvil shape that characterizes the top of mature thunderstorm clouds. The base of the cloud, meanwhile, grows to several miles across. Precipitation begins to fall from the thunderstorm cloud when ice crystals or water drops within the cloud reach a critical mass. That is, they become large enough to overcome the updrafts that have previously confined them to the tops of the clouds. As the precipitation falls, it pulls air with it. These downward blasts of air, felt at the surface as cool gusts, are called downdrafts.

The updrafts of air continue to bring warm, humid air into the thunderstorm throughout the mature stage. These updrafts create a situation in which there are columns of rising air adjacent to columns of descending air. The rising air builds up the storm cloud while the descending air returns the cloud's moisture to Earth.

Weather report: Where thunderstorms occur

More than fourteen million thunderstorms take place throughout the world each year. For the most part, they occur in warm, humid areas. The world's greatest concentrations of thunderstorms are in Brazil's Amazon Basin, the Congo Basin of equatorial Africa, and in the islands of Indonesia. In each of these areas, thunderstorms occur on more than one hundred days each year.

Thunderstorms occur, with varying frequency, all throughout the United States. Central Florida's Gulf Coast has thunderstorms more often than any other U.S. location. Thunderstorms occur there on 130 days per year, on average. On the other extreme is the Pacific Coast, which sees thunderstorms on only five to ten days per year, and Alaska, which only has one thunderstorm every three to five years.

In between those two extremes are the following annual averages: 1) Florida's Gulf Coast, plus the Gulf Coasts of Alabama and Mississippi, have thunderstorms on 80-100 days; 2) the rest of the southeastern United States has thunderstorms on 60-80 days; 3) the central portion of the Rockies has thunderstorms on 50-70 days; 4) the Corn Belt (Iowa, Indiana, and Illinois) and Great Plains states (states just east of the Rockies) have thunderstorms on about 50 days; 5) the portion of the Midwest that lies east of Iowa, as well as the mid-Atlantic states and New England, have thunderstorms on 20-40 days; 6) the desert Southwest also generally has thunderstorms on 20-40 days but the rate is terrain-dependent; 7) the far West, has the fewest thunderstorms with 0-20 days.

As the storm progresses, the updrafts weaken and downdrafts strengthen by entrainment. Entrainment is the process by which cool, unsaturated air next to a cloud gets pulled into the cloud. As this dry air mixes with air in the cloud, the relative humidity (amount of water vapor in the air mass) in the cloud is lowered. Some of the water droplets evaporate, a process that absorbs latent heat from the cloud as water changes from liquid to gas. This has the opposite effect that condensation had in the cumulus stage. Specifically, evaporation cools the rising air and slows its ascent. It also cools the downdrafts. Since cold air is denser and heavier than warm air, this cooling causes the downdrafts to fall faster.

The thunderstorm peaks in intensity at the end of the mature stage. As downdrafts begin to dominate updrafts, the thunderstorm yields the heaviest rain, the most frequent lightning, and the strongest winds. If the thunderstorm produces hail or tornadoes, they will occur at the end of the mature stage.

Dissipating stage

In the dissipating stage of a thunderstorm, precipitation falls from the entire cloud base. Downdrafts overtake updrafts, preventing warm, moist air from rising up into the cloud. In effect, the downdrafts cut off the thunderstorm's fuel supply. Without a constant influx of moisture from below, the cloud begins to evaporate. During the dissipating stage, rain becomes light and winds become weak. An hour or so after the cumulus stage began, the storm cloud dissipates, leaving only wispy traces high in the sky. The traces are of cirrus clouds, at high levels of the troposphere. These clouds are so thin as to be nearly transparent. The cool, refreshing air brought by downdrafts during the mature stage subsides. The rain evaporates, which increases the humidity of the air. The heat and humidity following the thunderstorm may create even more oppressive conditions than those that existed before the thunderstorm.

The preceding explanation of a thunderstorm's life cycle applies to a single convective cell of a thunderstorm. A convective cell is a unit within a thunderstorm cloud that contains updrafts and downdrafts. While some thunderstorms are of the single cell variety, most thunderstorms contain several convective cells and are called multicell thunderstorms. In the case of a multicell thunderstorm, convective cells are simultaneously in various stages of development. Old cells die and new cells form as the storm moves over the ground and encounters fresh sources of warm, moist air. The life cycle of each convective cell lasts thirty minutes to sixty minutes. A multicell thunderstorm may last several hours.

Types of thunderstorms

Thunderstorms are classified by a number of criteria. The first criterion is the mechanism that triggers its formation. Another criterion is whether the thunderstorm is isolated or part of a cluster of thunderstorms. Finally, thunderstorms are classified on the basis of their severity. These groupings often overlap. For instance, a thunderstorm that forms along a cold front may be weak or severe, and may occur singly or in a line of thunderstorms.

Air mass thunderstorms

An air mass thunderstorm, the most common type of thunderstorm, is one that forms within a single mass of warm, humid air. Air mass thunderstorms are relatively weak, meaning they don't produce hail or strong winds, and die out quickly. They do, however, produce lightning and sometimes destructive downward gusts of wind called downbursts. Air mass thunderstorms most often form in the late afternoon, at the warmest time of day. In regions outside of the tropics, they form only in summer.

For any thunderstorm to develop, air must be lifted to the level at which it is saturated, meaning the moisture within it condenses and forms a cloud. The air does not rise to its condensation level automatically. Rather, it requires a lifting mechanism. In air mass thunderstorms, that lifting mechanism is the intense heating of small areas on the surface, which produces rising pockets of air called thermals. This heating is most often accompanied by a convergence, or movement inward, of surface winds and a resultant uplift of air. The air ascends to a point where there is a divergence of winds aloft.

An air mass thunderstorm occurs in isolation from other thunderstorms. It may be composed of a single convective cell. More often, however, it contains multiple cells.

Air mass thunderstorms are common in the central and eastern United States in spring and summer. They are initiated by the northward flow of humid, tropical air masses from the Gulf of Mexico, the Caribbean, and the Atlantic Ocean near Bermuda. On an average, these thunderstorms occur one afternoon out of every three on the Gulf Coast of Florida.

Orographic thunderstorms

An orographic thunderstorm, also called a mountain thunderstorm, is a type of air mass thunderstorm that is initiated by the flow of warm air up along a mountainside. Such storms occur most commonly on slopes with greatest exposure to the Sun.

As a slope is heated, the air next to it is also heated. That warm air rises and cools and the moisture within it condenses to form cumulus clouds. Given a constant influx of warm, moist air and an unstable atmosphere, these clouds will develop into cumulonimbus clouds.

Orographic thunderstorms are also relatively weak. They are common in the Rocky Mountains, as afternoon breezes lift warm air up the mountainsides. Because of this lifting mechanism, mountainous regions in the United States are hit with more thunderstorms than any region outside southeastern states.

Frontal thunderstorms

Frontal thunderstorms form along the edge of a front. They occur most often when a cold front is displacing a maritime tropical (warm and moist) air mass that has remained stationary for several days. An advancing cold front wedges underneath an existing warm air mass, thrusting the warm air upward.

Less frequently, a frontal thunderstorm is initiated by an advancing warm front, the leading edge of a warm air mass. Such storms occur only if advancing warm air, which glides up and over the residing cold air mass, is entering a particularly unstable air layer.

Frontal thunderstorms may occur any time of day or night and at any time of year, except for very cold winter days. They are most likely to form in warm weather, when convection is enhanced by the heating of the ground. Frontal thunderstorms that occur in the winter may yield snow and are generally far weaker than their summertime counterparts.

Though they often have stronger winds and heavier rain than air mass thunderstorms, frontal thunderstorms are relatively weak systems. Under certain conditions, thunderstorms produced along a front are severe. This is the case when a series of thunderstorms, called a squall line, arises in a band running parallel to the front.

Frontal thunderstorms are most common in the Great Plains states and the Midwest, where cold fronts from Canada overtake warm, moist air from the Gulf of Mexico. More than half the world's tornadoes occur in this region.

Mesoscale convective complexes

A mesoscale convective complex (MCC) is a group of thunderstorms that forms a nearly circular pattern over an area that is about one thousand times the size of an individual thunderstorm. An MCC may cover an area greater than 50,000 square miles (130,000 square kilometers), or the size of a small state.

The combined effect of the individual thunderstorms of the MCC is to produce an airflow that favors the formation of new thunderstorms. MCCs most often arise in warm weather and at night. More than fifty MCCs per year form over the central and eastern United States.

Thunderstorms continually form and dissipate within an MCC. The overall pattern persists for up to twenty-four hours and moves very slowly, usually less than 20 mph (30 kph). MCCs yield significant amounts of rainfall. In the Great Plains states and the Midwest, MCCs produce around 80 percent of the rainfall during the growing season. Around half of all MCCs are severe, spawning some combination of tornadoes, flash floods, hailstorms, and high winds.

Severe thunderstorms

The National Weather Service defines a thunderstorm as "severe" if it has one or more of the following elements: wind gusts of at least 58 mph (93 kph); hailstones at least 3/4 inch (2 centimeters) in diameter; or tornadoes or funnel clouds. Severe thunderstorms may also be accompanied by flash floods.

Severe thunderstorms are formed in the same way as more moderate thunderstorms: by the rising of moist air into an unstable atmosphere. A strong cold front is frequently the force that provides the vigorous uplift of warm air required to produce a severe thunderstorm. At the same time, the moist surface air is pulled upward when a divergence, or flow away from a central point, occurs in the winds aloft. This divergence triggers the convergence, or coming together, of surface winds beneath that point. The surface winds then rise to the area of divergence above.

One condition that gives rise to some of the largest and most severe thunderstorms is that an inversion is present for much of the day. An inversion is the increase of air temperature with height, through some portion of the atmosphere. The presence of a warm air layer aloft acts as a lid that prevents warm, humid surface air from rising. In other words, an inversion produces a stable atmosphere. As a result, only shallow cumulus clouds can form.

Sometimes on a summer day when an inversion has occurred, surface air will become heated to the point at which it is warmer than the warm air aloft. Pockets of warm air will then burst through the upper warm layer, creating towering cumulonimbus clouds. Once the warm air has burst through the inversion layer, the clouds rapidly develop into severe thunderstorms.

An important factor in the formation of a severe thunderstorm is that updrafts are not weakened by falling precipitation. Such updrafts are produced in one of two ways. First, the updrafts are so strong that they keep all precipitation suspended in the cloud top for a long time, while the thunderstorm builds. Second, the updrafts are tilted so that precipitation falls alongside them, rather than into them. The updrafts become tilted by strong upper-level winds. When the updrafts are tilted, precipitation falls into dry air alongside the updrafts, rather than directly into the updrafts.

Updrafts that are not weakened by falling precipitation are able to continue building the cloud top upward to greater and greater heights. Meanwhile, the precipitation falls into dry air adjacent to the updrafts and partially or completely evaporates. The dry air becomes cooler and denser and plunges downward.

The updrafts in a severe thunderstorm travel at speeds of 50 mph (80 kph) or greater. They remain strong for far longer than they do in a weaker thunderstorm. Sometimes the updrafts are so powerful that they rise above the troposphere, the lowest atmospheric layer, and penetrate the stratosphere, the layer above. This condition is called overshooting.

One effect of the strong updrafts in a severe thunderstorm is to keep hailstones suspended in the cloud for longer than usual. During that time the hailstones receive several coatings of ice and become quite large. When they become so heavy that they can not be supported by updrafts, the large hailstones either descend with a downdraft or are tossed through the side of the cloud by an updraft.

The downdrafts in a severe thunderstorm are also very strong. When strong, cool downdrafts reach the ground they further intensify the storm by displacing the warm, moist air and forcing it back up into the cloud. In this way, the storm is continually rejuvenated and can persist for several hours. Sometimes the warm, moist air that is forced upward has the effect of producing new thunderstorms. When strong downdrafts occur in the spreading "anvil" at the top of the fully developed cloud, they may produce pouchlike mammatus projections on the anvil's underside.

The dividing line between cold downdrafts and warm air at the surface is called the gust front. Similar to a cold front, an advancing gust front is characterized by winds that are strong, shifting, and cold. The winds along a gust front can reach speeds of 55 mph (88 kph) or greater. In dry, dusty areas they carry debris along with them and create dust storms or sandstorms.

In some cases, a gust front can be clearly identified by the roll cloud that follows directly behind it. A roll cloud looks like a giant, elongated cylinder lying on its side that, as its name implies, is rolling forward. This cloud occupies a narrow vertical layer of air. The top of the cloud is prevented from developing upward by stable air at the base of the thunderstorm.

Another type of cloud associated with a gust front is a shelf cloud. A shelf cloud is fan-shaped with a flat base. It forms along the edge of the gust front as warm, humid air is thrust upward and encounters the stable air layer, the layer through which an air parcel cannot rise or descend. In contrast to a roll cloud, which is a distinct formation, a shelf cloud is attached to the underside of the cumulonimbus cloud. Particularly violent winds blow on the surface beneath a shelf cloud.

Squall lines

Most thunderstorms that are classified as "severe" exist in a band of thunderstorms called a squall line. A squall line may form either along a cold front or up to 200 miles (320 kilometers) in front of it. A squall line is particularly ominous in appearance. It looks like a churning, solid bank of fast-moving, low, dark clouds. A squall line may stretch for hundreds of miles (hundreds of kilometers). It moves along at speeds approaching 50 mph (80 kph).

Thunderstorms may form along a cold front when the cold front wedges beneath a warm, moist air mass. If the air mass being displaced is sufficiently moist, this upward thrust can cause vertical cloud development and thunderstorms.

When the squall line is ahead of the cold front, it is known as a prefrontal squall line. Two processes may lead to the formation of a prefrontal squall line. One process involves the lifting of warm, moist air by upper-level winds. When upper-level winds encounter a cold front, they flow over it. Once they have crossed the cold front, the upper-level winds then dip downward again. This sets in motion a wave pattern of upper-level air flow. As the wave again flows upward, some 100 to 200 miles (160 to 320 kilometers) ahead of the cold front, it promotes the uplift of the warm, moist surface air.

A prefrontal squall line may also form if the cold front is preceded by two air masses: a warm, dry air mass and a warm, moist air mass. In this case, thunderstorms don't form directly along the cold front, since the cold front is advancing on a dry air mass. However, the dry air mass is being pushed forward into the moist air mass, lifting the moist air upward. In that case, the squall line forms many miles ahead of the cold front.

Supercell storms

Supercell storms are the most destructive and long-lasting of all severe thunderstorms. They may continue for several hours and produce one strong tornado after another, as well as heavy rain and hail the size of golf balls. A supercell storm blazes a trail of destruction stretching 200 miles (329 kilometers) or more. For these reasons, the supercell has earned the title "The King of Thunderstorms."

A supercell storm arises from a single, powerful convective cell. It forms along a cold front that is pushing its way through a mass of very warm, humid air. A supercell storm may form in isolation or at the end of a squall line. Most supercell storms occur in spring and early summer, when temperature contrasts between warm and cold air masses is greatest.

The formation of a supercell thunderstorm requires a very specific vertical arrangement of air layers. At the surface, the cold and warm air masses are rotating around a central area of low pressure. At an altitude of about 5,000 feet (1,500 meters), above the surface low, there must be a layer of warm, moist air blowing towards the north. Above that, at about 10,000 feet (3,000 meters), is a layer of cold, dry air moving across from the southwest. This layer is called the dry tongue.

Located at the next highest layer, at about 18,000 feet (5,500 meters), are the upper-air westerlies. These winds aloft progress from west-to-east in a wave-like pattern of ridges and troughs. A ridge is a northward crest wherein exists a high-pressure area, and a trough is a southward dip wherein exists a low-pressure area. In this layer of air, the low-pressure center of a trough must be located just to the west of the warm surface air. Finally, at about 30,000 feet (9,000 meters) the jet stream produces an area of maximum speed where winds diverge (spread apart). At this level, surface air converges (comes together) and begins to rise.

A supercell storm reaches immense proportions due to strong winds aloft. The motion of the upper-level winds tilts the storm so that updrafts remain free from falling precipitation. The precipitation falls downward into the dry air, creating downdrafts that force additional warm, moist air upward. The updrafts, in turn, continue adding fuel to the storm, causing the thunderstorm cloud to surge to tremendous heights.

Another necessary ingredient in the formation of a supercell is wind shear. Wind shear describes a condition in which layers of wind increase in speed, and change direction, with height. When a layer of air is sandwiched between two other layers, each traveling at a different speed and direction, the sandwiched layer starts to roll.

The sandwiched air layer becomes a rotating horizontal roll, like a rolling pin. That rotating roll is then turned upright, like a barber pole, by powerful updrafts. As updrafts continue to blow through the now vertical, spinning column, the updrafts themselves begin to rotate. Wind shear creates a region of rotating updrafts within a supercell, called a mesocyclone. Mesocyclones are, on average, 10 miles (16 kilometers) in diameter although the diameters of the largest may reach 250 miles (400 kilometers). In addition to providing power to the thunderstorm, a mesocyclone is a necessary component in the formation of tornadoes.

Thunderstorm-associated phenomena

A number of destructive and potentially deadly elements are associated with thunderstorms. Lightning is the most common, but hail, flash floods, tornadoes, and strong downdrafts of wind (including macro-bursts, microbursts, and derechos) are also familiar to people who live in thunderstorm-prone areas. Lightning, flash floods, and downdrafts are discussed in this section. To learn about hail and hailstorms, see "Precipitation." For an explanation of derechos, see "Local Winds." Because of the complexity of tornadoes, they are discussed in their own chapter.

Weather report: Lightning safety

If you hear thunder, you are in the vicinity of lightning. Thunder should be considered a signal to seek shelter immediately. The best way to remain safe from lightning strikes is go inside a sturdy building. A shed or flimsy structure will not protect you from lightning. Once indoors, until the storm has passed do not do the following: talk on the telephone (cordless and cellular phones are safe); take a bath or shower; handle electrical appliances, computers, or plumbing fixtures. It's safest to unplug all electrical appliances except a radio, so you can be alerted to severe weather.

If you are not near a building when lightning threatens, the next safest option is to get into your car (as long as it's not a convertible!) and keep the windows rolled up. Do not touch the metal sides of the car.

If you are outdoors, far from buildings and vehicles, go to the lowest spot in the area and crouch down. However, do not crouch in or touch standing water. Keep away from trees, fences, and poles. If you are in the woods, stay away from the tallest trees.

If you are outdoors and feel your skin tingle, feel your hair stand on end, or hear clicking sounds, lightning may be about to strike. In that case, the safest position is to crouch down on the balls of your feet. Place your hands on your knees and your head down between your hands. If possible, pick up one foot and balance on the other. Do not lie down—the idea is to remain low while minimizing your body contact with the ground.

Once you have reached the safest possible spot, remain there until the storm has passed. If lightning strikes nearby, it does not mean the danger is over. Lightning may strike the same spot more than once during a storm. In fact, the Empire State Building in New York City was struck by lightning fifteen times during one thunderstorm.

The worst place to be in a thunderstorm is in the water. If you are boating or swimming, hurry back to land and seek shelter. Other dangerous places to be when lightning strikes include: under a tree; on an athletic field or golf course; on a bicycle, tractor, or riding lawnmower; or on a mountain.

If you witness someone being struck by lightning, immediately call for emergency medical assistance. Attend to the person right away. It is not true that someone struck by lightning carries an electrical charge. Check for breathing and a pulse. In many cases it will be necessary to administer cardiopulmonary resuscitation (CPR).

Lightning and thunder

In order to qualify as a "thunderstorm," a rain shower or snow shower must be accompanied by lightning and thunder. Lightning is a short-lived, bright flash of light that heats the air through which it travels to about 50,000°F (28,000°C). Compare this to the surface of the sun, which is about 11,000°F (6,000°C)! Thunder is the sound wave that results when the intense heating causes the air to expand explosively.

Who's who: Benjamin Franklin

Benjamin Franklin was born in 1706 in Boston, Massachusetts, that at that time was a British colony. Franklin was the fifteenth child out of a total of seventeen. Because his family was poor, the young Franklin had only two years of formal education. Franklin made up for this by educating himself. He went on to become a scientist, diplomat, author, publisher, and inventor.

Franklin was a pioneer in the study of electricity. He first conducted experiments using a Leyden jar, which is a glass jar filled with water and plugged with a rubber stopper. It contains a metal rod inserted through the stopper, one end of which extends into the water. The other end of the rod is connected to a machine that generates an electric charge. Using the Leyden jar, Franklin studied the nature of static electricity in water and the glass that enclosed the water.

The crackling noise made by electricity in the Leyden jar reminded Franklin of the crackling of thunder. This observation led him to wonder if lightning was also a form of electrical discharge. Late in 1752, in Philadelphia, Pennsylvania, Franklin conducted his famous kite-flying experiment to test this hypothesis.

He fashioned a kite from two wooden sticks and a large silk handkerchief. He attached a metal key to the kite string, just above the point where he held the string, and set the kite flying during a thunderstorm. The storm-generated electricity traveled down the rain-drenched string, to the key. When Franklin touched the key, he felt a shock.

Fortunately, Franklin had the foresight to run a wire from the key to the ground, so the electric charge would run into the ground. If he had not grounded his experiment in this way, the electrical discharge might have killed him. Franklin was also fortunate that lightning did not strike his kite directly. If that had happened, the grounding wire would probably have not protected him from a lethal electric shock.

Three years prior to his famous experiment, Franklin had invented lightning rods as a way to protect tall structures from lightning strikes. A lightning rod is a metal pole that is attached to the tallest point of a building and connected, by an insulated conducting cable, to a metal rod buried in the ground. Franklin's invention caught on quickly. Most tall structures, to this day, are topped with lightning rods.

Franklin's weather observations went far beyond the topic of lightning. In 1743, Franklin was the first to conclude that a local storm was not an isolated event, but rather was due to the largescale circulation of air masses. He made this discovery 175 years before meteorologists in Scandinavia discovered that rotating fronts produce large, organized storm systems. Franklin noticed that a storm had followed a path from Philadelphia to Boston—that is, from the southwest to the northeast. During the storm, however, the surface winds were blowing from the northeast. Franklin concluded that since the local storm had arrived from a direction counter to that of the local winds, it must not be local in nature, but part of a larger storm system.

At any given moment, approximately one hundred lightning flashes are occurring worldwide. Lightning kills between 75 and 100 people in the United States each year and causes about 550 injuries. This is a greater number of deaths than those resulting from hurricanes or tornadoes. Lightning also is responsible for around ten thousand brushfires and forest fires annually, particularly in the western United States, western Canada, and Alaska. In addition, tens of millions of dollars in damage is caused to electrical utility equipment. The total property damage due to lightning in the United States alone is several hundred million dollars per year.

Lightning is most often produced by cumulonimbus clouds during the mature stage of a thunderstorm. However, it can also arise from other clouds, including: cumulus clouds; stratus clouds; clouds produced by volcanic eruptions; or even billowing clouds of sand produced during sandstorms.

WORDS TO KNOW

mammatus:

round, pouchlike cloud formations that appear in clusters and hang from the underside of a larger cloud.

moist adiabatic lapse rate:

the variable rate at which the temperature of a saturated air parcel changes as it ascends or descends through the atmosphere.

multicell thunderstorm:

a thunderstorm system that contains several convective cells.

orographic thunderstorm:

a type of air mass thunderstorm that's initiated by the flow of warm air up a mountainside. Also called mountain thunderstorm.

precipitation:

water in any form, such as rain, snow, ice pellets, or hail, that falls to Earth's surface

radar:

an instrument that detects the location, movement, and intensity of precipitation, and gives indications about the type of precipitation. It operates by emitting microwaves, which are reflected by precipitation.

relative humidity:

the amount of water vapor in an air mass relative to the amount of water in a saturated air mass of the same temperature.

ridge:

a northward crest in the wavelike flow of upper-air westerlies, within which exists a high pressure area.

roll cloud:

a cloud that looks like a giant, elongated cylinder lying on its side, that is rolling forward. It follows in the wake of a gust front.

saturated:

air that contains all of the water vapor it can hold at a given temperature; 100 percent relative humidity.

severe thunderstorm:

a thunderstorm with wind gusts of at least 58 mph (93 kph); hailstones at least 3/4 inch (2 centimeters) in diameter; or tornadoes or funnel clouds.

shelf cloud:

a fan-shaped cloud with a flat base that forms along the edge of a gust front.

squall line:

a moving band of strong thunderstorms.

stable air layer:

an atmospheric layer through which an air parcel cannot rise or descend.

Lightning, for all of its harmful effects, is generally beneficial to life on Earth. First, it makes possible the conversion of normal oxygen to ozone. Ozone in the upper atmosphere protects plants and animals from harmful ultraviolet radiation. Second, lightning breaks down oxygen and nitrogen in the air, producing ammonia and nitrogen oxides. These chemicals react with rainwater to form nitrogen compounds, which are natural fertilizers. Over 100 million tons (90 million metric tons) of nitrogen compounds fall to the ground each year.

If it were not for lightning, however, life might not exist on Earth. Many scientists hypothesize that lightning initiated the series of chemical reactions in the oceans that led to the formation of life.

WORDS TO KNOW

stepped leader:

an invisible stream of electrons that initiates a lightning stroke. A stepped leader surges from the negatively charged region of a cloud, down through the base of the cloud, and travels in a stepwise fashion toward the ground.

stratosphere:

the second-lowest layer of Earth's atmosphere, from about 9 to 40 miles (15 to 65 kilometers) above ground.

supercell storm:

the most destructive and long-lasting form of a severe thunderstorm, arising from a single, powerful convective cell. It is characterized by strong tornadoes, heavy rain, and hail the size of golf balls or larger.

thermal:

a pocket of rising, warm air that is produced by uneven heating of the ground.

thunderstorm:

a storm resulting from strong rising air currents; characterized by heavy rain or hail along with thunder and lightning.

tornado:

a violently rotating column of air that reaches the ground and is attached to a cumulonimbus cloud; it is nearly always observable as a funnel cloud.

tropopause:

the boundary between the troposphere and the stratosphere, between 30,000 and 40,000 feet (9,000 and 12,000 meters) above ground.

troposphere:

the lowest atmospheric layer, where clouds exist and virtually all weather occurs.

trough:

a southward dip in the wavelike flow of upper-air westerlies, within which exists a low-pressure area.

unsaturated:

air that has less than 100 percent relative humidity.

updraft:

a column of air blowing upward, inside a vertical cloud.

virga:

rain that falls from clouds but evaporates in midair under conditions of very low humidity.

warm front:

the leading edge of a moving warm air mass.

wind shear:

a condition in which a layer of air is sandwiched between two other layers, each of which is traveling at a different speed and/or direction, causing the sandwiched air layer to roll.

How lightning is produced

In order for lightning to occur, there must be two objects, or regions, carrying different electrical charges. If an object gains electrons, it is said to be negatively charged. If an object loses electrons it is said to be positively charged.

Lightning is a surge of electrons and ions (electrically charged atoms) between regions with opposite electrical charges. The majority of lightning occurs within a single cloud. Most of the rest occurs between a cloud and the ground. Cloud-to-cloud and cloud-to-air lightning occurs less frequently.

Under normal conditions, such as on a clear day, the ground is negatively charged while the upper air is positively charged. When a cumulonimbus cloud forms, the charge distribution changes. The ground beneath the developing cloud becomes positively charged by induction, the process by which excess electrical charges in one object causes the accumulation of opposite electrical charges in another nearby object. A narrow region at the base of the cloud, as well as the upper portion of the cloud, also become positively charged. However, in the lower portion of the cloud, just above the base, there exists a negatively charged, saucer-shaped region. This region may be about 1,000 feet (300 meters) thick and several miles (several kilometers) in diameter.

Under most conditions, air acts as an insulator, meaning electricity does not readily flow through it. During the mature stage of a thunderstorm, however, the electrical charge differential between the cloud and the ground becomes so great that the resistance of air breaks down. Specifically, air loses its insulating properties when the electric field grows stronger than 915,000 volts per foot (about 3 million volts per meter). Then, electricity, which appears as lightning, surges between differently charged regions in order to neutralize the opposing charges.

Meteorologists are still not sure how the distribution of charges in cumulonimbus clouds happens. However, it is clear that the separation of charges requires strong updrafts which carry water droplets upward. As they rise into colder air, the droplets of water become supercooled droplets, that is liquid water that is below freezing (14°F to −4°F or −10 to −20°C). These supercooled droplets collide with ice crystals to form a soft ice-water mixture called graupel.

According to the electrostatic induction theory, opposite charges are driven to different parts of the cloud by strong updrafts and the formation of graupel. The collisions between the supercooled water droplets and the ice crystals results in a slight positive charge being transferred to the ice crystals, and a slight negative charge to the graupel. Updrafts drive the lighter ice crystals upwards, causing the cloud top to accumulate increasing positive charge. The heavier negatively charged graupel falls towards the middle and lower portions of the cloud, building up an increasing negative charge. Charge separation and accumulation continue until the electrical potential becomes sufficient to initiate lightning discharges.

Cloud-to-ground lightning

Lightning that travels between a cloud and the ground accounts for only 20 percent of all lightning, yet this type of lightning has been studied more extensively than any other. Cloud-to-ground lightning is considered the most important by researchers since it is the only type that endangers people and objects on the ground.

Weather report: The color of lightning

Lightning takes on a range of colors, depending on atmospheric conditions.

• Blue lightning within a cloud indicates the presence of hail.
• Red lightning within a cloud indicates the presence of rain.
• Yellow or orange lightning indicates a large concentration of dust in the air.
• White lightning is a sign of low humidity in the air. Since forest fires break out when the air is relatively dry, white lightning is the most likely type to ignite forest fires.

Although lightning lasts only for two-tenths of a second and appears as a mere flash of light, it is quite a complex process. It begins when an invisible stream of electrons, called a stepped leader, surges from the negatively charged region of the cloud, down through the base. This is called the stepped leader because it travels in a stepwise fashion down toward the ground. Each portion of the stepped leader covers about 60 to 300 feet (20 to 100 meters) in less than a millionth of a second. Then it stops for about 50 millionths of a second before starting off in a new direction. The stepped leader creates a branching pattern, ionizing a path through the air as it goes.

When the stepped leader reaches a point about 300 feet (100 meters) above the ground, a "positive streamer" (a flow of positive ions upward) forms leading upward toward the stepped leader. If the electric field is sufficiently strong, the positive streamer will quickly evolve into a hotter "current leader." The current leader moves upward from the ground and connects with the stepped leader coming down from the cloud, completing a conductive path between the cloud and the ground.

When the two leaders connect, a powerful stroke of electrical current surges up from the ground toward the cloud. This surge of current is called a return stroke. It typically comes from a tall, pointed object, such as an antenna or flagpole, since the induced positive charges on the ground accumulate on tall, pointed objects.

A large concentration of electrons are discharged to the ground through this completed electrical circuit. Then positive ions from the ground shoot back up to the cloud. The upsurge of positive ions generates the bright flash commonly considered "lightning."

The return stroke is 2 to 7 inches (5 to 8 centimeters) in diameter. It travels at nearly one-third the speed of light and takes a mere ten-thousandth of a second to reach the cloud. This flow of positive ions partially neutralizes the charge difference between the cloud and the ground.

In the approximately one-tenth of second that follows, several (usually two to four, but sometimes as many as twenty) more lightning strokes may occur along the ionized channel. These strokes, which serve to discharge the remaining buildup of electrons near the base of the cloud, are initiated by surges from the base of the cloud called dart leaders.

Dart leaders, like stepped leaders, are intercepted by return strokes when they get closer than 300 feet (100 meters) to the ground. These return strokes occur about fifty-thousandths of a second apart. They are individually indistinguishable and appear as a flickering light in the wake of the initial return stroke. The dart leaders cease when the charge differential between the cloud and the ground has been neutralized.

In less than 10 percent of all cases of cloud-to-ground lightning, a positively charged stepped leader surges from the upper portion of the cloud. It travels downward to a negatively charged area on the ground. These powerful discharges occur most commonly during winter storms and produce a flash of light similar to a return stroke.

Ground-to-cloud lightning occurs less frequently than cloud-to-ground lightning. This form of lightning begins with the ascent of a stepped leader, usually positively charged, from the ground. As the stepped leader approaches the cloud above, it triggers the release of a return stroke from the cloud. Ground-to-cloud lightning is most often initiated from very tall points on the surface, such as mountaintops or the tops of towers or antennae.

How close is a thunderstorm?

While we see a lightning flash at almost the exact instant it occurs, we don't hear the thunder until a short time later. The reason for this delay is that lightning travels at the speed of light (186,282 miles per second or 299,914 kilometers per second), which is about one million times faster than the speed of sound (1100 feet per second or 330 meters per second).

One way to tell how close you are to a thunderstorm is to determine the time lapse between the lightning and the thunder. The rule to remember is that it takes thunder about five seconds to cover one mile (three seconds for one kilometer). Therefore, if you hear thunder seven seconds after you see the flash of lightning, the thunderstorm is 1.4 miles (2.3 kilometers) away (7 divided by 5 equals 1.4).

You can tell, in general terms, whether a thunderstorm is near or far, by the quality of the thunder. If it sounds like a sharp crack or clap, the storm is close. If that sound is immediately followed by a loud bang, the storm is very near—closer than 330 feet (100 meters).

The thunder from distant storms produces a rumbling sound. One reason for this effect is that the sound waves are bouncing off hills or buildings before reaching you. Another reason is that you first hear the sound from the part of the lightning near the ground, which is closer to you, after which you hear the sound from the upper part of the lightning, which is farther away.

Other kinds of lightning

Cloud-to-cloud lightning is the most common form of lightning. It occurs either within a single cloud or between two clouds. In the former case, the lightning runs between the negatively charged lower portion of the cloud and the positively charged upper portion of the cloud. This type of lightning illuminates and provides a brilliant view of a cumulonimbus cloud. The whole cloud lights up spectacularly.

Lightning that runs between two clouds occurs less frequently than it does within a single cloud. Intercloud lightning represents a discharge of electrons from the lower portion of one cloud to the upper portion of an adjacent cloud.

Cloud-to-air lightning is the flow of electricity between areas of a cloud and the surrounding air which have opposite charges. This form of lightning is relatively weak and often occurs at the same time as a cloud-to-ground stroke. Usually, this lightning travels a path between an area of negative charge in the surrounding air and the positively charged top of the cloud. Because cloud-to-air lightning occurs at great heights, it is almost always too distant to have an audible thunder component.

Both cloud-to-cloud lightning and cloud-to-air lightning are often referred to as sheet lightning. Sheet lightning illuminates a cloud or a portion of a cloud. The cloud blocks the distinct pattern of the lightning from view, so the lightning appears as a bright sheet.

Ball lightning is the rarest and most mysterious form of lightning. It has never been photographed but has been witnessed by numerous individuals throughout history. It is reported to look like a dimly-to-brightly lit sphere, ranging from 0.4 to 40 inches (1 to 100 centimeters) in diameter. It lasts between one and five seconds and either hangs in the air or floats horizontally at a rate of about 10 feet (3 meters) per second. It either dissipates silently or with a bang.

The cause of ball lightning is unknown, but many theories have been proposed. One recent theory suggests that it is an "electromagnetic knot" created by linked lines of magnetic force that form in the wake of an ordinary cloud-to-ground lightning strike. Some scientists suggest that ball lightning does not exist, but is merely an optical illusion experienced by an individual who has just witnessed a stroke of lightning.

Forked lightning is much more common. It occurs when a return stroke originates from two different places on the ground at once. This creates two separate ionized channels and the appearance of being "forked."

Lightning that appears to sway from a cloud is called ribbon lightning. It is produced when the wind blows the ionized channel so that its position shifts between return strokes.

A flash of lightning that resembles a string of beads is called bead lightning. This type of lightning may be the result of a fragmenting ionized channel. An alternative explanation is that part of the lightning stroke is obscured by clouds or falling rain.

Silent lightning from a distant storm, generally more than about 10 miles (16 kilometers) away is called heat lightning or summer lightning. This lightning is not accompanied by thunder since it is too far away for the sound to reach the observer. It often occurs on hot summer nights when the sky above is clear. Heat lightning sometimes appears orange, due to the scattering of light by dust particles in the air.

Weather report: Lightning rods

Lightning rods are metal poles used to protect buildings from lightning strikes. A lightning rod is attached to the tallest point of a structure and connected, by an insulated conducting cable, to a metal rod buried in the ground. As a cloud with a large accumulation of negative charge passes over head, a positive charge is induced under the cloud on the building or tower, and on the ground. The principle behind lightning rods is that the induced charge collects at the sharp tip of the rod. The electrical field around the sharp tip of the rod becomes so intense that the air nearby begins to ionize. The ionized air harmlessly dissipates the accumulated induced positive charge before lightning can strike. In the event lightning does strike, it may be attracted to the rod and conducted safely into the ground.

A lightning rod provides protection to a cone-shaped area around and beneath it. The tip of the cone is located at the top of the rod. The radius of the base of the cone is equal to the height of the rod. Thus, the taller the lightning rod, the greater the area it protects.

Downbursts

A downburst is an extremely strong, localized downdraft beneath a thunderstorm. It blasts down from a thunderstorm cloud like water pouring out of fully opened tap. When this vertical wind hits the ground it spreads horizontally. It then travels along the ground, destroying objects in its path.

Red sprites, blue jets, and elves

Sprites, jets, and elves are three different and poorly understood phenomena that are associated with lightning in clouds. They were first observed by military and airline pilots, shuttle astronauts, and others flying at high altitudes. At first, they were all dismissed by meteorologists as some sort of optical illusion, but have since been photographed by specially equipped television cameras.

Red sprites are extensive but weak luminous red flashes that appear directly above an active thunderstorm system and occur at the same time as cloud-to-ground or cloud-to-cloud lightning strokes. While dim, they are just barely visible by the unaided eye under proper conditions. Sprites appear to be only associated with the rarer positive cloud-to-ground lightning strokes which typically are significantly displaced from the electrically active cores of thunderstorms. Sprites have not been observed to be associated with negative cloud-to-ground strikes (the usual polarity). They are likely due to some sort of electrical discharge or electromagnetic pulse into the upper atmosphere.

Elves also appear to be associated with energetic cloud-to-ground strikes of either polarity. They appear as wide areas of luminosity that occur high above the cloud tops. They may be caused by strong electromagnetic pulses shot up into Earth's ionosphere (a region within Earth's stratosphere where many atoms have lost one or more electrons).

Blue jets are a different high altitude phenomenon, distinct from sprites, but also observed above active thunderstorms. They are also dim, but are visible with the unaided eye and by using low light television systems. Blue jets originate from the tops of the clouds directly above electrically active core regions of thunderstorms. Following emergence, they propagate upward in a narrow cones with vertical speeds of roughly 60 miles per second (100 kilometers per second).

Red sprites, blue jets, and elves have not been observed until recently because they are dim, and rare. They are only associated with around 1 percent of lightning strokes. However, sprites and jets are barely visible to the unaided eye under proper conditions.

The best way to attempt to view these phenomena would be by observing a thunderstorm at a distance of 100-200 miles (200-300 km) from a relatively high vantage point, such as the top of a hill or side of a mountain. The sky would have to be completely dark and clear (well after twilight and without moonlight). There must be no intervening clouds. Your eyes would need to be completely dark-adapted. If you can clearly see the Milky Way, then your eyes are sufficiently dark-adapted and other conditions are optimal.

Now use a piece of dark paper to block the light from the thunderstorm (so that your eyes are not distracted by lightning or lose their dark-adaptation) and carefully gaze at a space above the storm about as wide as your fist held at arm's length. Remember, the sprite or jet will be a very brief flash just on the edge of perceptibility. But with luck and patience, you may be rewarded by a sight that very few people have seen.

Downbursts may or may not be accompanied by rain. Dry downbursts generally occur beneath virga, streamers of rain that evaporate in midair. Downbursts are capable of knocking down trees, damaging buildings, and leveling crops, as well as kicking up dust and debris into a "cloud" that tumbles along the ground. In many cases, damage caused by downbursts has been mistakenly attributed to tornadoes.

Downbursts are divided into two categories, macrobursts and micro-bursts, depending on their size. Downbursts may result in tornado-force, straight-line winds with long lives. These winds are called derechos. For a discussion of derechos, turn to the chapter entitled "Local Winds."

Macrobursts

A macroburst is a downburst that creates a path of destruction on the surface greater than 2 miles (4 kilometers) wide. The winds of a macroburst travel at around 130 mph (210 kph) and last for up to thirty minutes. A macroburst (as well as a microburst) may either follow in the wake of, or its leading edge can develop into, a gust front.

Microbursts

A microburst is a smaller downburst that a macroburst, yet it is potentially more dangerous. The path of destruction created by a microburst is between several hundred yards and 2 miles (4 kilometers) wide. Its winds, which only last about ten minutes, may exceed 170 mph (270 kph). Like macrobursts, microbursts may evolve into gust fronts.

Who's who: Tetsuya Theodore Fujita

Although Tetsuya Theodore Fujita is most famous for developing the scale of tornado intensity which bears his name, his primary area of research has been downbursts. Fujita was the first to identify these destructive downdrafts of wind. His research on downbursts has been particularly relevant to aviation safety, since microbursts, the smallest and most intense form of downbursts, pose an extreme hazard to aircraft.

Fujita was born in 1920 in Kitakyushu City, Japan. He graduated from Meiji College of Technology with the equivalent of a bachelor's degree in mechanical engineering in 1943. Soon thereafter, Fujita was hired on at the college as an assistant professor of physics.

In 1945, the Japanese cities of Hiroshima and Nagasaki were devastated by atom bombs dropped by U.S. airplanes. Three weeks later, Fujita was part of a research team sent to those cities to survey the damage. Fujita noticed that the destruction was in the shape of a starburst. The hub was located directly beneath the bomb and spokes radiated outward. Fujita also calculated the height from which the bombs must have been dropped to create such a pattern. These findings became relevant in Fujita's later work on downbursts.

In 1949, Fujita moved to Tokyo to pursue his doctorate in atmospheric science at Tokyo University. In 1953, at the invitation of professor and thunderstorm specialist Horace R. Byers, Fujita moved to the United States to join the faculty of the University of Chicago.

Fujita's main topic of research soon became the destructive potential of storm-related winds, particularly tornadoes. Based on his surveys of tornado damage, Fujita created the Fujita Intensity Scale for tornadoes in the late 1960s. Fujita's scale consists six categories of tornado intensity, based on the ground damage created by the tornado. His scale provided the first objective, uniform way of assessing tornado strength.

In April 1974, Fujita took a plane ride to survey the damage caused by a massive outbreak of tornadoes. Flying over West Virginia, he noticed the same starburst pattern of destruction he had seen in Japan. Fujita proposed that in that area, the damage had been created not by a tornado, but by powerful downdrafts produced by thunderstorms. He then coined the term "downbursts."

At first, Fujita's findings were met with skepticism by his fellow meteorologists. It was commonly accepted at the time that thunderstorms produce downdrafts of air, but it was believed that downdrafts weaken significantly before reaching the ground. Fujita conducted a research project to put his hypothesis to the test. Over a forty-two-day period in the spring and summer of 1978, Fujita and his team of researchers detected fifty microbursts in Chicago's western suburbs.

Fujita retired from teaching in 1991. He passed away quietly in his sleep on November 19, 1998.

Microbursts receive more attention than macrobursts because of the hazard they pose to airplanes during takeoff or landing. Microbursts are accompanied by abrupt changes in the speed or direction of wind at various heights, known as wind shear. Wind shear is something that every pilot seeks to avoid, since it can spell disaster for aircraft. Due to their small size, microbursts elude detection by airport radar, an instrument which operates by bouncing microwaves off of weather phenomena.

As a plane enters a microburst, it first experiences a strong headwind that sends it upward. Soon thereafter, the plane experiences a strong tailwind which forces it downward. In the thirty-year period from 1964 to 1994, about thirty planes have crashed as a result of microbursts. Microbursts are the second-leading cause of airplane crashes, only exceeded by pilot error.

Microbursts occur with an alarming frequency. In a 1978 study, conducted by Tetsuya Theodore Fujita, fifty microbursts were detected over forty-two days in Chicago's western suburbs. Another study was conducted near Denver's Stapleton International Airport over an eighty-six-day period in the summer of 1982. A total of 186 microbursts were detected.

Flash floods

A flash flood is a sudden, intense, localized flood caused by persistent torrential rainfall. In the 1970s, flash floods replaced lightning as the number-one weather-related killer in the United States. Each year throughout the 1980s, flash floods killed an average of 110 people and were responsible for an average of $3 billion in property damage. In the first half of the 1990s the number of deaths due to flash floods rose to an annual average of 140.

Flash floods can be caused by warm-weather thunderstorms that are either slow-moving or stationary. The reason for their lack of movement is that the winds aloft are nearly calm. These storms unleash huge quantities of rain over one location. In contrast, thunderstorms which move more quickly spread their rain across larger areas and don't produce flash floods.

[See AlsoClimate; Clouds; Flood; Human Influences on Weather and Climate; Local Winds; Precipitation; Tornado; Weather: An Introduction ]

For More Information

BOOKS

Chambers, Catherine. Thunderstorm. 2nd ed. Portsmouth, NH: Heinemann, 2007.

Thunderstorm Project. The Thunderstorm: Report of the Thunderstorm Project. 2nd ed. Honolulu, HI: University Press of the Pacific, 2002.

PERIODICALS

Darack, Ed. "Florida's Central Gulf Coast: Lightning Capital of North America." Weatherwise. (Jan/Feb 2007): pp. 14-18.

WEB SITES

Schultz, David M., and Roger Edwards, eds. Electronic Journal of Severe Storms Meteorology. 〈http://www.ejssm.org/〉 (accessed March 23, 2007).

"Thunderstorms and Lightning." Federal Emergency Management Administration. 〈http://www.fema.gov/hazard/thunderstorm/index.shtm〉 (accessed March 23, 2007).

Wilhelmson, Bob, et al. "Types of Thunderstorms." University of Illinois: Weather World 2010 Project. 〈http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/type/home.rxml〉 (accessed March 23, 2007).

Thunderstorm

Thunderstorm development

Hail, lightning, and tornadoes

Resources

A thunderstorm, or an electrical storm, is a strong disturbance in the atmosphere that brings heavy rain, lightning, and thunder to areas from one to hundreds of kilometers across. An average thunderstorm may release as much energy as 10,000,000 kilowatt-hours— a large thunderstorm may release 100 times as much energy.

Thunderstorms are formed when humid air near the surface begins rising and cooling. The rising air forms clouds. Storms develop when the clouds cool enough to bring about the growth of rain droplets or ice crystals. Eventually the growing drops or crystals fall out of the cloud as precipitation. Strong updrafts and downdrafts that are inside a thunderstorm can cause static charges to build up in the cloud. Charges of opposite sign (electrical polarity) accumulate in different parts of the cloud until a spark occurs between them, resulting in the jagged bolts of lightning associated with thunderstorms. Severe thunderstorms may include hail, tornadoes, and damaging straight line winds, making these storms among nature’s most destructive.

Thunderstorm development

Thunderstorms develop in the same process that forms the puffy clouds of summer skies, cumulus clouds. These clouds form when humid air (that is, air with an abundance of water vapor) near the surface is pushed up by being forced over a mountain range, a front, strong solar heating of the surface, or some other means. As the air rises through the atmosphere, it expands and cools. Eventually the rising air cools to the point where its water vapor condenses to form droplets of liquid water. A huge collection of these tiny suspended droplets forms a cloud. At this stage, the rising air is visible as a cumulus cloud, called a convective cloud since it forms by convection (vertical air movement). During fair weather, the convective clouds stop their vertical growth at this point and do not bring rain.

To form a thunderstorm from a convective cloud several conditions are necessary. Most importantly the atmosphere must be unstable. In an unstable atmosphere the air temperature drops rapidly with height, meaning any bubble of air that begins rising and cooling will remain warmer than its surroundings. At every point in its ascent, the rising air acts like a hot air balloon: since it is warmer and less dense than the surrounding air, it continues to rise.

A second requirement for a strong thunderstorm is plenty of humid air. This condition supports the growth of cloud droplets and actually fuels the rising air through latent heat. The water vapor in the air comes from the evaporation of liquid water somewhere—most likely the oceans. To evaporate the water into vapor, energy is required, just as heat must be added to a kettle to make its water boil. This energy carried with the water vapor wherever it goes is latent or hidden heat. If and when the vapor condenses to form liquid water, the latent heat will be released back into the environment. Thus, when the water vapor in rising air condenses to form water droplets, a significant amount of heat is released to the surrounding air. Heating the air makes it less dense and increases the tendency of the air bubble, now a cloud, to rise.

As the air continues to rise and cool, droplets within the cloud begin to grow by coalescence (sticking together). In the clouds of colder climates, droplets may freeze to form ice crystals, which grow as more and more water vapor condenses on them. The droplets or ice crystals, known as precipitation particles, only grow as long as they can be supported by the updrafts. When they grow too large, they begin to fall out of the cloud as drizzle or raindrops. If the updrafts in the cloud are vigorous enough, much larger precipitation will be formed. In a thunderstorm the uplift process is so strong that the cloud grows to the height of the entire lower atmosphere (about 40,000 ft [12 km] above the surface) allowing large raindrops and hailstones to form.

At least two distinct types of thunderstorms can be observed. Over warm humid areas such as the Gulf of Mexico, the air-mass thunderstorm is the most common. These thunderstorms grow from converging cumulus clouds that rise and cool as described above. As the storm matures, rain begins to fall from the upper part of the cloud. The falling precipitation causes downdrafts. This downward moving air eventually overwhelms the rising air. The downdrafts effectively shut off the uplift necessary for the storm to grow, so the storm dissipates as the air sinks and no more rain is formed. These types of thunderstorms are

common over the Florida peninsula, where they bring showers and lightning strikes but rarely any hail or damaging winds, unless frontal action is nearby.

Potentially more severe thunderstorms form in temperate regions such as the central and eastern United States. Called frontal thunderstorms, these storms often form ahead of the advancing edge of a cold air mass (a cold front). In the summer months, the air ahead of the cold front is usually warm humid air that is highly unstable. The denser cold air forces the warmer lighter air ahead of it to rise, forming convective clouds and, eventually, rain. As in an air mass thunderstorm, the falling rain causes downdrafts in the cloud. Unlike the air mass storm, a frontal thunderstorm is arranged so that it is intensified by the downdrafts. The downdrafts become strong gusts of down-flowing air. When they reach the ground, the downdrafts spread out and force more warm humid air to begin rising into the thunderstorm. This condition provides the storm with more latent heat, strengthening the cloud’s updrafts, increasing its wind speeds, and improving the chances of heavy rain and hail. The storm advances into the warm air, vacuuming up humid air, and transforming it into a very organized system of powerful updrafts and down-drafts. After the storm and the front passes, the affected area is often affected by the cold air behind the front where temperatures and humidities are usually much lower.

Hail, lightning, and tornadoes

Strong updrafts in a thunderstorm support the growth of large rain drops and ice crystals. In a severe storm, some of the ice crystals may be dragged down by the downdrafts, then swept up again by updrafts.

Ice particles may be circulated several times through the storm cloud in this manner, picking up water with each cycle. In a process called riming, rainwater freezes onto the ice particles and eventually grows to be large hailstones. Hailstones continue to be recirculated through the cloud until they grow large enough to fall out under their own weight, falling against the strong updrafts. If located in the appropriate part of the storm, hailstones can grow to impressive sizes. Hail as large as 5.5 in (14 cm) in diameter has been recorded.

Another product of the vigorous up and down drafts in the storm cloud is lightning. Lightning is a giant spark caused by a buildup of static electrical charges, a larger version of the spark one gets by touching a metal doorknob after walking across a cloth carpet. By processes that still are not understood fully, thunderstorm clouds build up a large separation of electric charge with positive charges located near the top of the cloud and negative charges concentrated

KEY TERMS

Air-mass thunderstorm —A thunderstorm typical of tropical areas that may produce heavy rain but rarely any hail or tornadoes.

Convective cloud —A cloud formed from the vertical uplift (convection) of surface air.

Frontal thunderstorm —Thunderstorms associated with cold fronts moving through warm humid air.

Latent heat —The heat given off when water vapor condenses to form liquid water.

Precipitation particles —Rain drops or ice crystals that have grown heavy enough to fall out, or precipitate, out of a storm cloud.

Riming —The freezing on contact of raindrops as they are collected by an ice pellet growing to a hailstone.

Unstable atmosphere —The condition of the atmosphere when air temperature drops rapidly with height. Such conditions support rising air and contribute to strong thunderstorms.

near the middle. Usually the cloud base has a smaller pocket of positive charge. Because the charges are separated, huge voltage differences within the cloud and between the cloud base and the ground often result. The voltage difference is equalized suddenly by a bolt of lightning between these areas. The spark heats the air in the lightning channel to over 54,000°F (30,000°C) causing a rapid expansion. The resulting sound is heard as thunder.

Severe thunderstorms also may form tornadoes, columns of air spinning at extremely high wind speeds. Tornadoes pack wind speeds of 220 mph (over 100 m/second) in a small area, making them capable of great destruction.

See also Air masses and fronts; Atmospheric circulation; Tornado.

Resources

BOOKS

Aguado, Edward. Understanding Weather and Climate. Upper Saddle River, NJ: Pearson/Prentice Hall, 2006.

Ahrens, C. Donald. Meteorology Today: An Introduction to Weather, Climate, and the Environment. Belmont, CA: Thomson/Brooks/Cole, 2006.

Danielson, Eric W., James Levin, and Elliot Abrams. Meteorology. 2nd ed. with CD-ROM. Columbus, OH: McGraw-Hill Science/Engineering/Math, 2002.

De Villiers, Marg. Windswept: The Story of Wind and Weather. New York: Walker, 2006.

Hughes, Monica. Weather Patterns. Chicago, IL: Heinemann Library, 2004.

Kramer, Stephen P. Lightning. Boston, MA: Houghton Mifflin, 2001.

Lutgens, Frederick K. The Atmosphere: An Introduction to Meteorology. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

Rakov, Vladimir A. Lightning: Physics and Effects. Cambridge, UK, and New York: Cambridge University Press, 2003.

Redmond, Jim. Thunderstorms. Austin, TX: Raintree Steck-Vaughn, 2002.

Renner, Jeff. Lightning Strikes: Staying Safe Under Stormy Skies. Seattle, WA: Mountaineers Books, 2002.

James Marti

Thunderstorm

A thunderstorm is a strong disturbance in the atmosphere bringing heavy rain, lightning , and thunder to areas from one to hundreds of kilometers across. Thunderstorms are formed when humid air near the surface begins rising and cooling. The rising air forms clouds . Storms develop when the clouds cool enough to bring about the growth of rain droplets or ice crystals. Eventually the growing drops or crystals fall out of the cloud as precipitation . Strong updrafts and downdrafts are inside a thunderstorm which cause static charges to build up in the cloud. Charges of opposite sign accumulate in different parts of the cloud until a spark occurs between them, resulting in the jagged bolts of lightning associated with thunderstorms. Severe thunderstorms may include hail, tornadoes, and damaging straight line winds, making these storms among nature's most destructive.

Thunderstorm development

Thunderstorms develop in the same process that forms the puffy clouds of summer skies, cumulus clouds. These clouds form when humid air (that is, air with an abundance of water vapor) near the surface is pushed up by being forced over a mountain range, a front, strong solar heating of the surface, or some other means. As the air rises through the atmosphere, it expands and cools. Eventually the rising air cools to the point where its water vapor condenses to form droplets of liquid water. A huge collection of these tiny suspended droplets forms a cloud. At this stage the rising air is visible as a cumulus cloud, called a convective cloud since it forms by convection (vertical air movement). During fair weather the convective clouds stop their vertical growth at this point and do not bring rain.

To form a thunderstorm from a convective cloud several conditions are necessary. Most importantly the atmosphere must be unstable. In an unstable atmosphere the air temperature drops rapidly with height, meaning any bubble of air that begins rising and cooling will remain warmer than its surroundings. At every point in its ascent the rising air acts like a hot air balloon : since it is warmer and less dense than the surrounding air it continues to rise.

A second requirement for a strong thunderstorm is plenty of humid air. This condition supports the growth of cloud droplets and actually fuels the rising air through latent heat . The water vapor in the air comes from the evaporation of liquid water somewhere—most likely the oceans. To evaporate the water into vapor, energy is required just as heat must be added to a kettle to make its water boil. This energy carried with the water vapor wherever it goes is latent or hidden heat. If and when the vapor condenses to form liquid water, the latent heat will be released back into the environment. Thus when the water vapor in rising air condenses to form water droplets a significant amount of heat is released to the surrounding air. Heating the air makes it less dense and increases the tendency of the air bubble, now a cloud, to rise.

As the air continues to rise and cool, droplets within the cloud begin to grow by coalescence (sticking together). In the clouds of colder climates droplets may freeze to form ice crystals, which grow as more and more water vapor condenses on them. The droplets or ice crystals, known as precipitation particles, only grow as long as they can be supported by the updrafts. When they grow too large, they begin to fall out of the cloud as drizzle or raindrops. If the updrafts in the cloud are vigorous enough, much larger precipitation will be formed. In a thunderstorm the uplift process is so strong that the cloud grows to the height of the entire lower atmosphere (about 40,000 ft [12 km] above the surface) allowing large raindrops and hailstones to form.

At least two distinct types of thunderstorms can be observed. Over warm humid areas such as the Gulf of Mexico the air-mass thunderstorm is the most common. These thunderstorms grow from converging cumulus clouds that rise and cool as described above. As the storm matures, rain begins to fall from the upper part of the cloud. The falling precipitation causes downdrafts. This downward moving air eventually overwhelms the rising air. The downdrafts effectively shut off the uplift necessary for the storm to grow, so the storm dissipates as the air sinks and no more rain is formed. These types of thunderstorms are common over the Florida peninsula bringing showers and lightning strikes but rarely any hail or damaging winds unless frontal action is nearby.

Potentially more severe thunderstorms form in temperate regions such as the central and eastern United States. Called frontal thunderstorms these storms often form ahead of the advancing edge of a cold air mass (a cold front). In the summer months the air ahead of the cold front is usually warm humid air that is highly unstable. The denser cold air forces the warmer lighter air ahead of it to rise forming convective clouds which eventually rain. As in an air mass thunderstorm, the falling rain causes downdrafts in the cloud. Unlike the air mass storm, a frontal thunderstorm is arranged so that it is intensified by the downdrafts. The downdrafts become strong gusts of down-flowing air. When they reach the ground the downdrafts spread out and force more warm humid air to begin rising into the thunderstorm. This provides the storm with more latent heat, strengthening the cloud's updrafts, increasing its wind speeds, and improving the chances of heavy rain and hail. The storm advances into the warm air, vacuuming up humid air, and transforming it into a very organized system of powerful updrafts and downdrafts. After the storm and the front passes, the affected area is often affected by the cold air behind the front where temperatures and humidities are usually much lower.

Hail, lightning, and tornadoes

Strong updrafts in a thunderstorm support the growth of large rain drops and ice crystals. In a severe storm some of the ice crystals may be dragged down by the downdrafts then swept up again by updrafts. Ice particles may be circulated several times through the storm cloud in this manner picking up water with each cycle. In a process called riming, rain water freezes onto the ice particles and eventually grows to be large hailstones. Hailstones continue to be recirculated through the cloud until they grow large enough to fall out under their own weight, falling against the strong updrafts. If located in the right part of the storm, hailstones can grow to impressive sizes. Hail as large as 5.5 in (14 cm) in diameter has been recorded.

Another product of the vigorous up and down drafts in the storm cloud is lightning. Lightning is a giant spark caused by a buildup of static electrical charges, a larger version of the spark one gets by touching a metal doorknob after walking across a cloth carpet. By processes that still are not understood fully, thunderstorm clouds build up a large separation of electric charge with positive charges located near the top of the cloud and negative charges concentrated near the middle. Usually the cloud base has a smaller pocket of positive charge. Separating charges results in huge voltage differences within the cloud and between the cloud base and the ground. The voltage difference is equalized suddenly by a bolt of lightning between these areas. The spark heats the air in the lightning channel to over 54,000°F (30,000°C) causing a rapid expansion. The resulting sound is heard as thunder.

Severe thunderstorms also may form tornadoes, columns of air spinning at extremely high wind speeds. Tornadoes pack wind speeds of 220 mph (over 100 m/second) in a small area, making them capable of great destruction.

See also Air masses and fronts; Atmospheric circulation; Tornado.

Resources

books

Battan, Louis J. Weather. Engelwood Cliffs: Prentice-Hall Inc., 1985.

Battan, Louis J. Weather in Your Life. New York: W.H. Freeman & Co., 1983.

Danielson, Eric W., James Levin, and Elliot Abrams. Meteorology. 2nd ed. with CD-ROM. Columbus: McGraw-Hill Science/Engineering/Math, 2002.

Hardy, Ralph, Peter Wright, John Kington, and John Gribben. The Weather Book. Boston: Little, Brown and Co., 1982.

McNeill, Robert. Understanding the Weather. Las Vegas: Arbor Publishers, 1991.

Mogil, H. Michael,and Barbara G. Levine. The Amateur Meteorologist. New York: Franklin Watts, 1993.

Wallace, John M., and Peter V. Hobbs. Atmospheric Science:An Introductory Survey. New York: Academic Press, 1977.

James Marti

KEY TERMS

Air-mass thunderstorm

—A thunderstorm typical of tropical areas which may produce heavy rain but rarely any hail or tornadoes.

Convective cloud

—A cloud formed from the vertical uplift (convection) of surface air.

Frontal thunderstorm

—Thunderstorms associated with cold fronts moving through warm humid air.

Latent heat

—The heat given off when water vapor condenses to form liquid water.

Precipitation particles

—Rain drops or ice crystals that have grown heavy enough to fall out, or precipitate, out of a storm cloud.

Riming

—The freezing on contact of raindrops as they are collected by an ice pellet growing to a hailstone.

Unstable atmosphere

—The condition of the atmosphere when air temperature drops rapidly with height. Such conditions support rising air and contribute to strong thunderstorms.

Thunderstorm

A storm is any disturbance in Earth's atmosphere with strong winds accompanied by rain or snow and sometimes thunder and lightning. Storms have a generally positive effect on the environment and on human societies because they are the source of most of the precipitation on which the planet depends.

The most common violent change in the weather is the thunderstorm. In the United States, thunderstorms usually occur in the late spring and summer. Thunderstorms are rare in the parts of the country where the air tends to be colder, such as the New England states, North Dakota, and Montana. They also are rare by the Pacific Ocean, where summers are dry. The southeastern states tend to have the most thunderstorms. Some parts of Florida experience thunderstorms on a average of 100 days a year. A thunderstorm may last up to two hours, but most thunderstorms peak after about 15 to 30 minutes.

How thunderstorms form

Thunderstorms develop by the same process that forms cumulus clouds, the puffy clouds of summer skies. These clouds form when a humid air mass (air with an abundance of water vapor) near the surface rises on currents of air called updrafts. As the air mass rises through the atmosphere it expands and cools. Eventually, the rising air cools to the point where its water vapor condenses to form droplets of liquid water, releasing heat in the process into the surrounding air. This latent heat, in turn, causes the air mass to rise ever more quickly. The upward movement of air in a storm cloud has been measured at more than 50 miles (80 kilometers) per hour.

As the upward movement of air continues, more moisture condenses out of the air mass and the suspended droplets form a large cloud. Depending on atmospheric conditions, a storm cloud of this type may rise to a height of anywhere from 6 to 9 miles (10 to 15 kilometers). In the clouds of colder climates, droplets may freeze to form ice crystals, which grow as more and more water vapor condenses on them. The droplets or ice crystals only grow as long as they can be supported by the updrafts. When they grow too large they begin to fall out of the cloud as drizzle or raindrops.

Words to Know

Latent heat: The heat given off when water vapor condenses to form liquid water.

Riming: The freezing on contact of raindrops as they collect on an ice pellet growing to a hailstone.

Updraft: Any movement of air away from the ground.

If the updrafts in the cloud are vigorous enough, much larger precipitation will be formed. In a severe storm, some of the ice crystals may be dragged down by the downdrafts, then swept up again by updrafts. Ice particles may be circulated several times through the storm cloud in this manner, picking up water with each cycle. In a process called riming, raindrop water freezes onto the ice particles, eventually producing large hailstones. Hailstones continue to be recirculated through the cloud until they grow large enough to fall out under their own weight. If located in the right part of the storm, hailstones can grow to impressive sizes. Hail as large as 5.5 inches (14 centimeters) in diameter has been recorded.

Lightning and thunder

Another product of the vigorous up and down drafts in the storm cloud is lightning. Lightning is a giant spark caused by a buildup of static electrical charges. By processes that still are not fully understood, thunderstorm clouds build up a large concentration of positive electrical charges near the top of the cloud and negative electrical charges near the middle. Usually the cloud base has a smaller pocket of positive charge. These opposite charges result in huge voltage differences within the cloud and between the cloud base and the ground. The opposite charges are strongly attracted to each other and when the air between them can no longer keep them apart, a discharge takes place—a bolt of lightning. Depending upon the location of the opposite charges, lightning can occur as cloud-to-ground lightning, cloud-to-cloud lightning, or cloud-to-air lightning.

The temperature of a lightning bolt exceeds 40,000°F (22,000°C). The surrounding air is superheated, causing it to expand and then contract rapidly. This expansion and contraction produces the sound vibrations heard as thunder.

It is possible to calculate how far away a storm is by counting the seconds between a lightning flash and a thunder clap. Since it takes thunder about 5 seconds to travel 1 mile (3 seconds to travel 1 kilometer), simply divide the counted seconds by 5 to determine the miles (by 3 to determine the kilometers). Normally, thunder cannot be heard more than 20 miles (32 kilometers) away.

[See also Air masses and fronts; Cyclone and anticyclone; Tornado ]

thunderstorm Electrical storm caused by the separation of electrical charges in clouds. Water drops are carried by updraughts to the top of a cloud, where they become ionized and accumulate into positive charges – the base of the cloud being negatively charged. An electrical discharge (a spark) between clouds, or between a cloud and the ground, is accompanied by light (seen as a lightning stroke) and heat. The heat expands the air explosively and causes it to reverberate and produce sounds and echoes called thunder.