Weather, Violent

Violent weather can occur anywhere. Depending on where an outbreak occurs, there will be different types of storms. However, not all storms are violent. There are, for example, thunderstorms that are not severe, and tornadoes that do not cause extensive damage. Common measurement scales do not provide an understanding of the strength of a weather disturbance. Consequently, meteorologists must use different scales to measure the strength of weather systems, and to make predictions about when violent weather may strike.

Thunderstorms

There are three ingredients necessary for a thunderstorm to form:

1. moisture must be present in the lower levels of the atmosphere;
2. cold air must be present in the upper atmosphere; and
3. there must be a catalyst to push the warm air into the cold air.

This catalyst is usually in the form of a front, which is the interface between air masses at different temperatures. As the warm air rises it cools and some of the water vapor turns into clouds. Eventually the air mass will reach an area where it is the same temperature as the area surrounding it, and thunderstorms can occur.

Thunderstorms can be measured as strong or severe. Some thunderstorms, though, may be neither. Severe thunderstorms are classified as having winds greater than or equal to 58 mph (miles per hour), and/or hail greater than or equal to three-quarters of an inch. If a severe thunderstorm is strong enough, it can become a supercell thunderstorm. A supercell thunderstorm is the type that is most likely to spawn a tornado. In a supercell, as the air moves upward it begins to rotate, which does not happen in other types of thunderstorms. When the whole cell rotates, it is called a mesocyclone.

Tracking Thunderstorms. Many meteorologists use Doppler radar to track thunderstorms. Christian Doppler suggested the idea behind Doppler radar over 150 years ago. It differs from traditional weather radar because it can detect not only where a storm is located, but also how fast the air inside the storm is moving.

Doppler radar transmits pulses of waves that are reflected by water droplets in the air. Some of the energy reflected by the droplets then returns to the radar unit and shows how the storm is moving and what it looks like. Doppler radar also shows where moisture is located in a storm because where there is a higher moisture content, there is greater reflectivity since there is more for the waves to reflect off of.

The ability to determine the structure of storms is particularly useful in the detection of severe weather. For example, on radar, areas of a storm containing hail will display denser than areas with just rain. Doppler radar is therefore an invaluable tool in storm detection and prediction. Meteorologists can monitor a storm across large areas and make sure the public stays informed about its progress.

Tornadoes

Tornadoes are formed in thunderstorms when winds meet at varying angles. If low-level winds flow in a southerly direction and upper-level wind flows towards the west, the air masses can interact violently. The two paths will begin to swirl against each other, forming a vortex. As this vortex increases it will appear as a funnel to someone on the ground. Once this funnel touches the ground, it becomes a tornado.

One of the recent scales created to measure the severity of a tornado is the Fujita scale. Developed in 1971, the Fujita scale is used to measure the intensity of a tornado by examining the damage caused after it has passed over man-made structures. Tetsuya Theodore Fujita (1920–1998) took into account the wind speed of a tornado and the damage it caused, and from that he was able to assign a number to represent the intensity of the tornado.

On the Fujita scale, an F2 tornado is classified as a significant tornado with winds between 113 and 157 mph. An F2 tornado can cause considerable damage. Roofs may be torn off frame houses, mobile homes may be demolished, and large trees can be uprooted. In an F5 tornado winds can reach between 261 and 318 mph. A tornado of this strength can lift frame houses from their foundation and carry them considerable distances, send automobile-sized objects through the air, and cause structural damage to steel-reinforced structures.

Not long after the inception of the Fujita scale, it was combined with the work of Allen Pearson to also consider the components of path length and width. Adding these two measurements together provides more accurate tornado readings.

The Fujita scale is not perfect. Wind speed does not always accurately correlate with the amount of damage caused by a tornado. For example, a tornado with a great intensity that touches down in a rural area may rank lower on the Fujita scale than a less intense tornado that touches down in an urban area. This is because a tornado is a rural area could cause less damage

since there is less to be damaged. An accurate F-scale rating often depends on the experience of the person who assigns the rating.

Though the Fujita scale does have flaws, it is an improvement over the rating system that had previously been in place. The scale is simple enough to be put into daily practice without substantial added expense, and a rating can be assigned in a reasonable amount of time.

Predicting Tornadoes. While tornadoes can occur during any time of year in the United States, they most frequently occur during the months of March through May. Organizations such as the National Weather Service keep detailed records of when and where tornadoes occur. From these records, they can make predictions about when the most favorable times are for tornadoes to occur in different areas of the country. Different areas have different times when tornado development is most likely. For example, by keeping records, it has been shown that the mid-section of the country is much more likely to have tornadoes than the west. The amount of tornadoes seen in the mid-section has earned it the nickname "tornado alley."

Hurricanes

Hurricanes form when there is a pre-existing weather disturbance, warm tropical oceans, and light winds. If these conditions persist, they can combine and produce violent wind, waves, and torrential rain.

A hurricane is a type of tropical cyclone. These cyclones are classified by their top wind speeds as a tropical depression, tropical storm, or a hurricane. In a hurricane, walls of wind and rain form a circle of spinning wind and water. In the very center of the storm there is a calm called the "eye." At the edges of the eye, the winds are the strongest. Hurricanes cover huge areas of water and are the source of considerable damage when they come onshore.

Like tornadoes, hurricanes are classified according to their wind speed. They are measured on the Saffir-Simpson hurricane scale. The number rating assigned from the scale is based on a hurricane's intensity. A category one hurricane has winds between 74 and 95 mph. This type of hurricane will not have a very high storm surge, and will cause minimal damage to building structures. In a category five hurricane, top sustained winds will be greater than 155 mph. The storm surge in a hurricane like this can be greater than 18 feet above normal, flooding will occur, and evacuation will be necessary in low-lying areas.

The number assigned to a hurricane is not always a reliable indicator of how much damage a hurricane can cause. Lower-category storms can cause more damage than high-category storms depending on where they make landfall. In most hurricanes, loss of life occurs due to flooding, and lesser hurricanes, and even tropical storms, can often bring more extensive rainfall than a higher-category hurricane.

Predicting Hurricanes. For years meteorologists have been tracking and keeping records on where and when hurricanes occur. The official start of hurricane season in the Atlantic Basin is June 1. The peak time for hurricane formation is from mid-August to late October. Hurricanes can form outside of the season, but it is uncommon. Scientists have been able to designate a season for hurricanes from careful observation of storms over many years.

Not only can scientists predict the favorable time of year for hurricanes to form, but they can also make predictions on the path a hurricane may take depending on where it forms at certain times in the season. To make these predictions, scientists must use statistical analysis to find out probabilities on where and when a hurricane can form.

Floods

Floods happen when the ground becomes so saturated with moisture that it cannot hold any more. Often, floods occur when too much rain falls in a short period of time, or when snow melts quickly. Initially the ground can absorb the moisture, but eventually there is a point where the soil cannot absorb anymore. At this point, water begins to collect above ground, and flooding occurs.

A flood is measured by how much the water is above the flood stage. Flood stages are different everywhere. Measurements of a flood can be done in inches, feet, or larger units depending on the severity. We have also begun to quantify a flood by the economic damage it causes.

Predicting Floods. The prediction of floods can be statistically described by frequency curves. Flood frequency is expressed as the probability that a flood of a given magnitude will be equaled or exceeded in any one year.

The 100-year flood, for example, is the peak discharge that is expected to be equaled or exceeded on the average of once in 100 years. In other words, there is a 1 percent chance that a flood of peak magnitude will occur in a given year. Similarly, a 50-year flood has a 2 percent chance of occurring in any given year, and a 25-year flood has a 4 percent chance. This recurrence interval represents a long-term average, and these types of floods could occur with a greater or lesser frequency. Many floods that people consider to be record-breaking are actually much lower in magnitude.

Violent weather is unavoidable. With reliable measurements and predictions, though, humans can increase their preparedness to minimize the damage and destruction violent weather can cause.

see also Predictions; Weather Forcasting Models.

Brook Ellen Hall

Internet Resources

Hurricane Basics. National Hurricane Center. <http://www.nhc.noaa.gov/HAW/basics/hurricane_basics.html>.

Questions and Answers about Thunderstorms. National Severe Storms Laboratory. <http://www.nssl.noaa.gov/edu/storm/>.

The Fujita Scale of Tornado Intensity. The Tornado Project. <http://www.tornadoproject.com/fscale/fscale.htm>.

The Saffir-Simpson Hurricane Scale. National Hurricane Center. <http://www.nhc.noaa.gov/aboutsshs.html>.

Tracking Severe Weather with Doppler Radar. Forecast Systems Laboratory. <http://www.fsl.noaa.gov/~osborn/sam/Activity1.html>.

Tracking Tornadoes. The Weather Channel. <http://www.weather.com/newscenter/specialreports/tornado/tracking/toto.html>.

HOW REALISTIC IS HOLLYWOOD?

It is possible to measure a tornado from the inside, just as the characters were trying to do in the 1996 movie Twister. The idea for the device that they used, Dorothy, actually came from a similar instrument, TOTO (TOtable Tornado Observatory), invented by Howie Bluestein. TOTO was retired after 1987 and is now on display in Silver Spring, Maryland, at a facility of the National Oceanic and Atmospheric Administration.