Weather and climate

Weather refers to the atmospheric conditions at a certain time or over a certain short period in a given area . It is described by a number of meteorological phenomena that include atmospheric pressure, wind speed and direction, temperature, humidity , sunshine, cloudiness, and precipitation . In contrast, climate refers to long-term, cyclic or seasonal patterns of temperature, precipitation, winds, etc.

Climates are often defined in terms of area, latitude , altitude, or other geophysical features. Although there are thousands of microclimate variations, climates can essentially be broken down into four basic types. Hot, moist climates feature high rainfall with often intense and rapid chemical weathering . Cold, moist climates still feature chemical weathering but because of the lower temperature, the rates are dramatically reduced from those encountered in hot, moist climates. Cold, dry climates feature the least weathering but mechanical weathering (e.g., ice wedging) does produce slow landscape evolution . Hot, dry climates often have intense mechanical weathering pressures (e.g., wind, sand-blasting, etc).

The effects of weather also contribute in shaping Earth's surface features. The impact of weather is most pronounced during the occurrence of extreme weather situations, such as prolonged periods of heat, cold, rain, drought , and smog conditions. In addition, shorter but intense events such as hurricanes, tornadoes, winter blizzards, freezing rain, and floods also produce often-dramatic effects on both the social and geologic landscape. The concern to reduce the impact of weather on public health and property provides an important motivation for the continued efforts by meteorologists and scientists to improve weather forecasting .

The study of meteorological phenomena related to both weather and climate changes is an important component in the development of chaos theory . Chaos theories are used to study weather-related complex systems in which, out of seemingly random, disordered processes, there arise new processes that are more predictable.

Most of the weather elements on which weather forecasting is based cannot be seen directly, they can only be observed by the effects they create. For the most part, weather variables are measured and recorded by instruments. For example, air subjects everything to considerable pressure. At sea level, the atmosphere exerts approximately 15 lb/in² (about 1 kg/cm²) of pressure. The standard instrument used to measure atmospheric pressure is the mercury barometer. The physics for the barometer dates to the classic experiments performed for the first time in 1643 by the Italian scientist Evangelista Torricelli (1608–1647). A column of mercury is held in a closed glass tube, then inverted and immersed into a mercury dish. The weight of the column is thus balanced by the atmospheric pressure and the length of the column affords a measure of that weight. The mean atmospheric pressure at sea level is 760 mmHg or 1,013 millibars. Pressure as well as air density decrease with increasing altitude and barometric pressure will rise or fall as a function of different weather systems. On weather maps, points of equal pressure are represented by isobars .

Wind, by its broadest definition, is any air mass in motion relative to Earth's surface. It is predominantly a horizontal movement. However, localized vertical air motion— updraft or downdraft—also occurs, for example in storms. Wind is described by two quantities: speed and direction. Wind velocity as measured by the anemometer is reported in mi/hr, knots, or km/hr. The wind direction is given by the compass bearing from which the wind blows, for example, a southerly wind blows from the south. The horizontal air movement near Earth's surface is controlled by four forces: the pressure gradient force, the Coriolis force, the centrifugal force, and the frictional forces. The existence of barometric differences in the atmosphere sets up the pressure gradient force that causes air to move from a higher to a lower pressure area. The Coriolis force is the apparent deflection of air mass caused by the rotation of Earth. Because of Earth's rotation, there is an apparent deflection of all matter in motion to the right of their path in the northern hemisphere and to the left in the southern hemisphere. For this reason, in the northern hemisphere, high-pressure systems (area of atmospheric divergence) rotate clockwise, low-pressure systems (areas of atmospheric convergence) counterclockwise. These rotational patterns are reversed in the southern hemisphere.

Temperature and humidity are crucial in defining the origins and types of air masses. The thermal properties of an air mass are determined by its latitudinal position on the globe, and its moisture content depends on the underlying surface, be it land or water . For example, polar air is cold and dry, whereas tropical air is hot and humid. In essence, the convergence of these two types of air masses is responsible for most global weather activities. The clash of these contrasting air masses leads to the formation of frontal wave depressions moving in an oscillating west-east pattern and steered by the upper-air jet stream . Hot, humid tropical air is also the source material that fuels the devastating force of hurricanes. Across the network of weather stations, readings of temperature and humidity are taken at regular intervals. Standard equipment in an instrumentation shelter consists of a dry and a wet bulb thermometer, and readings from the two are used to establish the dew point . A pair of special thermometers measures the maximum and minimum temperatures occurring during day and nighttime. The hygrometer measures the relative humidity of the air. In fully-automated stations, electronic sensors measure and transmit weather information.

In addition to temperature and humidity, daily weather forecasts inform the public about the heat index during summer and about the wind chill index during the winter. These indicators warn the about the possible dangers to human health resulting from exposure to summer heat and winter cold. By combining temperature and humidity, the heat index gives a measure of what temperatures actually feel like. In terms of human health, an increased heat index corresponds to physical activity being more exhausting, resulting in possible heat-related illnesses, cramps, exhaustion, or heatstroke. By contrast, the wind chill factor relates the risk of cold to exposed skin, which may lead to frostbite and hypothermia. The wind chill factor takes into account the effect of wind speed on temperature. For example, a temperature of 20°F (−6.66°C) at a wind-speed of 20 mph (32.18 km/hr) will feel like −10°F (−12.2°C). Humidity is the one factor that not only creates weather activity, but also makes life on Earth possible.

Water exists in one of the following three phases: vapor, liquid, or ice. Water vapor, the invisible gaseous form of water, is always present in the atmosphere; it is defined as the partial pressure of the atmosphere and therefore, like air pressure, it is measured in mmHg. Water vapor supplies the moisture for dew and frost, for clouds and fog , and for wet and frozen forms of precipitation.

The visible weather elements are, of course, sunshine, clouds, and precipitation. Traditionally, the forecasting of weather was mainly based on the observation of clouds, because their size, shape, and location are the visible indicators of air movement and of changes in water going from vapor to liquid or ice. The first important contribution to the classification of clouds was made in 1802 by the English scientist Luke Howard . Based on his observations, clouds were grouped according to three basic shapes: cumulus (heaps), stratus (layers), and cirrus (wispy curls). He also attached the term nimbus to clouds associated with precipitation. From this basic scheme has evolved the modern classification system of clouds by which the lower 10 mi (16 km) of the atmosphere are divided into three layers of clouds characterized by their water phase, i.e., low clouds consisting of water droplets, middle clouds containing a mixture of water droplets and ice crystals , and high clouds entirely made up of ice crystals. While some types of clouds are confined to one layer—such as stratus, stratocumulus and smaller type cumuli in the lower layer, altocumulus and altostratus in the middle layer, and cirrus and cirrostratus in the higher layer—other types can occupy two layers, namely, the nimbostratus and the swelling cumulus cloud which can reside in both lower and middle layers, as well as the cirrocumulus found in the middle and higher layers. A third type can expand through all three layers, such as the huge cumulus congestus cloud and of course, the cumulonimbus with its characteristic anvil.

Warm and cold fronts are also distinct in their cloud cover. The first signs of an approaching warm front are the cirrus and cirrostratus clouds, followed by the obscuring altostratus and the thick nimbostratus with continuous precipitation, and occasionally with the formation of patches of stratus clouds. After the passage of the warm front, precipitation ceases and the cloud cover breaks up. The typical cloud of cold fronts is the cumulonimbus and, depending on the instability of the air, nimbostratus. Precipitation will vary from brief showers to heavy, prolonged downpours with thunder and lightning .

The weather's immediate impact on public health has been demonstrated numerous times by severe events like hurricanes, tornadoes, floods, snow and ice storms, and prolonged periods of extreme heat or cold. In past years, considerable research efforts have been deployed to gain a better understanding of the physics of hurricanes and tornadoes. Better forecasting the path of severe weather systems and broadcasting early warnings has helped decrease the occurrence of weather-related deaths and injuries. Concerns are now increasingly focused on the weather's indirect influence on human health. It has been observed that certain weather situations provide conditions that will, for example, foster the proliferation of insects and consequently the spread of disease. This was the case in 1999 in the eastern regions of the United States, where weeks of drought and heat created the perfect breeding conditions for mosquitoes carrying a type of encephalitis virus. Weather conditions can also heighten the effects of pollution. For example, air pollutants trapped in fog or smog may cause severe respiratory problems. The interrelationship of weather and environmental health issues lends urgency for more meteorology research in order to develop the accurate forecasting capabilities required to lower the impact of adverse weather and climate changes on public health.

See also Air masses and fronts; Atmospheric chemistry; Atmospheric circulation; Atmospheric composition and structure; Atmospheric inversion layers; Atmospheric pressure; Drought; El Niño and La Nina phenomena; Hydrologic cycle; Isobars; Jet stream; Land and sea breeze; Lightning; Ocean circulation and currents; Seasonal winds; Thunder; Tornado; Tropical cyclone; Weather forecasting methods; Weather radar; Weather satellite; Wind chill; Wind

Weather

Solar energy

Humidity, clouds, and precipitation

Atmospheric pressure and winds

Terrestrial characteristics

Topographic factors

Weather and climate

Resources

Weather is the condition of the atmosphere at any given time and place. Weather conditions are determined by six major factors: air temperature, air pressure, humidity of the air, amount and kind of cloud cover, amount and kind of precipitation, and speed and direction of the wind. Weather condition patterns for a region or the entire planet can be charted on a weather map containing information about all six of these factors. This information often can be used to produce a weather forecast, which is a prediction of weather conditions at some future time for some given region.

The study of weather is known as meteorology. No exact date can be given for the beginnings of this science, because humans have studied weather conditions for many centuries. Indeed, the word meteorology itself goes back to Meterologica, a book written by the Greek natural philosopher Aristotle in about 340 BC. Many scholars date the rise of modern meteorology to the work of a Norwegian father and son team, Vilhelm and Jakob Bjerknes. They were the first to develop the concept of masses of air moving across Earth’s surface, affecting weather conditions as they moved. They also created the first widespread system for measuring weather conditions throughout their native Norway.

The six factors determining weather conditions result from the interaction of four basic physical elements: solar radiation, Earth’s atmosphere, Earth itself, and natural landforms on Earth’s surface.

Solar energy

The driving force behind all meteorological changes taking place on Earth is solar energy. The outer portions of Earth’s atmosphere receive an average of 2 calories per square cm per minute. This value is known as the solar constant. Although the solar constant changes over very long periods of time, it does not vary enough to affect the general nature of Earth’s weather over short periods of time.

Thirty percent of all solar energy is lost to space by scattering and by reflection off clouds and Earth’s surface. Another 19% is absorbed by gases in the atmosphere and by clouds. About a quarter of it (25%) reaches Earth’s surface directly; another quarter (26%) eventually reaches the surface after being scattered by gases in the atmosphere.

An important factor in determining the fate of solar radiation is its wavelength. Shorter wavelengths tend to be absorbed by gases in the atmosphere (especially oxygen and ozone) whereas radiation of longer wavelengths tends to be transmitted to Earth’s surface.

Solar radiation that reaches Earth’s surface is absorbed to varying degrees, depending on the kind of material on which it falls. Because dark and rough surfaces absorb radiation better than light and smooth surfaces, soil tends to absorb more solar radiation than water.

Solar energy that reaches Earth’s surface is re-radiated back to the atmosphere as heat, also referred to as infrared radiation. Infrared radiation consists of much longer wavelengths. This re-radiated energy is likely to be absorbed by certain gases in the atmosphere such as carbon dioxide and nitrous oxide. This absorption process, the greenhouse effect, is responsible for maintaining the planet’s annual average temperature.

Humidity, clouds, and precipitation

The absorption of solar energy by Earth’s surface and its atmosphere is directly responsible for most of the major factors making up weather patterns. For example, when the water in oceans, lakes, rivers, streams, and other bodies of water is warmed, it tends to evaporate and move upward into the atmosphere. The amount of moisture found in the air at any one time and place is called the humidity. Humans are very sensitive to this characteristic of weather.

Water that has evaporated from Earth’s surface (or escaped from plants through the process of transpiration) rises to an altitude in the atmosphere at which the air around it is cold enough to cause condensation. When moisture condenses into tiny water droplets or tiny ice crystals, clouds are formed.

Clouds are an important factor in the development of weather patterns. They tend to reflect sunlight back into space. Thus, an accumulation of cloud cover may contribute to a decrease in heat retained in the atmosphere.

Clouds give rise to various types of precipitation. As water droplets or ice crystals collide with each other, they coalesce and form larger particles. Eventually, the particles become large enough and heavy enough to overcome upward drafts in the air and fall to Earth as precipitation. The form of precipitation that occurs (rain, snow, sleet, hail, etc.) depends on the atmospheric conditions through which the water or ice falls.

Atmospheric pressure and winds

Solar energy also is responsible for different atmospheric pressures across the planet’s surface, and the winds that result from these different pressures. Because Earth’s surface differs in color and texture from place to place, some locations will be heated more intensely by solar radiation than others. Warm places usually heat the air above them, setting convection currents into movement that carry masses of air upward into the upper atmosphere. Those same convection currents then carry other masses of air downward from the upper atmosphere toward Earth’s surface.

In regions where warm air moves upward, the atmospheric pressure tends to be low. Downward air movements are associated with higher atmospheric pressures. These higher or lower atmospheric pressures can be measured by a barometer. Barometers provide information not only about current pressures but about possible future weather patterns as well. The existence of areas with different atmospheric pressures accounts for the movement of air, which is felt as wind.

Terrestrial characteristics

The sun provides the energy by which weather patterns can develop, and certain features of Earth itself determine the precise forms in which those patterns may be exhibited. One example has already been provided above. Earth’s surface is highly variable, ranging from oceans to deserts to cultivated land to urbanized areas. The way solar energy is absorbed and reflected from each of these regions is different, accounting for variations in local weather patterns.

Other characteristics of the planet account for more significant variations in weather patterns. These characteristics include such features as the tilt of Earth on its axis in relation to its plane of revolution, and the variations in Earth’s distance from the sun.

The fact that Earth’s axis is tilted at an angle of 23 1/2° to the plane of its orbit means that the planet is heated unevenly by the sun. During the summer, sunlight reaching the Northern Hemisphere strikes more nearly at right angles than it does in the Southern Hemisphere. In the winter, the situation is reversed.

The elliptical shape of Earth’s orbit around the sun also affects weather conditions. At certain times of the year the planet is closer to the sun than at others. This variation means that the amount of solar energy reaching the outer atmosphere will vary from month to month depending on Earth’s location in its path around the sun.

Earth’s rotation on its own axis also influences weather patterns. If Earth did not rotate, air movements on the planet would probably be relatively simple: air heated along the equator would rise into the upper atmosphere, travel northward toward the poles, be cooled, and then return to Earth’s surface at the poles.

Earth’s rotation causes the deflection of these theoretically simple air movements. Instead of a single overall equator-to-poles air movement, global winds are broken up into smaller cells. In one cell warm air rises above the equator, moves northward in upper

KEY TERMS

Humidity —The amount of water vapor contained in the air.

Meteorology —The study of Earth’s atmosphere and the changes that take place within it.

Orographic —A term referring to effects produced when air moves across a mountain range.

Solar constant —The rate at which solar energy strikes the outermost layer of Earth’s atmosphere.

Solar energy —Any form of electromagnetic radiation that is emitted by the sun.

Topography —The detailed surface features of an area.

altitudes, is cooled, and returns to Earth in the regions around 30° north and south latitude. A second cell consists of air that moves upward in the regions around 60° north and south latitude, across the upper atmosphere, and then downward at about 30° north and south latitude. The final cell contains winds traveling upward at 60° north and south latitude and then downward again at the poles.

Topographic factors

Irregularities on Earth’s surface affect weather. A mountain range can dramatically affect the movement of approaching air masses. Suppose that a mass of warm moisture-filled air is forced to ascend one side of a mountain range. As the air is pushed upward it cools off and moisture begins to condense out, first in the form of clouds then as precipitation. Thus, the windward sides of mountains typically receive more rain and snow than the leeward sides.

As the air mass continues over the top of the mountain range, it does so without its moisture. The winds that sweep down the far side of the range will tend to be warm and dry. The term orographic is used to describe changes in weather patterns like these induced by mountain ranges.

Weather and climate

The terms weather and climate often are used in conjunction with each other, but refer to different phenomena. Weather involves atmospheric conditions that currently prevail or that exist over a relatively short period of time. Climate refers to the average weather pattern for a region (or for the entire planet) over a much longer period of time. Many authorities use a time period of at least three decades to distinguish between weather and climate.

Changes in weather patterns are easily observed. It may rain today and be clear tomorrow. Changes in climate patterns are much more difficult to detect. If the summer of 1997 is unusually hot, there is no way of knowing if that fact is part of a general trend towards warmer weather or a single variation that will not appear again for some time.

See also Atmospheric temperature; Seasons; Weather forecasting; Weather mapping.

Resources

BOOKS

Ahrens, Donald C. Meteorology Today. Pacific Grove, Calif.: Brooks Cole, 2006.

Bluestein, H.B. Tornado Alley: Monster Storms of the Great Plains (reprint edition). Oxford, United Kingdom: Oxford University Press, 2006.

Palmer, Tim and Renate Hagedorn, ed. Predictability of Weather and Climate. New York: Cambridge University Press, 2006.

David E. Newton

WEATHER AND CLIMATE

WEATHER AND CLIMATE. The history of the climate during the early modern age is largely centered on the climatic deterioration known as the "Little Ice Age." Much evidence testifies to a significant degradation of atmospheric conditions from the perhaps uniquely favorable circumstances of the High Middle Ages to the cooler, wetter, and less stable weather of the early modern period. No consensus exists with regard to the nature or the chronology of this phenomenon, the value of the sources available to investigate it, or its impact upon European societies. Nevertheless, the recognition of the importance of climate as a historical factor has led researchers to revisit many well-traveled paths of European history. Their efforts have become particularly relevant considering twenty-first-century fears of global warming.

The sources that historians draft to document the climate of the fifteenth to eighteenth centuries may be arranged in two main categories: literary and iconographic documents and serial and/or quantifiable data. In turn, this second group of sources may itself be divided into direct and indirect records. The value and the limitations of all relevant sources are still debated. References to weather conditions are found in many diaries, almanacs, chronicles, letters, professional accounts, and scientific and military logs. Yet this information is very heterogeneous and thinly and unevenly distributed across the continent and the centuries. It is inevitably subjective and likely to recall extreme or rare occurrences (similar comments may be directed at the pictorial records that testify to various effects of the weather). More systematic and more intentional direct records of weather conditions are rare, particularly early in the period. Their great merit is to enable the construction of data series, yet the lack of standardized measures of temperatures and other climatic variables greatly complicates the task of researchers.

To complete this rich yet insufficient medley of references, historians turn to indirect evidence. Some of it requires refined scientific analyses, ranging from the mapping of tree rings to carbon dating and the assaying of soil or ice cores. A second category of proxy sources includes evidence of weather-dependent economic output, principally crops. Municipal rolls of market prices, institutional accounts of harvests, church tithes registers, or seigneurial records may all reflect variations in local weather conditions. However, both agricultural production itself and the transactions that produced these records were also shaped by economic, political, and cultural tensions. (Agronomists also warn of the intrinsic complexity of the relation between weather and output.) For instance, the dates of grape harvests have always been linked to competitive pressures and evolving tastes as well as spring and summer conditions, just as flood reports are shaped by water levels but also by demographic pressures, hydraulic works, or fiscal imperatives. Increasingly rigorous standards have been applied to the reconstitution of early modern climates, demanding advanced dissections of the effect of weather upon the documented variables and sophisticated statistical testing of the resulting figures.

Several significant cross-disciplinary collaborations substantiate the existence of a negative turn in the weather during the early modern era and also expose its complexity. Its outside limits range from the fourteenth to the nineteenth centuries, although its beginnings are obviously less documented than its end. Naturally, no uniform weather pattern stretched across this long period or across all regions of Europe; this calls for the study of fine regional and chronological distinctions. Temperatures are perhaps better known than precipitation amounts, and great variability as well as episodes of extreme weather are emerging as key findings. Charts of growing seasons and growing ranges have been drawn and compared with the more favorable conditions of the High Middle Ages and the well-documented contemporary era. To date, the geography and chronology of the early modern climate "pessimum" (severe deterioration) remain the object of much valuable work.

Historians speculate on the origins of this climatic deterioration, notably turning to factors such as solar, volcanic, or even human activity, but they are chiefly interested in its consequences. Its impact upon food production is at the center of many debates, because of its crucial importance to many aspects of early modern social, economic, and even political life. Inquiries into the demographic impact of the Little Ice Age continue to enrich our understanding of related subjects such as famines, epidemics, and epizootics. Increasingly, historians separate the consequences of sharp and brutal but short events from those of medium-term, interannual, and decadal or secular trends and underscore the distinctions to be made between the great climatic zones of Europe. They also contrast the impact of weather in secure agricultural areas with that in marginal lands of all sorts and have started to acknowledge the importance of microclimates. New knowledge of climatic patterns is also being applied to many long-standing historical concerns: the "general crises" of the fourteenth and seventeenth centuries; large-scale migration patterns, and, occasionally, the disappearance of whole communities; popular rebellions; economic trends ranging from the southward retreat of vineyards to the shifting of fishing grounds and the great inflation of the sixteenth century; and some of the key advances of the early modern age, such as the agricultural revolution. Finally, climate history has also entered the field of cultural studies, with explorations of the role of climate in shaping popular beliefs and traditions reflected in language, ceremonies, superstitions, and even witch-hunts.

The implications of research on the history of climate are many. Even those who remain skeptical of the solidity of such probes will agree that they serve to highlight and explain the importance and the diversity of human responses to environmental challenges. Research devoted to the early modern climate can also speak to early modern communities' ability to diversify their crops, their landholding patterns, the attempts of authorities to mitigate the impact of brutal episodes, the role played by growing commercial networks and related levels of specialization, the flexibility or rigidity of certain social structures, and the reasons behind important evolutions of landscapes. In the course of these investigations, several fundamental assumptions have been questioned, such as the vulnerability of preindustrial communities to climatic fluctuations, and even the stability of the natural environment in which they functioned.

The strongest objections to the work of climate historians revolve around the value of the data and methods used. But there are also regular denouncements of the risks of determinism associated with these (and other) probes into environmental history. This is particularly so because of a longstanding tradition linking the supposedly favorable climate of Europe with the successful projection of European power across the oceans. Many aspects of the European environment have been and are still advanced to justify what has been called the "European Miracle," ranging from its (mostly) temperate nature and the (relative) absence of large-scale destructive episodes, to its very diversity. All such theses stand accused of ignoring or underestimating the historically crucial element of human agency and, most significantly, of simplifying the great complexity of climate patterns and their impact upon land and people.

Such Eurocentric interpretations of the influence of climate upon societies are not new. Emboldened by the growing reach of their information networks, early modern thinkers linked geography and climate with social and cultural development in several ways, just as they started to reflect on the possibility of climatic variations over time. These reflections could join speculations on the relative merits of ancient and modern societies, or the clustering of geniuses. They could also enter the realm of religious thought, through hypotheses on "geological times" or the universal decline of the earth's ability to support life, as well as daring interpretations of some key episodes of the Scriptures; those, on the contrary, who argued the immutability of climate opened the door for more enlightened plans for improving lives. The same period also marked the beginnings of a more systematic and more scientific interest in recording weather patterns. This trend made clear the need for more reliable thermometers and other instruments and heralded the eventual science of meteorology, although, as is common during the early modern era, cultural groups other than the elite of princely scientific societies remained active in their own ways. The interest in climate, like that in many other aspects of nature, helped mark social and regional identities. Late in the period, attention turned to the potential impact of human activities upon the natural environment and climate. Large-scale or particularly acute instances of deforestation fueled the theory of desiccation, predicated upon the idea that forests attracted, retained, and redistributed atmospheric moisture. Some applied it on a grand scale, speculating, for instance, on the decline of Classic societies or the future of the North American climate after settlement. Others turned to the small but revealing scale of tropical islands. In these settings, free of some of the traditional bounds that had developed in Europe, novel measures emerged that may be seen as forerunners of the science of ecology and the protectionist measures that would grow in the nineteenth and twentieth centuries.

Research in climate history is an established component of environmental history. Like other aspects of this new field, it calls for decidedly multidisciplinary approaches, and it struggles to overcome the fundamental objections associated with the ever-recurrent temptation of deterministic interpretations of history. In the context of an early modern era rich in sources, it greatly enriches our understanding of material and social life and contributes to the development of ever more refined models of the links between nature and culture.

See also Agriculture ; Environment ; Forests and Woodlands ; Scientific Instruments .

BIBLIOGRAPHY

Blaut, James M. "Environmentalism and Eurocentrism." Geographical Review 89 (July 1999): 391–408.

Flohn, Hermann, and Roberto Fantechi, eds. The Climate of Europe, Past, Present, and Future: Natural and Man-Induced Climatic Changes, A European Perspective. Dordrecht and Boston, 1984.

Jankovi'c, Vladimir. Reading the Skies: A Cultural History of English Weather, 1650–1820. Chicago, 2000.

Jones, P. D., et al., eds. History and Climate: Memories of the Future? New York, 2001.

Journal of Interdisciplinary History 10, nos. 2 and 4 (1979–1980).

Le Roy Ladurie, Emmanuel. Times of Feast, Times of Famine: A History of Climate since the Year 1000. Translated by Barbara Bray. Garden City, N.Y., 1971.

Pfister, Christian, Rudolf Brázdil, and Rüdiger Glaser, eds. Climatic Variability in Sixteenth-Century Europe and Its Social Dimension. Dordrecht and Boston, 1999.

Pierre Claude Reynard

Weather

Weather is the state of the atmosphere (mass of air surrounding Earth) at a particular place and point in time. Rain showers, gusty winds, thunderstorms, cloudy skies, droughts (prolonged period of dry weather), snowstorms, and sunshine are all examples of weather conditions. Weather scientists, called meteorologists, use measurable factors like atmospheric pressure (pressure caused by weight of the air), temperature, moisture, clouds, and wind speed to describe the weather. Meteorologists make predictions of future weather based on observations of present regional weather patterns and past trends. Weather prediction, or forecasting, is an important part of meteorology (weather science). Advance warning of such weather phenomena as extreme hot and cold temperatures, heavy rainfall, drought, and severe storms can protect people's property and save lives.

The weather patterns that a region experiences over tens, hundreds, or thousands of years are called climate. For example, the northeastern United States experiences a wide range of weather during an average year. Below-freezing temperatures and heavy snowfall are typical weather conditions in winter, while warm temperatures and afternoon thunderstorms are common in the summer. Communities of plants and animals (ecosystems) adapt over thousands of years to survive the weather extremes of their particular climate. In New England, plants lie inactive, mammals grow shaggy coats, and birds fly south during the cold dark winter. In the spring, trees pull sap from their roots and grow leaves, animals bear young, and seeds germinate in time to take advantage of mild temperatures and long, sunny days in the summer. Climate change happens over hundreds and thousands of years, but weather varies from day to day, hour to hour, and sometimes from minute to minute.

Weather conditions: pressure, temperature, and moisture

The atmosphere presses down on Earth's surface. (There is no atmosphere in outer space. Without their pressurized space suits, astronauts' bodies would explode.) The weight of the column of air molecules above a surface is called atmospheric pressure. The average weight of the atmosphere on one square inch of ground at sea level is 14.7 pounds. People do not feel this pressure because their senses are adjusted to it and the human body is designed to withstand it.

Meteorologists use an instrument called a barometer to measure pressure, and atmospheric pressure is also called barometric pressure. Evangelista Torricelli (1608–1647), an Italian physicist, invented the barometer in 1643. His instrument, "Torricelli's tube," was a glass tube full of dense, liquid mercury with its end in an open dish of mercury. His barometer works the same way that mercury barometers work in modern day. Air pressing down on the mercury in the dish pushes some of the mercury upwards into the glass tube. As air pressure increases, the mercury in forced into the tube and the column of mercury rises. When air pressure decreases, the mercury flows back into the dish and the column of falls. Barometric pressure is often measured in inches of mercury. When a weather forecaster says the mercury is falling, it means that air pressure is falling, and bad weather may be approaching.

Atmospheric pressure differs from one place on Earth to another due to temperature, moisture, and topography (physical surface features). Pressure decreases with elevation. There are many fewer air molecules above a square foot (kilometer) on the summit of Mt. Everest than above a square foot of Waikiki Beach. Air currents, better known as winds, blow from areas of high pressure to areas of low pressure. Rapidly changing

Weather Forecasting

Meteorologists use weather indicators like barometric pressure and cloud types, maps of large-scale weather patterns, and data from previous years to predict upcoming temperature, moisture, and severe weather conditions. News outlets like television, radio, and newspapers broadcast forecasts to the public. In emergencies, they broadcast severe weather warnings and information on how to take shelter during floods and storms. In the United States, the National Weather Service provides specific forecasts for pilots and ship captains who need more detailed information. They also provide storm warnings and recommendations for emergency procedures during severe weather. The U.S. Farm report provides weather information specifically for farmers who use forecasts to plan their planting, harvesting, and irrigation schedules.

Weather prediction is a tricky business. In some places, atmospheric conditions lead to continuously changing weather. Mark Twain said of New England, "If you don't like the weather, wait ten minutes." In other places, weather patterns are generally so predictable that changes take people by surprise. Winter-weary residents of North Dakota say, "If summer happens on a Saturday, we'll have a picnic." While everyone from pilots to party planners knows that weather forecasts are never perfect, people still depend on forecasts to help plan for the future. Weather forecasts provide vital advance warning of severe weather such as hurricanes, tornadoes, and floods that can pose a serious threat to lives and property.

Modern meteorologists use a variety of techniques to measure atmospheric conditions and generate forecasts. Maps of barometric pressures, temperatures, and rainfall amounts help meteorologists spot weather trends like cyclones, anticyclones, fronts, air masses, and storm systems. Weather radars, satellite maps, and computer-generated models of future weather help weather scientists make precise measurements, continuously updated forecasts, and more complex predictions. Other methods have been used by observant people for hundreds of years For example, some flowers close their petals before it rains, high clouds and falling atmospheric pressure signal an approaching cold front, and dogs sense thunder before humans.

Other weather lore is probably more myth than reality. Many people still believe the theory that a groundhog's shadow predicts an additional six weeks of winter, or that cows lie down before it rains.

patterns of winds, precipitation (any form of water falling), clouds, and storms develop around moving high and low pressure centers in Earth's atmosphere.

Temperature affects air pressure and moisture in the atmosphere. Warmer air expands and rises, so pressure falls beneath rising columns of warm air. Warm air also holds more moisture, in the form of water vapor, than cool air. Rising warm air in low pressure zones often carry water vapor high into the atmosphere. When the warm air begins to cool, the moisture condenses into droplets or freezes into ice crystals and clouds form. Precipitation and storms are common in low-pressure centers. As air cools it contracts, causing air pressure to rise under the sinking air. Because cool air holds less moisture, and because sinking air masses are usually already dry, high pressure areas usually are low in humidity (air moisture).

High and low pressure systems

Major east and west-blowing winds blow high and low-pressure weather systems around Earth. High-pressure systems, also called anticyclones, consist of winds spiraling out from a high-pressure center under sinking, dry air. Low-pressure systems, or cyclones, have low-pressure centers and winds that spiral toward their centers. High-pressure systems are called anticyclones. A cyclone has a column of warm air rising from its center. In anticyclones, the air sinks toward the center and warms as it descends. In the northern hemisphere (half of the Earth), anticyclones spin clockwise and cyclones spin counterclockwise, and the reverse is true in the southern hemisphere. Because air travels from high to low pressure areas, high-pressure anticyclones often follow low-pressure cyclones.

In North America, the jet stream (high-speed winds that race around the planet at about five miles above the Earth) blows cyclones and anticyclones from west to east. In general, cyclones bring intense weather in the form of rain, snow, clouds, and storms. Dry, clear, calm weather usually accompanies the passage of anticyclones. (The parched residents in deserts of the American Southwest might look forward to the clouds and rain storms a cyclone brings. A southward dip in the Jet Stream causes a near-permanent zone of high pressure over Arizona, New Mexico, and Southern California, and moisture-bearing weather systems tend to bypass the region.) Trade winds (persistent tropical winds that blow generally toward the west) blow low-pressure systems that develop in the tropical Atlantic Ocean west toward the Caribbean Sea and east coast of the United States. These tropical cyclones feed on warm ocean waters and can develop into massive storm systems called tropical storms and hurricanes.

Air masses and fronts

An air mass is a large body of air that has similar temperatures and moisture content throughout. Several air masses contribute to weather patterns in North America: cold, dry air over northern Canada; hot, dry air in the American Southwest; cool, moist air moving east over the Pacific Northwest; and warm, moist air traveling north from the Gulf of Mexico.

The boundaries between air masses are called fronts. A cold front occurs where a cold air mass is moving in to replace warm air. Clouds, precipitation, and storms are common at cold fronts. The incoming cold, dense mass lifts the warm, moist air and creates unstable conditions where moisture rapidly condenses and winds organize clouds into storms. Once a cold front has passed, temperatures and humidity drop and a high-pressure system moves in. A warm front precedes an incoming warm air mass. Warm fronts bring moisture and higher temperatures. Stationary fronts separate unmoving air masses.

A typical cyclone in the American Mid-West is a rotating pinwheel of three air masses and three fronts moving east toward the Atlantic Ocean. Cold, dry air flows south from Canada behind cool, moist air flowing from the Pacific Northwest. Warm air from the Gulf of Mexico moves north and contributes moisture to the system. Thunderstorms and blizzards develop along cold fronts.

Laurie Duncan, Ph.D.

For More Information

Books

Day, John A., et al. Peterson First Guide to Clouds and Weather. Boston: Houghton Mifflin, 1999.

Vasquez, Tim. Weather Forecasting Handbook. 5th ed. Austin, TX: Weather Graphics Technologies, 2002.

Williams, Jack. The USA Today Weather Book: An Easy-To-Understand Guide to the USA's Weather. 2nd ed. New York: Vintage, 1997.

Websites

"Meteorology, the Online Guides." Weather World 2010, University of Illinois at Urbana-Champagne Department ofAtmospheric Sciences.http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/home.rxml (accessed on August 17, 2004).

"National Weather Service." National Oceanic and Atmospheric Administration.http://www.nws.noaa.gov/ (accessed on August 17, 2004).

"Weather." National Oceanic and Atmospheric Administration.http://www.noaa.gov/wx.html (accessed on August 17, 2004).

Weather

Weather can be defined as the condition of the atmosphere at any given time and place. Weather conditions are determined by six major factors: air temperature , air pressure , humidity of the air, amount and kind of cloud cover, amount and kind of precipitation , and speed and direction of the wind . Weather condition patterns for any one region or for the whole planet can be charted on a weather map containing information about all six of these factors. This information often can be used to produce a weather forecast, a prediction of weather conditions at some future time for some given region.

The study of weather is known as meteorology . No exact date can be given for the beginnings of this science, since humans have studied weather conditions for many centuries. Indeed, the word meteorology itself goes back to Meterologica, a book written by the Greek natural philosopher Aristotle in about 340 b.c. Many scholars date the rise of modern meteorology to the work of a Norwegian father and son team, Vilhelm and Jakob Bjerknes. The Bjerknes's were the first to develop the concept of masses of air moving across the earth's surface, affecting weather conditions as they moved. They also created the first widespread system for measuring weather conditions throughout their native Norway.

The six factors determining weather conditions result from the interaction of four basic physical elements: the Sun , the earth's atmosphere, the earth itself, and natural landforms on the earth's surface.

Solar energy

The driving force behind all meteorological changes taking place on the earth is solar energy . Each minute, the outer portions of the earth's atmosphere receive an average of 2 calories/sq cm. This value is known as the solar constant. Although the solar constant changes over very long periods of time, it does not vary enough to affect the general nature of the earth's weather over short periods of time.

The solar energy reaching the outer atmosphere may experience a variety of fates. Thirty percent of all solar energy is lost to space by means of scattering and by reflection off clouds and the earth's surface. Another 19% is absorbed by gases in the atmosphere and by clouds. About a quarter of it (25%) reaches the earth's surface directly; another quarter (26%) eventually reaches the surface after being scattered by gases in the atmosphere.

An important factor in determining the fate of solar radiation is its wavelength. Shorter wavelengths tend to be absorbed by gases in the atmosphere (especially oxygen and ozone ) while radiation of longer wavelengths tends to be transmitted to the earth's surface.

Solar radiation that reaches the earth's surface is absorbed to varying degrees, depending on the kind of material on which it falls. Since darker colors and rougher surfaces absorb radiation better than lighter colors and smoother surfaces, soil tends to absorb more solar radiation than water .

Solar energy that reaches the earth's surface is re-radiated back to the atmosphere as heat , also referred to as infrared radiation. Infrared radiation consists of much longer wavelengths. This re-radiated energy is likely to be absorbed by certain gases in the atmosphere such as carbon dioxide and nitrous oxide. This absorption process, the greenhouse effect , is responsible for maintaining the planet's annual average temperature.

Humidity, clouds, and precipitation

The absorption of solar energy by the earth's surface and its atmosphere is directly responsible for most of the major factors making up weather patterns. For example, when the water in oceans, lakes, rivers , streams, and other bodies of water is warmed, it tends to evaporate and move upward into the atmosphere. The amount of moisture found in the air at any one time and place is called the humidity. Humans are very sensitive to this characteristic of weather.

Water that has evaporated from the earth's surface (or escaped from plants through the process of transpiration ) rises to an altitude in the atmosphere at which the air around it is cold enough to cause condensation. When moisture condenses into tiny water droplets or tiny ice crystals, clouds are formed.

Clouds are an important factor in the development of weather patterns. They tend to reflect sunlight back into space. Thus, an accumulation of cloud cover may contribute to a decrease in heat retained in the atmosphere.

Clouds are also the breeding grounds for various types of precipitation. As water droplets or ice crystals collide with each other, they coalesce, and form larger particles. Eventually, the particles become large enough and heavy enough to overcome upward drafts in the air and fall to the earth as precipitation. The form of precipitation that occurs (rain, snow, sleet, hail, etc.) depends on the atmospheric conditions through which the water or ice falls.

Atmospheric pressure and winds

Solar energy also is directly responsible for the development of differing atmospheric pressures at various locations on the planet's surface, and the winds that result from these differences. Since the earth's surface is different in color and texture from place to place, some locations will be heated more intensely by solar radiation than others. Warm places usually heat the air above them, setting convection currents into movement that carry masses of air upward into the upper atmosphere. Those same convection currents then carry other masses of air downward from the upper atmosphere toward the earth's surface.

In regions where warm air moves upward, the atmospheric pressure tends to be low; downward air movements are associated with higher atmospheric pressures. These higher or lower atmospheric pressures can be measured by a barometer . Barometers provide information not only about current pressures but about possible future weather patterns as well.

The existence of areas with different atmospheric pressures accounts for the movement of air, which we call wind. Wind is simply the movement of air from a region of high pressure to one of lower pressure.

Terrestrial characteristics

If the Sun provides the energy by which weather patterns can develop, certain features of the earth itself determine the precise forms in which those patterns may be exhibited. One example has already been provided above. Earth's surface is highly variable, ranging from oceans to deserts to cultivated land to urbanized areas. The way solar energy is absorbed and reflected from each of these regions is different, accounting for variations in local weather patterns.

Other characteristics of the planet account for more significant variations in weather patterns. These characteristics include such features as the tilt of Earth on its axis in relation to its plane of revolution, and the variations in Earth's distance from the Sun.

The fact that Earth's axis is tilted at an angle of 23 1/2° to the plane of its orbit means that the planet is heated unevenly by the Sun. During the summer, sunlight reaching the Northern Hemisphere strikes more nearly at right angles than it does in the Southern Hemisphere. In the winter, the situation is reversed.

The elliptical shape of the earth's orbit around the Sun also affects weather conditions. At certain times of the year the planet is closer to the Sun than at others. This variation means that the amount of solar energy reaching the outer atmosphere will vary from month to month depending on Earth's location in its path around the Sun.

Even Earth's rotation on its own axis influences weather patterns. If Earth did not rotate, air movements on the planet would probably be relatively simple: air heated along the equator would rise into the upper atmosphere, travel northward toward the poles, be cooled, and then return to the earth's surface at the poles.

Earth's rotation causes the deflection of these theoretically simple air movements. Instead of a single overall equator-to-poles air movement, global winds are broken up into smaller cells. In one cell warm air rises above the equator, moves northward in upper altitudes, is cooled, and returns to the earth in the regions around 30° north and south latitude. A second cell consists of air that moves upward in the regions around 60° north and south latitude, across the upper atmosphere, and then downward at about 30° north and south latitude. The final cell contains winds traveling upward at 60° north and south latitude and then downward again at the poles.

Topographic factors

Irregularities on Earth's surface also affect weather. A mountain range can dramatically affect the movement of approaching air masses. Suppose that a mass of warm moisture-filled air is forced to ascend one side of a mountain range. As the air is pushed upward it cools off and moisture begins to condense out, first in the form of clouds then as precipitation. This side of the mountain range will experience high rates of precipitation.

As the air mass continues over the top of the mountain range, it does so without its moisture. The winds that sweep down the far side of the range will tend to be warm and dry. The term orographic is used to describe changes in weather patterns like these induced by mountain ranges.

Weather and climate

The terms weather and climate often are used in conjunction with each other, but they refer to quite different phenomena. Weather involves atmospheric conditions that currently prevail or that exist over a relatively short period of time. Climate refers to the average weather pattern for a region (or for the whole planet) over a much longer period of time (at least three decades according to some authorities).

Changes in weather patterns are easily observed. It may rain today and be clear tomorrow. Changes in climate patterns are much more difficult to detect. If the summer of 1997 is unusually hot, there is no way of knowing if that fact is part of a general trend towards warmer weather or a single variation that will not appear again for some time.

See also Atmospheric temperature; Seasons; Weather forecasting; Weather mapping.

Resources

books

Ahrens, C. David, Rachel Alvelais, and Nina Horne. Essentials of Meteorology: An Invitation to the Atmosphere. Belmont, CA: Brooks/Cole, 2000.

Bramwell, Martyn. Weather. New York: Franklin Watts, 1994.

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

Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa. The Atmosphere: An Intorduction to Meteorology. 8th ed. New York: Prentice-Hall, 2000.

Lynott, Robert E. How Weather Works and Why. Gadfly Press, 1994.

Watts, Alan. The Weather Handbook. Dobbs Ferry: Sheridan House, 1994.

Williams, Jack. The Weather Book. New York: Vintage Books, 1997.

periodicals

"Temperature And Rainfall Tables: July 2002." Journal of Meteorology 27, no. 273 (2002): 362.

"Weather Extremes: July 2002." Journal Of Meteorology 27 no. 273 (2002): 361.

David E. Newton

KEY TERMS

Humidity

—The amount of water vapor contained in the air.

Meteorology

—The study of the earth's atmosphere and the changes that take place within it.

Orographic

—A term referring to effects produced when air moves across a mountain range.

Solar constant

—The rate at which solar energy strikes the outermost layer of the earth's atmosphere.

Solar energy

—Any form of electromagnetic radiation that is emitted by the Sun.

Topography

—The detailed surface features of an area.