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Weather and Climate

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): 391408.

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, 16501820. 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 (19791980).

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

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Weather and Climate

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 (16081647). 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 downdraftalso 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 layersuch 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 layerother 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

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Weather and the Ocean

Weather and the Ocean

Much of the weather experienced on land has its origins over the oceans. Weather is the state of the atmosphere at any given time and place. Earth's oceans and atmosphere are in constant contact, sharing water, gases, and energy. The conditions of one directly affect the conditions of another. Unfortunately for weather predictors, these complex interactions behave according to chaos theory. That is, the outcome of any equation that attempts to describe them is so sensitive to tiny differences in starting conditions that the results appear to be random, or at least very difficult to predict.

Uneven heating of Earth creates circulation cells in the atmosphere. Circulation cells exist over each hemisphere, north and south. They are responsible for two-thirds of the heat transfer from tropical to polar regions. As air heats over the equator, it rises and cools. Water vapor condenses and falls as rain in the equatorial zone, drying the air mass as it migrates north or south from the equator, cooling and becoming denser than the air around it. The air mass begins to drop near the subtropical regions at about 30 degrees latitude and is drawn south by the rising tropical air.

Two circulation cells are created north and south of the equator, termed Hadley cells. Between 30 degrees and 60 degrees latitude north and south are the Ferrell cells, which are formed in much the same way except that they rotate the opposite way, north to south. Over the poles, from 60 degrees to 90 degrees latitude, lie the polar cells, again circulating opposite from the Ferrell cells, south to north.* The jet streams are zones of fastmoving west-to-east winds in the upper atmosphere between the Ferrell and Polar cells. Regions of rising air exhibit low pressure and wet weather, whereas areas of downward movement are often dry with high pressure and clear skies.

The Coriolis effect is caused by movement of air over a rotating Earth. As a result, air masses appear to curve clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, creating wind belts that drive the atmosphere around the Earth. In the Hadley cells, winds travel to the west, bending to the right in the north and left in the south. In the Ferrell cells, winds reverse and flow west to east, again bending to the right in the north and left in the south. The polar cell reverses again and flows east to west, also being influenced by the Coriolis effect. These moving air masses are responsible for the creation and distribution of weather systems throughout the world.

Heat Transfer

Wherever the Sun is perpendicular to Earth's surface, the most heat absorption takes place. Equatorial and tropical regions have a net gain of heat, whereas polar regions experience a net loss. Both air and water currents redistribute heat over Earth. The Sun warms the surface of the ocean and land, which in turn warm the atmosphere from the bottom up. Wherever the atmosphere contacts warm water, evaporation occurs and water vapor and energy are transferred to the air mass.

As the moisture-laden air mass rises to high altitudes or passes over a high landmass, it cools and the water vapor condenses and falls as precipitation.

The direction of air movements and the temperature of the ocean water determine the direction storm fronts take as well as their intensity.

Hurricanes, Typhoons, and Cyclones

A tropical cyclone, variably known as a hurricane, typhoon, or cyclone, is a huge rotating air mass, typically having very low pressure, high winds, and torrential rains. Tropical cyclones are the largest storm systems on Earth.

Air always moves from areas of high pressure towards areas of low pressure. The speed of the airflow increases as the pressure difference between the two air cells increases and their proximity decreases.

Hurricanes begin as low-pressure cells that break off from the equatorial low-pressure belt. They begin to spin due to the Coriolis effect and pick up large amounts of water vapor and heat energy as they pass over the warm tropical water. When wind velocity within the storms reaches 120 kilometers (77 miles) per hour, tropical storms are upgraded to hurricane status. In large hurricanes, wind speeds have reached 400 kilometers (250 miles) per hour.

Hurricanes form only in the late summer and fall, when water temperatures reach at least 26 degrees Celsius (79 degrees Fahrenheit). They travel with the trade winds flowing east to west. Most hurricanes last 5 to 10 days and remain in the tropical region. Some storms, however, pass into the middle latitudes where they can cause great destruction along the east and west coasts of the Americas.

El Niño and La Niña

Changes in the ocean temperature can affect weather patterns around the world. One of these cyclic changes is the El Niño/La Niña effect. El Niños occur when there is an abnormal warming of the ocean waters in the middle and eastern equatorial Pacific and Atlantic Oceans.

During normal years, consistent trade winds blow east to west across the ocean surface along the tropical region. If the trade winds along the equator slow or cease, the warm water is allowed to flow back to the middle and eastern Pacific. This layer of warm, nutrient-poor water prevents cold-water upwelling in the eastern Pacific. Without this source of the nutrients, which nourish the algal base of the food chain, the effect on ocean biology is significant. The areas of tropical storm generation are also shifted to the east. The track of the jet stream and approaching storm systems moves south from the wet Pacific Northwest to the dry areas of the Southwest, causing drought in the northern United States and floods in the south.

As trade winds increase, the warm water is pushed back to the west, allowing cold nutrient-rich ocean water to rise from below. This is an example of the La Niña effect, which defines a cooling of ocean surface waters. It generally signals decreased storm activity for the lower latitudes and increased storm activity in the higher latitudes.

see also Climate and the Ocean; El NiÑo and La NiÑa;Ocean Currents.

Ron Crouse

Bibliography

Aherns, C. Donald. Essentials of Meteorology, An Invitation To the Atmosphere. Minneapolis/St. Paul, MN: West Publishing Company, 1993.

Garrison, Tom. Oceanography, An Invitation to Marine Science. New York: Wadsworth Publishing Company, 1996.

Summerhayes, C. P., and S. A. Thorpe. Oceanography, An Illustrated Guide. New York: John Wiley & Sons, 1996.

Thurman, Harold V., and Alan P. Trujillo. Essentials of Oceanography. Upper Saddle River, NJ: Prentice Hall, 1999.

Internet Resources

National Climate Data Center. National Oceanic and Atmospheric Administration. <http://lwf.ncdc.noaa.gov/oa/ncdc.html>.

* See "Climate and the Ocean" for a diagram showing these circulation cells.

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Weather

Weather

Weather is the state of the atmosphere at any given time and place, determined by such factors as temperature, precipitation, cloud cover, humidity, air pressure, and wind. 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 thousands of years. Weather conditions can be regarded as a result of the interaction of four basic physical elements: the Sun, Earth's atmosphere, Earth itself, and natural land-forms on Earth.

Solar energy and Earth's atmosphere

The driving force behind all meteorological changes taking place on Earth is solar energy. Only about 25 percent of the energy emitted from the Sun reaches Earth's surface directly. Another 25 percent reaches the surface only after being scattered by gases in the atmosphere. The remaining solar energy is either absorbed or reflected back into space by atmospheric gases and clouds.

Solar energy at Earth's surface is then reradiated to the atmosphere. This reradiated energy is likely to be absorbed by other gases in the atmosphere such as carbon dioxide and nitrous oxide. This absorption processthe greenhouse effectis responsible for maintaining the planet's annual average temperature.

Humidity, clouds, and precipitation. The absorption of solar energy by Earth's surface and atmosphere is directly responsible for most of the major factors making up weather patterns. When water on the surface (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.

When this moisture reaches cold levels of the atmosphere, it condenses into tiny water droplets or tiny ice crystals, which group together to form clouds. Since clouds tend to reflect sunlight back into space, an accumulation of cloud cover may cause heat to be lost from the atmosphere.

Words to Know

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.

Solar energy: Any form of electromagnetic radiation that is emitted by the Sun.

Topography: The detailed surface features of an area.

Clouds also are the breeding grounds for various types of precipitation. Water droplets or ice crystals in clouds combine with each other, eventually becoming large enough to overcome upward drafts in the air and falling to Earth as precipitation. The form of precipitation (rain, snow, sleet, hail, etc.) depends on the atmospheric conditions (temperature, winds) through which the water or ice falls.

Atmospheric pressure and winds. Solar energy also is directly responsible for the development of wind. When sunlight strikes Earth's surface, it heats varying locations (equatorial and polar regions) and varying topography (land and water) differently. Thus, some locations are heated more strongly than others. Warm places tend to heat the air above them, causing that air to rise upward into the upper atmosphere. The air above cooler regions tends to move downward from the upper atmosphere.

In regions where warm air moves upward, the atmospheric pressure tends to be low. Downward air movements bring about higher atmospheric pressures. Areas with different atmospheric pressures account for the movement of air or wind. Wind is simply the movement of air from a region of high pressure to one of lower pressure.

Earth, land surface, and the weather

Earth's surface ranges from oceans to deserts to mountains to prairies 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.

However, the tilt of Earth on its axis and it's varying distance from the Sun account for more significant weather variations. The fact that Earth's axis is tilted at an angle of 23.5 degrees to the plane of its orbit means that the planet is heated unevenly by the Sun. During the summer, sunlight strikes the Northern Hemisphere more directly than it does the Southern Hemisphere. In the winter, the situation is reversed.

At certain times of the year, Earth 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 would move in a single overall equator-to-poles cycle. Earth's rotation, however, causes the deflection of these simple air movements, creating smaller regions of air movement that exist at different latitudes.

Weather and climate

The terms weather and climate often are used in place of each other, but they refer to quite different phenomena. Weather refers to the day-today changes in atmospheric conditions. 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).

[See also Air masses and fronts; Atmosphere, composition and structure; Atmospheric circulation; Atmospheric pressure; Clouds; Cyclone and anticyclone; Drought; El Niño; Global climate; Monsoon; Thunderstorm; Weather forecasting; Wind ]

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weather

weather, state of the atmosphere at a given time and place with regard to temperature, air pressure (see barometer), wind, humidity, cloudiness, and precipitation. The term weather is restricted to conditions over short periods of time; conditions over long periods, generally at least 30–50 years, are referred to as climate.

The earliest evidence of scientific activity in the field of meteorology, the study of the earth's atmosphere, especially as it relates to weather forecasting, is from the 4th cent. BC; Aristotle wrote what is probably the first treatise on the subject. The first attempt to chart weather from reports over a considerable area was made (1820) in Europe by H. W. Brandes, but it was not until after the invention of the telegraph that the rapid collection of weather data from remote stations became possible.

In the United States, a government weather service was established (1870) under the army Signal Corps. In 1891 the weather service was transferred to the U.S. Weather Bureau under the Dept. of Agriculture, and it later came (1940) under the jurisdiction of the Dept. of Commerce. The U.S. Weather Bureau has since been renamed the U.S. National Weather Service and transferred to the National Oceanic and Atmospheric Administration. The central forecast office is the National Meteorological Center (NMC), in Suitland, Md.; first-order stations are located chiefly in the larger cities, and numerous substations for special purposes (e.g., observing river stages, measuring depth of snow, and maintaining records of climate) are distributed throughout the country.

Devices used for meteorological observations include rockets, weather satellites, radiosondes, barometers, anemometers, weather vanes, psychrometers, thermometers, and radar. By means of high-speed telecommunications, information from all over the world is sent to the NMC, where the data is decoded and plotted. These data are used to create weather maps based on simultaneous weather observations at different atmospheric levels over any desired geographic region. On a typical map the various weather elements are shown by figures and symbols; isobars are drawn to show areas of low pressure (cyclones) and high pressure (anticyclones); fronts (boundaries between air masses) and areas of precipitation are indicated.

By using computer models based on mathematical formulations of the dynamics of the atmosphere, weather charts are also produced as prognostics of future weather patterns. The many simplifying assumptions required in these formulations, as well as the incompleteness of weather data, limit the accuracy of the computer predictions; though as advances in computer systems occur, these models are becoming more complete and, hence, more accurate. Meteorologists interpret and modify such prognostics according to their knowledge of the prognostics' reliability and their familiarity with local influences, such as topography and proximity to large bodies of water, in order to derive the best possible weather forecasts.

Forecasts are disseminated by television, radio, telephone, newspapers, and the Internet. Detailed forecasts can usually be made only for a short future period (generally 48 hr or less). Forecasts for up to five days can usually predict departures from normal temperature and precipitation fairly well; longer-range predictions are more general and less accurate, being based on the known normal weather of the area. Mathematical models, particularly those run on supercomputers, have helped to understand weather changes, including general global circulation patterns, and how perturbations in the atmosphere and oceans effect the weather.

See J. R. Eagleman, Weather Concepts and Terminology (1989); J. Farrand, Jr., Weather (1990); H. M. Conway and L. L. Liston, Weather Handbook (1990); R. C. McNeill, Understanding the Weather (1991); S. H. Schneider, Encyclopedia of Climate and Weather (2 vol., 1996); J. L. Fry et al., The Encyclopedia of Weather and Climate Change (2010).

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Weather

417. Weather

See also 27. ATMOSPHERE ; 85. CLIMATE ; 87. CLOUDS ; 246. LIGHTNING ; 345. RAIN ; 375. SNOW ; 387. SUN ; 394. THUNDER ; 420. WIND

aerographics
the study of atmospheric conditions. Also aerography . aerographer , n.
aerology
1. Obsolete. the branch of meteorology that observed the atmosphere by using balloons, airplanes, etc.
2. meteorology. aerologist , n. aerologic, aerological , adj.
aeromancy
1. the art or science of divination by means of the air or winds.
2. Humorous weather forecasting.
barograph
a barometer which automatically records, on a rotating cylinder, any variation in atmospheric pressure; a self-recording aneroid.
barometrography
the branch of science that deals with the barometer.
barometry
the art or science of barometric observation.
chonophobia
an abnormal fear or dislike of snow.
climatology
the science that studies climate or climatic conditions. climatologist , n. climatologic, climatological , adj.
cryophobia
an abnormal fear of ice or frost.
frontogenesis
the meeting of two masses of air, each with a different meteorological composition, thus forming a front, sometimes resulting in rain, snow, etc.
frontolysis
the process by which a meteorological front is destroyed, as by mixture or deflection of the frontal air.
homichlophobia
an abnormal fear of fog.
hyetology
Rare. the branch of meteorology that studies rainfall. hyetologist , n. hyetological , adj.
hyetophobia
an abnormal dislike or fear of rain.
hytherograph
a graph that shows the relationship between temperature and either humidity or precipitation.
irroration
Obsolete. 1. the process of moistening with dew.
2. the condition of being bedewed.
meteorology
the study of weather and its changes, especially with the aim of predicting it accurately. meteorologist , n. meteorologie, meteorological , adj.
microbarograph
a barograph for recording small fluctuations of atmospheric pressure.
nephology
the scientific study of clouds. nephologist , n.
ombrology
the branch of meteorology that studies rain. ombrological , n.
pluviography
the branch of meteorology that automatically measures rainf all and snowfall. pluviographic, pluviographical , adj.
pluviometry
the branch of meteorology concerned with the measurement of rainf all. pluviometric, pluviometrical , adj.
pluvioscope
an instrument for measuring rainfall; a rain gauge.
pluviosity
raininess. pluvious , adj.
telemeteorography
the recording of meteorological conditions at a distance, as in the use of sensing devices at various points that transmit their data to a central office. telemeteorographic , n.
udometry
the measurement of rainfall with any of various types of rain gauges. udometric , adj.
udomograph
a self-registering rain gauge.
vacuometer
an instrument used for comparing barometers at varying pressures against a Standard barometer.
weatherology
Informal. meteorology, especially weather forecasts for radio or television.

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weather

weath·er / ˈwe[voicedth]ər/ • n. the state of the atmosphere at a place and time as regards heat, cloudiness, dryness, sunshine, wind, rain, etc.: if the weather's good, we can go for a walk. ∎  a report on such conditions as broadcast on radio or television. ∎  cold, wet, and unpleasant or unpredictable atmospheric conditions; the elements: stone walls provide shelter from wind and weather. ∎  [as adj.] denoting the side from which the wind is blowing, esp. on board a ship; windward: the weather side of the yacht. Contrasted with lee. • v. [tr.] 1. wear away or change the appearance or texture of (something) by long exposure to the atmosphere: [tr.] his skin was weathered almost black by his long outdoor life | [as adj.] (weathered) chemically weathered rock. ∎  [intr.] (of rock or other material) be worn away or altered by such processes: the ice sheet preserves specimens that would weather away more quickly in other regions. ∎  [usu. as n.] (weathering) Falconry allow (a hawk) to spend a period perched on a block in the open air. 2. come safely through (a storm). ∎  withstand (a difficulty or danger): this year has tested industry's ability to weather recession. ∎  Sailing (of a ship) get to the windward of (a cape or other obstacle). 3. make (boards or tiles) overlap downward to keep out rain. ∎  (in building) slope or bevel (a surface) to throw off rain. PHRASES: in all weathers in every kind of weather, both good and bad.keep a weather eye on observe very carefully, esp. for changes or developments.under the weather inf. slightly unwell or in low spirits.

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weather

weather condition of the atmosphere with respect to heat or cold, calm or storm, etc. OE.; (with adverse implication) XII; direction of wind (perh. — ON.) XIV. OE. weder = OS. wedar (Du. weer), OHG. wetar (G. wetter), ON. veðr :- Gmc. *weðram :- either IE. *wedhrom (OSl. vedro good weather) or IE. *wetróm (Lith. vétra storm, OSl. větrǔ wind); prob. f. *wē̌- WIND1. Comp. weathercock vane in the form of a cock. XIII.
Hence weather vb. tr. and intr. in various uses concerning exposure to wind and weather XV; earlier in weathering XII.

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weather

weather State of the atmosphere at a given locality or over a broad area, particularly as it affects human activity. Weather refers to short-term states (days or weeks) as opposed to long-term climate conditions.

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weather

weather The state of the atmosphere (e.g. temperature, pressure, and humidity) and associated phenomena (e.g. precipitation and wind) occurring at a specified time and place.

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Weather

WEATHER

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weather

weatherblather, foregather, gather, slather •farther, father, lather, rather •grandfather • stepfather • godfather •forefather •altogether, feather, heather, leather, nether, tether, together, weather, wether, whether •bather • sunbather •bequeather, breather •dither, hither, slither, swither, thither, whither, wither, zither •either, neither •bother, pother •Rhondda • mouther • loather •smoother, soother •another, brother, mother, other, smother, t'other •grandmother • stepmother •godmother • housemother •stepbrother • further

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