Climate is the general state of the \Tatmosphere\t over a long period of time. Whereas weather is the expression of day-to-day conditions, climate is a composite of averages and extremes during a specified number of years. Like weather, climate results from energy and mass exchanges within the atmosphere and between the atmosphere and the Earth's surface. As a factor in the natural environment, climate not only affects world patterns of vegetation, soils, and water resources, but also directly or indirectly influences every human endeavor. Climate determines an area's suitability for settlement and for agriculture, manufacturing, transportation, and other economic activities. Knowledge of past climatic fluctuations has helped to explain ice ages, changes in sea level, famines, and migrations. Increasing evidence indicates that human impact on the environment is causing local and perhaps worldwide changes in climate. CLIMATOLOGY The study of climate is ancient. Greek philosophers replaced supernatural explanations with a concept of climate based on latitude and the inclination of the earth's axis. (The Greek word klima means the slope of the Earth with respect to the Sun and approximates the modern concept of latitude.) About 400 BC \THippocrates\t compiled On Airs, Waters and Places, the first medical climatology. Exploration, trade, and early scientific discoveries made fundamental contributions to climatology, but the modern science began with the invention of meteorological instruments. \TGalileo\t devised a thermometer in 1593; his pupil Evangelista \TTorricelli\t discovered the principle of the mercurial barometer in 1643. A wide range of sciences currently consider the effects of climate. The practical uses of climatological information have in turn resulted in an increasing need to understand the physical nature of climate, especially as a basis for forecasting or modifying its future trends. Increasingly complex instruments, widespread observation, faster communication, and other technological advances have aided research on the nature and causes of climate and have accelerated specialization of climatology. Major Branches Physical climatology seeks explanations of climatic phenomena through understanding of atmospheric processes, either with reference to large-scale transfer of heat, mass, and momentum (dynamic climatology), or in terms of observed climatic elements--solar radiation, temperature, humidity, cloudiness, precipitation, winds, and visibility (synoptic climatology). Descriptive climatology, or climatography, analyzes observational data, often using statistical and cartographic techniques. Regional climatology, an extension of descriptive climatology, is concerned with the classification and mapping of different types of climate. Applied climatology uses the data and principles of physical and descriptive climatology to solve climate-related problems--in health, industry, agriculture, architecture, and so on. Classification of Climates No two places on Earth have the same climate. In order to communicate information about different climates, an organized system of generalizations--that is, a classification--must be used. The three main approaches to climatic classification are genetic, empiric, and applied. Genetic systems group climates according to their presumed causes--for example, tropical, highland, continental, and monsoonal. The ancient Greek division of the world into torrid, temperate, and frigid zones was an early attempt at genetic classification based on the effects of latitude on temperature. Empiric classifications identify climates in terms of their observable characteristics, treated either singly or in combination. Empiric classifications commonly fix numerical limits for descriptive categories with reference to temperature, precipitation, sunshine, wind, or other elements. Applied, or technical, classifications employ any criteria relevant to the effects of climate on other phenomena. Climates may thus be defined according to their influence on landforms, vegetation, agriculture, human comfort, energy consumption, air pollution, or urban environments. Any climatic classification must provide for different scales of space and time. Broadly generalized regional climates encompassing large areas are macroclimates and may be subdivided into mesoclimates of intermediate scale. Microclimates are climates of small areas, often limited to shallow surface layers of the atmosphere for the purposes of applied studies. Paleoclimates--climates of the far distant past--are usually subdivided according to geologic or biologic time scales (see \Tpaleoclimatology\t). Evidence from archaeology, recorded history, and statistical analyses of observational data have fixed the limits of climatic periods since humans first appeared. The best known and most widely used climatic classification is that developed by Wladimir \TKoppen\t in the early 20th century. Koppen defined values of temperature and precipitation that set boundary conditions between major vegetation formations. The accompanying map shows the world distribution of thirteen main classes of climate, based on a modification of Koppen's system by the American geographer Glenn T. Trewartha. Numerous other classifications have been devised for climate research and for applied studies of natural resources, economic activities, or environmental problems. No single system can suit all purposes; rather, the system of organization and selection of criteria depend on the intended use: explanation, description, or application of climatic knowledge. WHY CLIMATES DIFFER Incoming solar radiation, or \Tinsolation\t, is the basic source of energy for atmospheric processes. The Earth's orbital revolution around the Sun and its rotation on a tilting polar axis produce seasonal and daily changes in the amount of insolation. Gases, clouds, and suspended particles in the atmosphere scatter and reflect about 26 percent of insolation into space. The Earth's surface reflects another 4 percent, although the proportion varies with the angle of the Sun and the reflectivity of different materials. The atmosphere and the Earth's surface together absorb about 70 percent of insolation, which is converted to the heat and kinetic energy that create weather and climate. Absorption by the atmosphere of energy emitted from the Earth's surface delays the energy's return to space, creating a \Tgreenhouse effect\t. Eventually all absorbed solar energy returns to outer space as long-wave radiation, maintaining a long-term global energy balance and a nearly constant average global temperature. The actual energy budget and resulting effects on climate at a given place depend on additional factors. Latitude determines the duration of daylight as well as the angle of the Sun's rays, which are more effective when the Sun is near the zenith. Altitude is also a factor in climate, because air temperature normally decreases with elevation at a rate of about 6 deg C/1,000 m (3.3 deg F/1,000 ft). General atmospheric and oceanic circulation systems redistribute heat and moisture, helping to prevent overheating in the tropics and intense cold near the poles. Prevailing winds, especially trade winds and westerlies, transfer temperature and moisture properties between the continents and the oceans. Because water is slower to heat and cool than land and affords a ready supply of moisture, regions downwind from oceans usually have more moderate temperatures and more precipitation than do the interiors of the continents. This maritime influence is marked in middle latitudes. Ocean currents and drifts further promote the transfer of heat. Areas lying in the paths of cyclonic storms are subjected to the associated variability of winds, temperature, and precipitation. Where prevailing winds, air masses, and traveling storms encounter mountains, the barrier effect retards movement, often forcing air to rise. This \Torographic\t effect causes cooling as the air expands and may induce greater precipitation on windward slopes, whereas the leeward slopes experience a \Train shadow effect\t. Mountain barriers can also slow the passage of cold, stable air masses, thereby protecting regions to the leeward. Local relief features and differences in slope or exposure affect the receipt of insolation, water runoff, and wind conditions. Daily differences in heating and cooling generate local mountain and valley winds and land-sea breezes. Inland bodies of water also can create daily breezes as well as influence temperature and humidity in their vicinity owing to the \Tlake\t EFFECT. CLIMATE CHANGE The elements of climate change through time as well as from place to place. Indirect evidence of climatic trends in the distant past is revealed in fossils, lake and ocean beds, peat bogs, glacial deposits, and soils. Widths of annual growth rings in trees correlate with temperature and rainfall fluctuations, especially along the drier margins of forests. Archaeological remains and written history offer clues to climatic conditions during the human era. Modern instrument records provide direct evidence of climatic change. For example, analysis of temperature data shows a slight warming in the northern hemisphere from about 1890 to 1940, followed by apparent cooling. No single explanation accounts for all the trends and fluctuations indicated by various types of evidence. Major theories fall into four broad categories incorporating changes in insolation patterns, changes in the atmosphere's content, the Earth's surface features, and human interference. Variations in the energy emitted by the Sun, as with sunspot activity, would alter the amount and perhaps the kind of radiation received by the Earth. The \LMilankovi\lch THEORY suggests that differences in the shape of the Earth's orbit, the tilt of the polar axis, and the time of year when the Earth is nearest the Sun effect changes in insolation. The periods of continental glaciation known as the \Tice ages\t have been linked to these effects by the Milankovitch theory. Substantial evidence now supports this link, although the theory continues to be debated. Within the atmosphere, fluctuations in the amount and distribution of gases, clouds, and solid particles would be expected to alter the energy budget. Among the possible natural causes of such fluctuations are volcanic eruptions, and studies of the effects of the 1982 eruption of Mexico's \TEl Chichon\t appear to support this concept. Drastic events such as comet or large meteor impacts, with resulting large-scale effects on climate, have also been suggested as causes of the mass \Lextinction\ls of life forms that have taken place in the Earth's history, but this idea remains unproven. The Earth's surface exerts immense influence on the heat and moisture budgets. Geologic changes in the size, position, and elevation of the continents have been studied as causes of paleoclimatic change. Changes in reflectivity resulting from shifting patterns of plant cover, water, or ice are other possible factors in climatic variability. Human activity has the potential of affecting large-scale climate patterns through the introduction of materials into the atmosphere and the depletion of forest cover. Scientists now clearly recognize that the greenhouse effect, mentioned above, is being enhanced by human activities, including the massive burning of fossil fuels and consequent increase of carbon dioxide in the atmosphere. The reduction of the ozone layer by industrial \Lfluorocarbon\ls also has become a serious concern, because the ozone layer serves as a shield against ultraviolet radiation and plays a role in maintaining the Earth's heat balance. The regional effects on climate of large urban centers have also been the subject of research for many years (see \Turban climate\t). CLIMATIC EFFECTS ON HUMAN BEINGS The climatic environment greatly affects human comfort and health, but the complexity of the human response to climate has resulted in misunderstanding and misinterpretation. Human beings are warm-blooded and thus maintain a constant body temperature in spite of wide variations in environmental temperature. A constant body temperature results from a balance between heat gain and loss. Imbalance causes several physiologic responses. Under cold conditions the body shivers to produce heat through kinetic energy and restricts the flow of blood to the extremities to limit heat loss. At high temperatures sweating is the main cooling mechanism, through the affects of evaporation. Several bioclimatological indices estimate degree of comfort. One of the most common, the effective temperature, is the temperature of saturated still air that has the same effect upon comfort as the air under investigation. A useful assessment of cold discomfort, the windchill index, measures the quantity of heat that the atmosphere can absorb. Many common diseases as well as mortality show a relationship to seasonal and other climatic variations. A direct cause-effect correlation is difficult to establish, however, because of the difficulty in differentiating weather-induced illness from other illnesses, and because of changes in sanitation and health education through time. Differences in tolerance as well as adaptation achieved through prolonged acclimatization further complicate physiological reactions to the atmospheric environment. Notable examples of acclimatization are: the ability of the native inhabitants of both Tierra del Fuego and the Australian desert to sleep outdoors in freezing temperatures, and the ability of the Indians native to the high Andean mountains to live with half the oxygen values found at sea level. Prolonged genetic selection appears to have developed functional differences, such as a greater number of sweat glands in races of tropical regions, and higher blood flow and surface temperature in Eskimos. Climate and Race Several empirical bioclimatic rules propose that various permanent physiological changes have occurred in nature to facilitate the survival of the species. Bergman's rule holds that subspecies living in colder climates attain a greater body size than those living in warmer climates. The volume of an object increases in proportion to the cube of its radius, whereas the surface area varies with the square of the radius. Because most heat loss takes place through the skin, a larger body would lose less heat. Allen's rule states that extremities are relatively short in the cooler regions of the range of a species because considerable heat loss takes place through these poorly insulated parts. The concentration of short people in low latitudes supposedly substantiates Bergman's rule, while the Eskimo, whose short stature is attributed to short legs, apparently conforms to Allen's rule. Different pigmentations have given rise to the theory that dark skin is a necessary protection in the tropics against excess vitamin D production. Elsewhere, dark skin would filter out too much radiation, resulting in disability from lack of vitamin D and survival of lighter-skinned people. These theories have not received full support. It is difficult to establish whether climate, or other factors (such as food), alone or in combination, are responsible for the differences. Furthermore, human modification of the climatic environment (through clothing and shelter) is so efficient that adaptation through marked physical changes was perhaps not necessary. Climate and Human Culture Writers such as Friedrich \TRatzel\t and Ellen Semple applied similar natural laws to explain differences in human culture and development. Perhaps the best-known writer in the field of climatic determinism was Ellsworth \THuntington\t, who described the role of climate in determining racial character, religion, and the rise and fall of civilization. His theory that life in cold and changeable climates has stimulated mental capacity and efficiency more than life in warmer regions has been criticized as an ethnocentric generalization that was based on limited data and that ignored contradictory evidence. The concept that climate changes affect migration patterns and cultural developments remains a working principle. Howard J. Critchfield Bibliography: Budyko, M. I., The Earth's Climate, Past and Future (1982); Critchfield, Howard J., General Climatology, 4th ed. (1983); Crowe, P. R., Concepts in Climatology (1971); Gregory, Stanley, ed., Recent Climatic Change (1988); Gribbin, John, Future Weather and the Greenhouse Effect (1982); Griffiths, J. F., and Driscoll, D. M., Survey of Climatology (1982); Grove, Jean, The Little Ice Age (1988); Kondratyev, K. Y., Climate Shocks (1988); Mather, John R., Climatology: Fundamentals and Applications (1974); Maunder, W. J., The Uncertainty Business (1986); Neiburger, Morris, et al., Understanding Our Atmospheric Environment, 2d ed. (1982); Oliver, J. E., and Hildore, J. J., Climatology (1984); Smith, Keith, Principles of Applied Climatology (1975); Trewartha, Glenn T., The Earth's Problem Climates, 2d ed. (1981).