Climate is the general state of the atmosphere 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.

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 B.C., Hippocrates 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. Galileo devised a thermometer in 1593. His pupil Evangelista Torricelli 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 by 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 establishing a 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. Evidence compiled 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 Köppen in the early 20th century. Köppen defined values of temperature and precipitation that set boundary conditions between major vegetation formations. The accompanying world map shows the world distribution of 13 main classes of climate, based on a modification of Köppen's system by the American geographer Glenn T. Trewartha. These climatic types fall into six major categories that combine broadly similar characteristics of temperature or precipitation and the associated natural vegetation: humid subtropical, dry, humid mesothermal, humid microthermal, polar, and highland.

The humid tropics are constantly warm and receive large annual totals of rainfall. In the tropical-wet type, adequate moisture throughout the year supports dense tropical rain forests such as those of the Amazon and Congo River basins. Monsoon climates prevail along certain tropical coasts where a strong onshore flow of moist air occurs during a distinctly wet summer season. Rainfall and soil-moisture storage generally are sufficient to maintain evergreen forests. Tropical wet-dry regions experience great contrasts in precipitation through the year. Summers are rainy, whereas winters are extremely dry. Tall grasses and scattered trees of the savanna vegetation are adapted to the alternating wet and dry periods. Representative savanna areas are the campos of Brazil and the sudan of northern Africa.

Dry climates extend from the tropics well into the middle latitudes and therefore vary widely in temperature characteristics. Steppe, or semiarid, climates occur along the slightly wetter margins of tropical deserts and in broad belts of western North America and central Asia. Shrubs and short grasses are the principal native vegetation. Desert, or arid, climates include the hot deserts of the tropics and subtropics and cold-winter deserts of middle latitudes. Thorny shrubs and scattered grasses are typical desert plants.

Humid mesothermal climates are found in the middle latitudes where winters are mild and short. Mediterranean climates are distinguished by rainy winters and hot, dry summers; another name for these regions of scrub forest along the west coasts of continents is "dry-summer subtropics." On continental east coasts in comparable latitudes are the subtropical humid climates, which have adequate rainfall throughout the year for forests. The hot, wet summers are much like those of the humid tropics, but winters are cooler. Poleward of Mediterranean-climate areas are marine-west-coast areas of cool forest climates with wet winters.

Humid microthermal climates extend in broad bands across North America and Eurasia. Winter temperatures are low, and they decrease with both latitude and distance from oceans. Interior areas experience great annual temperature ranges. Continental humid climates may be subdivided into "long" and "short" types, reflecting latitudinal effects on the growing season. Tall grasses and mixed forests are the natural vegetation. Farther poleward, subarctic climates are associated with the boreal forest (taiga), consisting mainly of conifers. Winters are very cold; summers are short and cool.

Polar climates form belts surrounding the North and South poles. Winters are long and dark; the short, cool summers have long daylight periods. Grasses and lichens of the tundra can grow on hummocky ground that overlies permanently frozen subsoil. Ice sheets cover most of Greenland and Antarctica. Temperatures during the short summers are not high enough to melt these ice caps, which are maintained by relatively light snowfall that may come in any season.

Regions with highland climates are mosaics of many different local climates that vary with slope, exposure, and altitude in the world's mountain ranges and plateaus. Windward slopes usually are wetter than those on the lee sides. In general, temperatures decrease and precipitation increases with altitude, resulting in a vertical zonation of plant life. Seasonal distribution of temperature and precipitation approximates that of adjacent lowland climates.

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.