Climate is the conditions of the atmosphere at a particular location over a long period of time; it is the long-term summation of the atmospheric elements (and their variations) that, over short time periods, constitute weather.

These elements are solar radiation, temperature, humidity, precipitation (type, frequency, and amount), atmospheric pressure, and wind (speed and direction).

From the ancient Greek origins of the word (kl’ma, "an inclination or slope"--e.g., of the Sun's rays; a latitude zone of the Earth; a clime), and from its earliest usage in English, climate has been understood to mean the atmospheric conditions that prevail in a given region or zone.

In the older form, "clime," it was sometimes taken to include all aspects of the environment, including the natural vegetation.

The best modern definitions of climate regard it as constituting the total experience of weather and atmospheric behaviour over a number of years in a given region.

Climate is not just the "average weather" (an obsolete, and always inadequate, definition). It should include not only the average values of the climatic elements that prevail at different times but also their extreme ranges, variability, and the frequency of various occurrences.

Just as one year differs from another, decades and centuries are found to differ from one another by a smaller but sometimes significant amount: climate is, therefore, time-dependent, and climatic values or indices should not be quoted without specifying what years they refer to.

This article treats the factors that produce weather and climate and the complex processes that cause variations in both.

Other major points of coverage include global climatic types and microclimates.

The article also considers both the impact of climate on human life and the effects of human activities on the climate.

For details concerning the disciplines of meteorology and climatology, see earth sciences.

See also the article atmosphere for further information about the properties and behaviour of the atmospheric system. Relevant data on the influence of the oceans and of atmospheric moisture on climate can be found in hydrosphere.

Solar radiation and temperature

Air temperatures have their origin in the absorption of radiant energy from the Sun. They are subject to many influences, including those of the atmosphere, ocean, and land, and are modified by them. As variation of solar radiation is the single most important factor affecting climate, it is considered here first.

Solar radiation

Distribution of radiant energy from the Sun

Nuclear fusion deep within the Sun releases a tremendous amount of energy that is slowly transferred to the solar surface, from which it is radiated into space.

The planets intercept minute fractions of this energy, the amount depending on their size and distance from the Sun.

A one-square-metre (11-square-foot) area perpendicular to the Sun at the top of the Earth's atmosphere, for example, receives about 1,365 watts of solar power.

(This amount is comparable to the power consumption of a typical electric heater.)

Because of the slight ellipticity of the Earth's orbit around the Sun, the amount of solar energy intercepted by the Earth steadily rises and falls by +/-3.4 percent throughout the year, peaking on January 3 when the Earth is closest to the Sun. Although about 31 percent of this energy is not used as it is scattered back to space, the remaining amount is sufficient to power the movement of atmospheric winds and oceanic currents and to sustain nearly all biospheric activity.

Most surfaces are not perpendicular to the Sun, and the energy they receive depends on the solar elevation angle (90 for the overhead Sun). This angle changes systematically with latitude, the time of year, and the time of day.

The noontime elevation angle reaches a maximum at all latitudes north of the Tropic of Cancer (23.5 N) around June 22 and a minimum around December 22.

South of the Tropic of Capricorn (23.5 S) the opposite holds true, and between the two tropics the maximum occurs twice a year.

When the Sun has a lower elevation angle, the solar energy is less intense because it is spread out over a larger area. Variation of solar elevation is thus one of the main factors that accounts for the dependence of climatic regime on latitude. The other main factor is the length of day.

For latitudes poleward of 66.5, the length ranges from zero (winter solstice) to 24 hours (summer solstice), whereas the Equator has a constant 12-hour day throughout the year.

The seasonal range of temperature consequently decreases from high latitudes to the tropics, where it becomes less than the diurnal range of temperature.

Effects of the atmosphere

Of the radiant energy reaching the top of the atmosphere, 46 percent is absorbed by the Earth's surface on average, but this value varies significantly from place to place, depending on cloudiness, surface type, and elevation.

If there is persistent cloud cover, as exists in some equatorial regions, much of the incident solar radiation is scattered back to space and very little is absorbed by the Earth's surface.

Water surfaces have low reflectivity (4-10 percent), except for low solar elevations, and are the most efficient absorbers. Snow surfaces, on the other hand, have high reflectivity (40-80 percent) and so are the poorest absorbers.

High-altitude desert regions consistently absorb higher than average amounts of solar radiation because of the reduced effect of the atmosphere above them.

An additional 23 percent or so of the incident solar radiation is absorbed on average in the atmosphere, especially by water vapour and clouds at lower altitudes and by ozone (O3) in the stratosphere.

Absorption of solar radiation by ozone shields the terrestrial surface from harmful ultraviolet light and warms the stratosphere, producing maximum temperatures of -15 to 10 C (5 to 50 F) at an altitude of 50 kilometres (30 miles).

Most atmospheric absorption takes place at ultraviolet and infrared wavelengths, so that more than 90 percent of the visible portion of the solar spectrum, with wavelengths between 0.4 and 0.7 micrometre (0.00002 to 0.00003 inch), reaches the surface on a cloud-free day. Visible light, however, is scattered in varying degrees by cloud droplets, air molecules, and dust particles.

Blue skies and red sunsets are in effect attributable to the preferential scattering of short (blue) wavelengths by air molecules and small dust particles. Cloud droplets scatter visible wavelengths impartially (hence clouds usually appear white) but very efficiently, so that the reflectivity of clouds to solar radiation is typically about 50 percent and may be as high as 80 percent for thick clouds.

The constant gain of solar energy by the Earth is systematically returned to space in the form of thermally emitted radiation in the infrared portion of the spectrum.

The emitted wavelengths are mainly between five and 100 micrometres, and they interact differently with the atmosphere compared to the shorter wavelengths of solar radiation.

Very little of the radiation emitted by the Earth's surface passes directly through the atmosphere. Most of it is absorbed by clouds, carbon dioxide, and water vapour and is then reemitted in all directions.

The atmosphere thus acts as a radiative blanket over the Earth's surface, hindering the loss of heat to space.

The blanketing effect is greatest in the presence of low clouds and weakest for clear, cold skies that contain little water vapour. Without this effect, the mean surface temperature of 15 C would be some 30 degrees colder.

Conversely, as atmospheric concentrations of carbon dioxide, methane, chlorofluorocarbons, and other absorbing gases continue to increase, in large part due to human activities, surface temperatures should rise because of the capacity of such gases to trap infrared radiation. The exact amount of this temperature increase, however, remains uncertain because of unpredictable changes in other atmospheric components, especially cloud cover.

An extreme example of such an effect (commonly dubbed the greenhouse effect) is that produced by the dense atmosphere of the planet Venus, which results in surface temperatures of about 475 C.

This condition exists in spite of the fact that the high reflectivity of the Venusian clouds causes the planet to absorb less solar radiation than the Earth.

Average radiation budgets

The difference between the solar radiation absorbed and the thermal radiation emitted to space determines the radiation budget of the Earth.

Since there is no appreciable long-term trend in planetary temperature, it may be concluded that this budget is essentially zero on a global, long-term average. Latitudinally, it has been found that much more solar radiation is absorbed at low latitudes than at high latitudes.

On the other hand, thermal emission does not show nearly as strong a dependence on latitude, so that the planetary radiation budget decreases systematically from the Equator to the poles.

It changes from being positive to negative at a latitude of about 40. The atmosphere and oceans, through their general circulation, act as vast heat engines, compensating for this imbalance by providing non-radiative mechanisms for the transfer of heat from the Equator to the poles.

While the Earth's surface absorbs a significant amount of thermal radiation due to the blanketing effect of the atmosphere, it loses even more through its own emission, and thus experiences a net loss of long-wave radiation.

This loss is only about 14 percent of the amount emitted by the surface and is less than the average gain of absorbed solar energy. Consequently, the surface has on average a positive radiation budget.

By contrast, the atmosphere emits thermal radiation both to space and to the surface, yet it receives long-wave radiation back from only the latter.

This net loss of thermal energy cannot be compensated for by the modest gain of absorbed solar energy within the atmosphere.

The atmosphere thus has a negative radiation budget, equal in magnitude to the positive radiation budget of the surface but opposite in sign. Non-radiative heat transfer again compensates for the imbalance, this time largely by vertical atmospheric motions involving the evaporation and condensation of water.

Surface-energy budgets

The rate of temperature change in any region is directly proportional to the region's energy budget and inversely proportional to its heat capacity.

While the radiation budget may dominate the average energy budget of many surfaces, non-radiative energy transfer and storage also are generally important when local changes are considered.

Foremost among the cooling effects is the energy required to evaporate surface moisture, which produces atmospheric water vapour.

Most of the latent heat contained in water vapour is subsequently released to the atmosphere during the formation of precipitating clouds, although a minor amount may be returned directly to the surface during dew or frost deposition.

Evaporation increases with rising surface temperature, decreasing relative humidity, and increasing surface wind speed.

Transpiration by plants also increases evaporation rates, which explains why the temperature in an irrigated field is usually lower than that over a nearby dry road surface.

Another important non-radiative mechanism is the exchange of heat that occurs when the temperature of the air is different from that of the surface. Depending on whether the surface is warmer or cooler than the air next to it, heat is transferred to or from the atmosphere by turbulent air motion (more loosely, by convection).

This effect also increases with increasing temperature difference and with increasing surface wind speed. Direct heat transfer may be an important cooling mechanism that limits the maximum temperature of hot, dry surfaces.

Alternatively, it may be an important warming mechanism that limits the minimum temperature of cold surfaces.

Such warming is sensitive to wind speed, so that calm conditions promote lower minimum temperatures.

In a similar category, whenever a temperature difference occurs between the surface and the medium beneath the surface, there is a transfer of heat to or from the medium.

In the case of land surfaces, heat is transferred by conduction. In the case of water surfaces, the transfer is by convection and may consequently be affected by the horizontal transport of heat within large bodies of water.

Average values of the different terms in the energy budgets of the atmosphere and surface are given in Figure 1.

The individual terms may be adjusted to suit local conditions and may be used as an aid to understanding the various temperature characteristics discussed in the next section.


Global variation of mean temperature

Global variations of average surface air temperatures are largely due to latitude, continentality, ocean currents, and prevailing winds.

The effect of latitude is evident in the large north-south gradients in average temperature that occur at middle and high latitudes in each winter hemisphere.

These gradients are due mainly to the rapid decrease of available solar radiation but also in part to the higher surface reflectivity at high latitudes associated with snow and ice and low solar elevations. A broad area of the tropical ocean, by contrast, shows little temperature variation.

Continentality is a measure of the difference between continental and marine climates and is mainly the result of the increased range of temperatures that occurs over land compared to water.

This difference is a consequence of the much lower effective heat capacities of land surfaces as well as of their generally reduced evaporation rates. Heating or cooling of a land surface takes place in a thin layer, the depth of which is determined by the ability of the ground to conduct heat.

The greatest temperature changes occur for dry, sandy soils because they are poor conductors with very small effective heat capacities and contain no moisture for evaporation.

By far the greatest effective heat capacities are those of water surfaces, owing to the mixing of water near the surface and the penetration of solar radiation that distributes heating to depths of several metres. In addition, about 90 percent of the radiation budget of the ocean is used for evaporation.

Ocean temperatures are thus slow to change.

The effect of continentality may be moderated by proximity to the ocean, depending on the direction and strength of the prevailing winds.

Contrast with ocean temperatures at the edges of each continent may be further modified by the presence of a north- or south-flowing ocean current.

For most latitudes, however, continentality explains much of the variation in average temperature at a fixed latitude as well as variations in the difference between January and July temperatures.

Diurnal, seasonal, and extreme temperatures

The diurnal range of temperature generally increases with distance from the sea and toward those places where solar radiation is strongest, in dry tropical climates and on high mountain plateaus (owing to the reduced thickness of the atmosphere to be traversed by the Sun's rays).

The average difference between the day's highest and lowest temperatures in January is 3 C (in July it is 5 C) in those parts of the British Isles nearest the Atlantic. It is 4.5 C in January and 6.5 C in July on the small island of Malta. At Tashkent, Uzbekistan, it is 9 C in January and 15.5 C in July, and at Khartoum, The Sudan, the corresponding figures are 17 C and 13.5 C. At Qandahar, Afg., which lies more than 1,000 metres above sea level, it is 14 C in January, 20 C in July, and exceeds 23 C in September and October when there is less cloudiness than in July.

Near the ocean at Colombo, Sri Lanka, the figures are 8 C in January and 4.5 C in July.

The seasonal variation of temperature and the magnitudes of the differences between the same month in different years and different epochs generally increase toward high latitudes and with distance from the ocean. Extreme temperatures observed in different parts of the world are listed in Table 1.

Variation with height

There are two main levels where the atmosphere is heated--namely, at the Earth's surface and at the top of the ozone layer (about 50 kilometres up) in the stratosphere. Radiation balance shows a net gain at these levels in most cases. Prevailing temperatures tend to decrease with distance from these heating surfaces (apart from the ionosphere and outer atmospheric layers where other processes are at work).

The world's average lapse rate of temperature (change with altitude) in the lower atmosphere is 0.6 to 0.7 C per 100 metres.

Lower temperatures prevail with increasing height above sea level for two reasons: (1) because there is a less favourable radiation balance in the free air, and (2) because rising air--whether lifted by convection currents above a relatively warm surface or forced up over mountains--undergoes a reduction of temperature associated with its expansion as the pressure of the overlying atmosphere declines. This is the adiabatic lapse rate of temperature, which equals about 1 C per 100 metres for dry air and 0.5 C per 100 metres for saturated air in which condensation (with liberation of latent heat) is produced by adiabatic cooling.

The difference between these rates of change of temperature (and therefore density) of rising air currents and the state of the surrounding air determines whether the upward currents are accelerated or retarded--i.e., whether the air is unstable so that vertical convection with its characteristically attendant tall cumulus cloud and shower development is encouraged, or whether it is stable and convection is damped down.

For these reasons, the air temperatures observed on hills and mountains are generally lower than on low ground except in the case of extensive plateaus, which present a raised heating surface (and on still, sunny days, when even a mountain peak is able to warm appreciably the air that remains in contact with it).

Circulation, currents, and ocean-atmosphere interaction

The circulation of the ocean is a key factor in air temperature distribution. Ocean currents that have a northward or southward component, such as the warm Gulf Stream in the North Atlantic or the cold Humboldt Current off South America, effectively exchange heat between low and high latitudes. In tropical latitudes the ocean accounts for a third or more of the poleward heat transport; at latitude 50 N the ocean's share is about one-seventh. In the particular sectors where the currents are located, their importance is of course much greater than these figures, which represent hemispheric averages.

A good example of the effect of a warm current is that of the Gulf Stream in January, which causes a strong east-west gradient in temperatures across the eastern edge of the North American continent.

The relative warmth of the Gulf Stream affects air temperatures all the way across the Atlantic, and prevailing westerlies extend the warming effect deep into northern Europe.

As a result, January temperatures of Troms¿, Nor. (6940' N), for example, average 24 C above the mean for that latitude. The Gulf Stream maintains a warming influence in July, but it is not as noticeable because of the effects of continentality.

The ocean, particularly in areas where the surface is warm, also supplies moisture to the atmosphere.

This in turn contributes to the heat budget of those areas in which the water vapour is condensed into clouds, liberating latent heat in the process, frequently in high latitudes and in locations remote from the ocean where the moisture was taken up.

The great ocean currents are themselves wind-driven--set in motion by the drag of the winds over vast areas of the sea surface, especially where waves increase the friction.

At the limits of the warm currents, particularly where they abut directly upon a cold current, as at the left flank of the Gulf Stream in the neighbourhood of the Grand Banks off Newfoundland and at the subtropical and Antarctic convergences in the oceans of the Southern Hemisphere, the strong thermal gradients in the sea surface result in marked differences in the heating of the atmosphere on either side of the boundary.

These temperature gradients tend to position and guide the strongest flow of the jet stream (see below Jet streams) in the atmosphere above and thereby influence the development and steering of weather systems.

Interactions between the ocean and the atmosphere proceed in both directions.

They also operate at different rates. Some interesting lag effects, which are of value in long-range weather forecasting, arise through the considerably slower circulation of the ocean. Thus, enhanced strength of the easterly trade winds over low latitudes of the Atlantic, north and south of the Equator, impels more water toward the Caribbean and Gulf of Mexico, producing a stronger flow and greater warmth in the Gulf Stream approximately six months later.

Anomalies in the position of the Gulf Stream-Labrador Current boundary, which produce a greater or lesser extent of warm water near the Grand Banks, so affect the energy supply to the atmosphere and the development and steering of weather systems from that region that they are associated with rather persistent anomalies of weather pattern over the British Isles and northern Europe.

Anomalies in the equatorial Pacific and in the northern limit of the Kuroshio (also called the Japan Current) seem to have effects on a similar scale. Indeed, through their influence on the latitude of the jet stream and the wavelength (that is, the spacing of cold trough and warm ridge regions) in the upper westerlies, these ocean anomalies exercise an influence over the atmospheric circulation that spreads to all parts of the hemisphere.

Sea-surface temperature anomalies that recur in the equatorial Pacific at variable intervals of two to seven years can sometimes produce major climatic perturbations.

Such an anomaly is known as El Ni–o (Spanish for "The Child"; it was so named by Peruvian fishermen who noticed its onset during the Christmas season).

During an El Ni–o event, warm surface water flows eastward from the equatorial Pacific, in at least partial response to weakening of the equatorial easterly winds, and replaces the normally cold upwelling surface water off the coast of Peru and Ecuador that is associated with the northward propagation of the cold Peru (or Humboldt) Current.

The change in sea-surface temperature transforms the coastal climate from arid to wet.

The event also affects atmospheric circulation in both hemispheres and is associated with changes in precipitation in regions of North America, Africa, and the western Pacific.

For further information, see the section El Ni–o/Southern Oscillation.

Short-term temperature changes

Many interesting short-term temperature fluctuations also occur, usually in connection with local weather disturbances.

The rapid passage of a mid-latitude cold front, for example, can drop temperatures by 10 C in a few minutes and, if followed by a sustained movement of a cold air mass, by as much as 50 in 24 hours, with life-threatening implications for the unwary.

Temperature increases of up to 40 in a few hours also are possible downwind of major mountain ranges when air that has been warmed by the release of latent heat on the windward side of a range is forced to descend rapidly on the other side (such a wind is variously called chinook, foehn, or Santa Ana).

Changes of this kind, however, involve a wider range of meteorological processes than discussed in this section. For a more detailed treatment, see below Atmospheric pressure and wind.