Climate



Climate, from the Greek word klima (κλίμα), is the statistical nature—both mean and variability—of weather at a given location for a fairly long period. The location may be as small as a town or as large as a planet, and the period may range from a few decades to millions of years. Therefore the term "climate" only makes sense when both the location and time period are well defined. Climate variables include precipitation (frost, fog, rain hail storms, snow), humidity (water vapour in the air), wind, amount of sunlight, temperature and all other measures of weather.

All planets with atmospheres have climates. This article focuses on the climate of Earth.

Factors and mechanisms influencing climate
Climate is influenced by:


 * Latitude—Solar radiation is on average strongest near the equator and weakest near the poles. The most obvious effect of this variation is on temperature. But this variation in solar heating affects many more aspects of climate through its influence on global atmospheric circulations.
 * Altitude (or how high an area is above sea level)—For exmple, temperature tends to decrease with height, and the windward sides of mountains tend to receive more rain or snow than the leeward (downwind) sides.
 * Distance from the ocean—Areas far away from an ocean have greater changes in temperature than areas close to the ocean. Climatologists have various measures of continentality that summarize this influence.
 * Land surface cover—Plants transpire water vapor through their stomata, an evolutionary adaptation that reduces leaf temperature. Aside from supplying moisture to the atmosphere the energy consumed in evaporating water from the plant interior is not available to heat the air, so that temperatures are cooler than they would be in the absence of vegetation. Urbanised areas, on the other hand, are often denuded of vegetation or nearly so.
 * Aerosols—Aerosol particles (airborne particles of liquid or solid) may be natural or man-made. Aerosols have two main effects on the energy balance of the earth and atmosphere: they directly scatter and absorb radiation, and they serve as condensation nuclei during cloud formation and thereby influence cloud reflectivity and lifetime. The first is referred to as the "direct" aerosol effect and the second as the "indirect" effect.

Factors and mechanisms influencing atmospheric circulation
Among other factors, climate strongly depends on atmospheric circulation. Some factors determining it are:


 * Solar radiation—Large atmospheric convection currents which contribute to weather patterns are generated at the equator, which gets the greatest amount of solar radiation, and the combined effect of the Earth’s rotation (Air acts like a fluid and is churned by the Earth’s rotation). This is a major circulation pattern known as Hadley cell. In addition, as the air is heated, it rises. As it rises it creates turbulent areas which generate wind and it loses heat and moisture. After it has lost sufficient amounts of heat, the cooled air settles, is then warmed by the heat radiating from the Earth’s surface and the process begins anew. Cooler air is also more dense that warmed air and the more dense air pushes the less dense air creating yet another movement of the atmosphere.
 * Coriolis Effect—The Earth's rotation means that the overlying air also rotates. For the Earth itself, and thus for still air, the sense of rotation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. In both cases the rotation is greatest at the poles and decreases to zero at the Equator. This angular momentum is conserved when air moves, causing a deflection to the right of the direction of motion in the Northern Hemisphere and to the left in the Southern Hemisphere.

Climatology
Climatology, the study of climates, is a combination of atmospheric science, hydrology, geology, meteorology, astrophysics and other fields. Global warming, a major focus of climatology, is studied to determine its causes and potential effects. See the main article about global warming for a discussion of the current theories concerning its causes.

Paleoclimatology
Paleoclimatology is the study of ancient climates. This is also true of ancient life forms whose presence may be determined from the fossil record. The ancient distribution of different rock types provide evidence of how the global climate has changed through time and how the continents have travelled across climatic belts. For example, coal deposits develop under wet conditions, bauxite deposits are evident where it was warm and wet, evaporites and calcretes occur under warm and dry conditions, and tillites where it was wet and cool.

Microclimate
Microclimates are distinctive, small-scale climates that are specific defined areas, such as valleys or regions on either side of a geological feature such as a mountain range that alters the precipitation, wind, sunlight, humidity to such an extent as to make the locale climatically different from the surrounding areas. A heavily wooded park within a industrialized urban setting will differ climatically from the surrounding buildings and paved areas, a valley branching off a large region of desertification may offer significant climatic variables.

Over time, plants and animals may adapt to these different microclimates to such an extent that they may thrive in a small defined area with a specific microclimate and yet if they are moved a short distance they will languish and show reduced growth and health, possibly even reduced abilty to multiply.

Microclimate may also refer to artificially controlled environments such as homes, office buildings, botanical gardens, archives or museums where specific requirements are not naturally available.

Natural microclimate
Needs expansion.

Artificial microclimate
Needs expansion.

Urban Regions
Urban heat islands are microclimates created by urban areas. The decrease in vegetation and exposed soil also increases temperature as a result of reduced evaporation from natural sources and, thus, reduced humidity. The process of evaporation uses latent heat and reduces temperature. Industry and vehicles also generate significant amounts of heat. Urban areas also generate pollution, introducing large concentrations of aerosol and naturally and non-naturally occurring atmospheric gases.