climate

The world's climatic zones. There are many systems of classifying climate. One system, that of Wladimir Köppen, was based on temperature and plant type. Other systems take into account the distribution of global winds.

The warming effect of the Earth's atmosphere is called the greenhouse effect. Radiation from the Sun enters the atmosphere but is prevented from escaping back into space by gases such as carbon dioxide (produced for example, by the burning of fossil fuels), nitrogen oxides (from car exhausts), and CFCs (from aerosols and refrigerators). As these gases build up in the atmosphere, the Earth's average temperature is expected to rise.

Combination of weather conditions at a particular place over a period of time – usually a minimum of 30 years. A climate classification encompasses the averages, extremes, and frequencies of all meteorological elements such as temperature, atmospheric pressure, precipitation, wind, humidity, and sunshine, together with the factors that influence them.

The primary factors that influence differences of climate between different areas of the globe are: latitude (as a result of the Earth's rotation and orbit); ocean currents; large-scale movements of wind belts and air masses over the Earth's surface; temperature differences between land and sea surfaces; topography; continent positions; and vegetation. The factors that determine the overall climate of the planet in the long term include the composition of the atmosphere, changes in the Earth's orbit, and changes in the angle of inclination of its axis. Climatologists have become especially concerned with the influences of human activity on climate change, among the most important of which, at both local and global levels, are those currently linked with ozone depleters, the greenhouse effect, and the consequent global warming.

Prevailing winds Regions are affected by different wind systems, which result from the rotation of the Earth and the uneven heating of surface air. As air is heated by radiation from the Sun, it expands and rises, and cooler air flows in to take its place. This movement of air produces belts of prevailing winds. Because of the rotation of the Earth, these are deflected to the right in the northern hemisphere and to the left in the southern hemisphere. This effect, which is greater in the higher latitudes, is known as the Coriolis effect. Because winds transport heat and moisture, they affect the temperature, humidity, precipitation, and cloudiness of an area. As a result, regions with different prevailing wind directions have different climates.

Temperature variations The amount of heat received by the Earth from the Sun varies with latitude and season. In equatorial regions, there is no large seasonal variation in the mean daily temperature of the air near the ground, while in the polar regions, temperatures in the long winters, when there is little incoming solar radiation, fall far below summer temperatures. The temperature of the sea, and of the air above it, varies little in the course of day or night, whereas the surface of the land is rapidly cooled by lack of solar radiation. This is due to the specific heat capacity – the amount of energy required to raise the temperature of land or sea. It takes about two and a half times as much energy to raise the temperature of the sea by 1°C as it does to raise the temperature of the land. This is because the sea is relatively transparent, and so sunlight warms a large volume of sea water, in contrast with a thin surface layer of land. Furthermore, a given volume of water requires a greater amount of heat to raise its temperature by one degree than does an equal volume of rock and soil.

Similarly, the annual change of temperature is relatively small over the sea but much greater over the land. Thus, continental areas are colder than maritime regions in winter, but warmer in summer. This results in winds blowing from the sea which, relative to the land, are warm in winter and cool in summer, while winds originating from the central parts of continents are hot in summer and cold in winter. On average, air temperature drops with increasing land height at a rate of 1°C/1.8°F per 90 m/300 ft – so even in equatorial regions, the tops of mountains can be snow-covered throughout the year.

Vegetation-based climates Rainfall is produced by the condensation of water vapour in air. When winds blow against a range of mountains the air is forced to ascend, resulting in precipitation (rain or snow), the amount depending on the height of the ground and the humidity of the air. Centred on the Equator is a belt of tropical rainforest, which may be either constantly wet or monsoonal (having wet and dry seasons in each year). On either side of this is a belt of savannah, with lighter seasonal rainfall and less dense vegetation, largely in the form of grasses. Then there is usually a transition through steppe (semi-arid) to desert (arid), with a further transition through steppe to what is termed Mediterranean climate with dry summers. Beyond this is the moist temperate climate of middle latitudes, and then a zone of cold climate with moist winters. Where the desert extends into middle latitudes, however, the zones of Mediterranean and moist temperate climates are missing, and the transition is from desert to a cold climate with moist winters. In the extreme east of Asia a cold climate with dry winters extends from about 70° N to 35° N. The polar caps have tundra and glacial climates, with little or no precipitation.

Climate changes Changes in climate can occur naturally or as a result of human activity. Natural variations can be caused by fluctuations in the amount of solar radiation reaching the Earth – for example, sunspot activity is thought to produce changes in the Earth's climate. Variations in the Earth's orbit around the Sun, known as the Milankovitch hypothesis, is also thought to bring about climatic changes. Natural events on the surface of the Earth, such as the eruption of volcanoes and the effects of El Niño, can result in temporary climate changes on a worldwide scale, sometimes extending over several months or even years. Human influences on the climate range from localized effects such as cloud seeding to produce rain, to the global effects of acid rain from industrial emissions; pollution; and the destruction of the rainforests. The study of past climates (palaeoclimatology) involves the investigation of climate changes from the ice ages to the beginning of instrumental recording in the 19th century.