The worldwide system of winds, which transports warm and cold air very great distances away from the source regions, influences significantly the climates of the world. This worldwide wind system is called the general circulation of the atmosphere, and it gives rise to the Earth's climate zones. Although the changing of the seasons and the positions of large oceans and continental landmasses affect these climate zones, they provide a general approximation to the different types of climate seen on Earth.
The Earth's general circulation arises as a result of the temperature difference between the equator and the poles. This latitudinal temperature gradient produces atmospheric pressure differences which generate winds that transport the equatorial heat north and south to higher latitudes. The Earth's rotation however, deflects the northerly and southerly components of this atmospheric circulation, by means of the Coriolis force, clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. Thus, global winds tend to be more easterly and westerly rather than northerly and southerly.
A number of climate zones or belts can be traced between the equator and the pole in each hemisphere. Centred roughly on the equator is the tropical or equatorial zone, a belt of relatively low atmospheric pressure and heavy rainfall associated with thunderstorms, due to the rising air. Historically, the zone became known to sailors as the Doldrums because, with the very light winds, ships would often spend many weeks stuck at sea.
At about 30 north and south of the equator is a subtropical climate belt of generally dry descending air, associated with high atmospheric pressure and clear skies. In the Northern Hemisphere, the belt is centred over the Sahara in Africa and is sometimes called the Azores High. Daytime surface temperatures can often exceed 40C, whilst at night, the extensive heat loss due to lack of cloud cover can lower temperatures close to freezing. The intense heat and lack of rainfall is typical of the desert climate which is commonly found in the subtropical zone. During the Northern Hemisphere summer, the subtropical zone moves northward to influence the Mediterranean region. Mediterranean climates are characterised by hot dry summers, but much cooler and wetter winters than truly subtropical climates nearer the equator.
Between the subtropical and equatorial zones trade winds blow, north-easterly in the Northern Hemisphere and south-easterly in the Southern Hemisphere. These regions are much drier than the equatorial zone, but receive more rainfall than the desert climates. These regions are often characterised by Savannah, scrub and grassland which blossoms during the rainy season and dies off during the prolonged dry season.
In the mid-latitudes around 50 to 60 north and south there is a belt of cyclonic low pressure, arising from the convergence of cold polar easterly winds and warm subtropical westerly antitrades. In the Northern Hemisphere, cyclonic depressions tend to develop in the North Atlantic and North Pacific. These regions are known respectively as the Icelandic and Aleutian Lows. They are characterized by relatively mild, moist winds that tend to bring frequent cyclonic precipitation (rain and snow), particularly along the west-facing side of continents. The precipitation tends to develop along warm and cold fronts, where cold air from the polar easterlies forces the warm, moist air of the westerlies to rise, which, on cooling, releases the moisture as clouds and ultimately rain and snow. Climate in the mid-latitudes is usually temperate.
At the highest latitudes in the polar regions, the cold air sinks producing high atmospheric pressure. The polar climates here are characterised by dry, icy winds that tend to radiate outward from the poles.