Strong, steady winds converge to fill the low pressure from the north and south, but the spinning of the Earth diverts the winds slightly to the right in the northern hemisphere and they become the north-east Trade Winds. In the southern hemisphere winds are diverted to the left, producing the south-west Trade Winds.
Air in the upper atmosphere flows away from the Equator and cools. Cooling air becomes denser, heavier and descends back to the surface on the polar sides of the 'Cell', causing zones of high pressure at about 30° north and south of the Equator (sub-tropical high pressure zone). Descending air warms and becomes drier, skies are cloudless and hot desert areas are found in these latitudes.
The ITCZ or low pressure zone moves north and south with the overhead sun. In June there will be low pressure (ITCZ) over the Tropic of Cancer (23.5°N). Warm, moist Trade Winds from over the Atlantic Ocean bring heavy summer rains. In December when the overhead sun is south of the Equator the sub-tropical high pressure zone moves south over the Tropic of Cancer. The hot descending air gives a winter dry season.
Around 60° north and south of the Equator the global circulation of air produces another zone of low pressure, where warm tropical air from the south west converges with cold polar air. Depressions form and at the fronts warm air is forced to rise over the cold denser air causing frontal rainfall. In the British Isles depressions can occur throughout the year providing a steady, reliable rainfall. Further south, in Rome, depressions are rare in summer, which is therefore drier and hotter than in the British Isles.
High pressure over the Poles, creates cold polar deserts.
Land and sea
Land masses become much hotter than the water in the oceans in summer, but lose heat much faster during the winter. Continental areas therefore have more extreme climates than coastal areas.
Hot land masses in summer warm the air above, which expands and rises creating low pressure areas and convectional currents. Summer thunderstorms are therefore more common in Berlin than in London and precipitation is more variable and unreliable through the year.
Relief
Temperature decreases with altitude. Warm, moist winds are forced to rise over the mountains, cool and cause relief rain. Mountains in the Scottish Highlands, the Alps, Japan and the Cameroon Mountains near Calibar all have more extreme temperatures and far wetter climates than the surrounding lowlands.
High, extensive mountain ranges, such as the Alps, Himalayas and the Andes, interrupt or block the global patterns of airflow and act as boundaries between very different climates either side.
Ocean currents
In the northern hemisphere ocean currents flowing northwards carry warm water from the tropics towards the polar areas. The North Atlantic Drift or Gulf Stream keeps winters in the British Isles warmer and the summers cooler than those in Central Europe.
Prevailing winds from the south west, passing over the ocean current are warmed and pick up large amounts of moisture. The precipitation, caused when the air is forced to rise over mountains or at fronts, provides the British Isles with a reliable precipitation all year.
Cold ocean currents from colder polar waters have a contrasting effect. Those flowing south along coasts in the northern hemisphere keep summer temperatures cooler and the air drier than normal for that latitude. Coastal fogs occur when warmer air from over the land is cooled by the cold water.
In tropical areas warm ocean currents, such as the Kuro Siwo which flows north towards Japan, are the source of the large amounts of energy and moisture that can result in the development of intense depressions or typhoons.
