Land & Sea

Climates around the world are shaped by the transfer of heat energy via the general circulation of the atmosphere. The idealised model of climate zones, pressure patterns and global wind belts that can be sketched however, is complicated by the position of continents and oceans. Land surfaces react quickly to heat gain and loss, becoming warm in summer, cold in winter. The oceans react far more slowly and during the summer they are cooler than the adjoining land, whilst in winter they are warmer.

High atmospheric pressure usually develops over cold areas were air is descending, whilst low pressure forms over warmer regions where air is rising. During the Northern Hemisphere winter a large high-pressure anticyclone develops over Asia, centred on Siberia where temperatures can fall to -40^oC. A weaker winter anticyclone develops over North America. In contrast, low-pressure centres known as depressions continue to develop in the warmer North Atlantic and North Pacific Oceans, centred respectively on Iceland and the Aleutian Islands. These are the Icelandic and Aleutian Lows.

In summer, the landmasses warm up, and the winter high pressure over Asia is replaced by a large low-pressure system, centred over northern India. The Icelandic and Aleutian Lows weaken and the subtropical highs become stronger. The build up of low pressure over Asia assists the development of the monsoon over the Indian Ocean, where the more usual northeast trade winds are replaced by southwesterlies which bring moist air and a prolonged wet season to India and Southeast Asia.

Similar modifications to pressure and wind patterns occur in the Southern Hemisphere, although these are less pronounced than in the Northern Hemisphere because of the absence of really large landmasses.

The moderating influence of the oceans helps to restrict extremes in climate in coastal areas of the world. The typical annual average temperature range in the UK is only about 10 to 15C, whereas in central Siberia or central Canada it can be over 40C. For this reason maritime climates influenced by airflow from the oceans are usually more pleasant than the continental climates of landmass interiors, although they are frequently wetter.