Pressure Patterns
The temperature difference between the equator and the poles generates the global general circulation which redistributes heat from low latitudes to high latitudes. The general circulation is made up of a number of large-scale circulation cells, which consist of rising and descending air. Such large-scale vertical movement of air generates pressure differences across the Earth, which assist the development of surface winds that transfer the heat. Latitudinal differences in pressure delineate a number of major pressure zones which correspond with zones of climate.
Low pressure develops when air rises. A major zone of low pressure exists at or near the equator, where the strong sunlight heats the Earth's surface and air in contact with it, causing it to rise. The climate associated with this zone of low pressure and rising air is tropical, with considerable rainfall in the form of daily thunderstorms. The tropical climate zone shifts north of the equator in summer and south of it in winter as the region of most intense solar radiation fluctuates with the seasons.
Air rising above the equator spreads out, cools, and descends (because colder air is heavier) in the subtropical zones at 25 to 40° latitude. Descending air increases atmospheric pressure, and subtropical climate is associated with generally clear skies, low rainfall, and high daytime temperatures of over 40°C. Many of the world's hot deserts are found in this subtropical high-pressure belt. As for the tropical belt of climate the subtropical zone expands towards the higher latitudes during the summer months.
Some of the descending air in the subtropical belt forms surface winds which return towards the equator. The rest travels towards the higher latitudes, delivering warm air masses to the temperate regions of the world at 50 to 60° latitude. Here they collide with cold polar air, and the warm air is forced to rise, developing a zone of low pressure. This temperate low-pressure zone is cyclonic in nature, and is associated with the development of frontal depressions. Such weather features are common to the UK and bring with them bouts of rain and intermittent sunshine. Depressions are more dominant in winter than in summer. During the warmer half of the year, subtropical high-pressure systems sometimes expand poleward to have a calming influence on the weather in the temperate latitudes.
The very low temperatures over the polar regions make the air there heavy, forcing it to descend, creating a zone of high pressure. The permanent and thick snow and ice cover in the Antarctic and Arctic regions give false impression that they receive a lot of snowfall. In fact, the polar regions can be as dry as the hot deserts of the subtropical climate zone, and it can take many years for even small amounts of snowfall to accumulate.
The positions of the world's continents and major oceans significantly influences the major pressure belts that develop from the general circulation of the atmosphere. Land gains and loses heat much more quickly than seawater. Consequently, the large landmasses of the Americas and Asia become much warmer in summer and much colder in winter. The extra surface heat in summer generates a continental region of low pressure, whilst in winter, the colder descending air gives rise to dominant high pressure anticyclones. In summer the development of continental low pressure significantly influences the pattern of monsoons that affect the weather of India and southern Asia, that would otherwise be much weaker if the subtropical high pressure remained persistent.
The development of the mid-latitude cyclonic depressions, at the convergence zone between polar and subtropical air masses, is also influenced by the land-sea distribution. In the Northern Hemisphere, depressions more readily develop in the Atlantic and Pacific Oceans. These zones are known respectively as the Icelandic and Aleutian Lows.
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