Tuesday, January 18, 2011

Climate & Atmospheric circulation

             Whereas the weather anywhere on Earth can fluctuate day by day, even hour by hour, in ways that are very hard to predict in detail, climate describes the general weather conditions of a location, season by season, year by year, averaged out to something that is constant and predictable. That is not to say that the climate has remained constant over geological time scales. There have been huge variations with the changing composition of the atmosphere, the output of the Sun and the changing positions of continents and ocean currents. For example, the onset of the Indian monsoon can be timed as coinciding with the initial uplift of the Himalayas which drive it. The climate is bound to change in the future too, with or without human help.


   The different regional climates on Earth are governed by atmospheric circulation and to some extent by oceans. Both are driven by heat from the Sun. the cells of atmospheric circulation reach several kilometers up into the troposphere. The deepest, called the Hadley cell, rises at the equator ad transports warm air north and south, returning cool warm air both north and south, returning cool air nearer the surface. The circulation continues across mid latitudes in the lower Ferrel cell. Above that, 5-10 Km high, and not affected by friction from the land’s surface, are jet streams. They form a series of constantly shifting waves around regions of low and high pressure.


           The Ferrel cell underneath thus becomes complicated by the Eddies. In a polar cell cold air sinks, flows out and is replaced by warmer air from above. Tropical and polar air meets at the polar front. Where warm air rises above cooler air to form a front, moisture in it condenses to form clouds and heavy rain and low pressure system that often sweeps in over Britain from the Atlantic. The position of the jet stream is important in determining whether the rain will fall on Iceland or London.