Friday, January 14, 2011

Seasons and Climatic Cycles


        The most obvious seasonal variations on Earth are due to the 23.5 degrees inclination of the Earth’s axis to the plane of its rotation around the Sun. This produces hot summers and cold winters as a simple consequence of the different amounts of sunshine received at mid and high latitudes. Neared the equator, seasonal temperature variations are less marked than variations in rainfall, so there the year is often divided into wet and dry seasons. Sri Lanka has two rainy seasons, in spring and autumn, because the equatorial rain belt passes over it twice.


         The most spectacular season is that of the monsoon in southern Asia. Between April and October warm south westerly winds blow in off the Indian Ocean laden with moisture. This is released as extremely heavy rain when the winds rise over the heated land and begin to cool.

        There are climatic cycles over much longer timescales. The eccentricity of the Earth’s orbit varies from nearly circular to more elliptical and back over a period of about 21,000 years. It currently occurs during the Southern Hemisphere summer but that will be reversed in 10,000 years. These cycles all produce long term climatic variations.
Some Climate Extremes:
                The least sunshine occurs at the north and south poles, where the Sun does not rise for 182 days of winter.
                The maximum sunshine received is in the eastern Sahara; more than 4,300 hours a year (97% of daylight hours)
            The coldest place on an average is Plateau station, Antarctica, at -89degrees C.
            The most rainfall in 24 hours fell on Cilaos in the Indian ocean in 1952, about 1870 mm.