The Atmosphere


The Earth and its atmosphere are one interconnected system which is controlled by the sun. Distinctive climatic regions are the result of atmospheric processes that operate within a global system. The basic force causing atmospheric motion is the variation in temperature between equatorial regions and the poles.


* It is the energy flows in the atmosphere that cause the variations in our weather. *



The Three-cell Model


The Three-cell Atmospheric Model

Originally it was believed that between equatorial regions and polar regions there was a single air system which controlled atmospheric movments and redistributed heat energy.


Later it was discovered that there were actually three air systems which made the three cell model (shown above).


The first cell nearest to the equator, called the Hadley cell and named after its discoverer, is the largest of the three. The three cell model also contains the ITCZ (more info) usually around 0° (the equator).


The three cell model continues to assume a uniform surface even when the discovery of the Coriolis effect. This effect causes moving objects to swing to the right in the northern hemisphere and left in the southern hemisphere.


The Coriolis effect means that the winds go westerly and easterly instead of north and south which was originally thought




The diagram above shows the earth as a whole, including the three cell model, pressures and winds. The red arrows indicate warm, rising air while the blue arrows show cool, decending air.



The Jet Stream & Rossby Waves


The first cell nearest to the equator is called the Hadley cell, named after its discoverer. The model also contains the ITCZ (inter tropical convergence zone) which is a belt of low pressure formed by the rise of warm air. The location of this ITCZ varies over time and moves north and south of the equator.


Between these cells, high up near the tropopause, there are the sub tropical jet stream and the polar jet stream. These are fast flowing relatively narrow air currents travelling around the earth.


These jet streams change direction around the earth and form a wave pattern (due to the Coriolis effect). These changes in direction were named after a Swedish meteorologist Carl-Gustaf Rossby who discovered them in 1930's.


The Rossby waves of the jet stream bring air masses with them, moving air masses around the globe and changing the weather. They also effect ocean currents majorly which change climates around the world e.g. Britain with the gulf stream.



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ITCZ (Inter tropical Convergence Zone)




The ITCZ is generally the area where the sun is directly overhead. It moves between the tropics (Cancer & Capricorn) throughout the year. Movements of the ITCZ effect the other air systems too.


Because of the intensity of the solar radiation, the equator, gets the most heat throughout the year. This results in rising air between the two Hadley cells which forms a low pressure zone. The ITCZ moves throughout the year causing a band around the earth. It also has very high levels of precipitation because of the warm air rising which then cools and form water vapour. This is why rain forests can be found along the equator.



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