What are the uses and limitations of the tri-cellular model?

Answer:-
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The tri-cellular model is a way of showing atmospheric circulation and helps us to understand the general circulation of the atmosphere.

There are 3 cells that make up the model:

1. The Hadley Cell which shows air circulation between 0º and 30º North and South of the Equator
2. The Ferrel Cell between 30º and 60º North and South of the Equator
3. The Polar Cell 60º upwards in both hemispheres.

The model helps us to understand how air circulates around the globe and so therefore the climatic conditions expected within each broad latitude belt. The equator is the central controlling system within the atmosphere. Warm air is forced to rise in the equatorial regions, as it rises it cools to produce cumulo-nimbus clouds. As the air cools uplift ceases and the air begins to move away from the equator. Further cooling allows the air to slow down and subside at roughly about 30º. This forms the descending arm of the Hadley Cell. As the air is descending, high pressure is created and as a result dry, clear skies result (the conditions associated with desert environments).

Upon reaching the earth’s surface some air is returned to the equator whilst the rest is uplifted and diverted poleward to form the Ferrel Cell. As air moves northwards over oceans collecting moisture it meets air moving south from the Polar Cell. The warmer air is forced to rise, this often occurs at 60º latitude, forming an area of low pressure and the rising arm of the Ferrel cell and polar cell. These unstable conditions produce depressions typical over these latitudes in areas such as Great Britain.

This overall pattern is crucial to an understanding of climatic factors around the globe. The pattern is also affected by the position of the overhead sun at certain times of the year. This apparent movement causes the seasonal shift of the heat from the Equator. When the Sun appears to be overhead at the tropic of Capricorn in December, the heat is greatest in this region (i.e. 30º South), therefore air rises here instead of sinking and as a result places that previously experience high pressure now experience low pressure, such as India.

In contrast as the earth’s position shifts again in June the sun now appears to be overhead in the northern hemisphere near the tropic of Cancer. This time areas such as Northern India are constantly heated and suffer intense low pressure. Air is drawn in to fill the void, which is humid and conducive to rainfall. This is the Monsoon period, a time that many countries are reliant upon for crop growing and the refilling of rivers and lakes.

The tri-cellular model is invaluable in understanding these atmospheric processes, however, it has its limitations. Being a model it is a 'perfect example scenario'; weather and climate does not fit such a mould. The model cannot account for local conditions in the atmosphere affected by distance from the sea, continentality, altitude, ocean currents, urban areas or local winds.