. The trade winds, westerly and easterlies are the permanent winds. These blow constantly throughout the year in a particular direction and they are global in nature. These play an important role in, and in turn get influenced too, formation of Weather Cell.
Headly Cell
The air at the Inter Tropical Convergence Zone (ITCZ) rises because of convection caused by high insolation and a low pressure is created. The winds from the tropics converge at this low pressure zone. The converged air rises along with the convective cell. (as in the figure, Headly Cell) It reaches the top of the troposphere up to an altitude of 14 km. and moves towards the poles. This causes accumulation of air at about 30o N and S. Part of the accumulated air sinks to the ground and forms a subtropical high (denoted as STH in fig). Another reason for sinking is the cooling of air when it reaches 30o N and S latitudes.
Down below near the land surface the air flows towards the equator as the Easterlies or Trade Winds. The easterlies from either side of the equator converge in the Inter Tropical Convergence Zone (ITCZ). Such circulations from the surface upwards and vice-versa are called cells. Such a cell in the tropics is called Hadley Cell. This is a hot cell, while polar cell is a colder cell.
Ferrel Cell
In the middle latitudes the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high. At the surface these winds are called WESTERLIES and the cell is known as the Ferrel cell.
Polar Cell
At polar latitudes the cold dense air subsides near the poles and blows towards middle latitudes as the polar easterlies. This cell is called the polar cell.
These three cells set the pattern for the general circulation of the atmosphere. The transfer of heat energy from lower latitudes to higher latitudes maintains the general circulation.