The circulation of air throughout our planet is often perceived as a fascinating, yet complex phenomenon. Air circulates within the troposphere layer of the atmosphere, where its motion is driven by imbalances in temperature and by forces in pressure gradients. The distribution of such circulation differs according to three cells that divide the Earth’s northern and southern hemispheres: the Hadley cell, the Ferrel cell, and the Polar cell.
The composition of the Hadley cell was defined by lawyer and amateur meteorologist, George Hadley, in the 18th century. Essentially, sunlight hits the equator perpendicularly, heating the air at the surface. This causes the air particles to expand and rise through the process of convection, diverging from the equator to high pressure regions 30 degrees north and south in latitude. The colder temperature within such high-pressure regions cools the air, which causes it to gain density and sink through the process of subsistence. These air masses then converge back toward the low-pressure region of the tropics. The Polar cell operates in the same manner as the Hadley cell, transferring warm air from the north and south 60th parallel to the poles and cold air from the poles back to the north and south 60th parallel. The movement of air within these two closed cells flows in the form of wind from the east, which is deemed as trade winds at the equator and polar easterlies at the poles. The Ferrel cell was theorized by 19th century meteorologist William Ferrel, which resides between the 30th and 60th north and south parallels. Air circulates in the opposite direction from the other two cells through winds from the west as westerlies, functioning as a zone of mixing.
The Coriolis Effect is produced by the Earth’s rotation rate, which changes with latitude and causes the appearance of air masses to deviate into different directions. As a result, winds in the Northern hemisphere churn rightwards in a clockwise motion, while air circulates leftward in a counter clockwise motion in the Southern hemisphere. In general, low pressure regions are characterized by heavy precipitation and cloud coverage; whereas, high pressure regions are described as drier with clear skies. Considering these patterns of air circulation provides great insight into the weather conditions of various environmental biomes, producing jungles, deserts, or forests throughout different latitudinal regions.
Kump, L. R., Kasting, J. F., and R. G. Crane. 2010. The Earth System. Prentice Hall. 3rd Edition.