George Hadley was born in London in 1685, and from an early age was interested in physics and astronomy. Although he studied, and practiced, law, his passion was weather. He had oversight of the meteorological reports presented to the Royal Society of London for seven years and in 1735 presented his own paper, “Concerning the Cause of the General Trade Winds.” The trade winds were of vital importance to European ships crossing the Atlantic and Edmond Halley, the famous astronomer, had tried to explain the origin of the winds in 1686.
George Hadley disagreed with Halley’s theory and postulated that the trade winds were the result of warm air rising from the equatorial regions, causing the tropical rains as it cooled and moved to both north and south. The cooled air would then fall in a region 30 degrees from the equator and cooler air would be pulled back in towards the equator, to allow the sequence to continue. Hadley concluded that the air moving back to the equator at surface came from the east, forming the winds known as the trade winds, without adequately explaining why.
The circulation of air rising at the equator, flowing towards the poles and falling again was called the Hadley Principle and the circulation pattern is now known as the Hadley Cell, with no distinction as to whether the cell is that to the north or south. As with so many aspects of scientific discovery this did not come about immediately but grew as more scientists developed their own theories about the phenomena, or added to knowledge about it.
The Coriolis Effect
Almost exactly a century later Gustave-Gaspard Coriolis developed his theory that the earth’s rotation affects the movement of air over the surface, especially at such large scales as the Hadley cells. This was the missing piece of the puzzle, and the reason that the equatorial air rises and falls in such a way that it causes surface winds to blow from east to west.
Dessicating Winds from Hadley Cells
Another aspect of the Hadley cells that has a bearing on human society is the region where the cooler, now dry, air falls back to the ground. Many of the desert regions lie 30 degrees from the equator, because of this dry air that causes zones of high pressure around the globe. You can click on the thumbnail to see the Hadley Cell mechanism, and any map will show the deserts where the dry air falls. You will clearly see deserts on a satellite image that are 30 degrees south and north of the equator.
Remembering George Hadley
We will not forget the contribution that Hadley made to our knowledge of the atmosphere. The Hadley Centre for Climate Prediction and Research is part of the UK Met Office, and the Hadley Crater is a crater on Mars. The Mons Hadley is on the moon, but this one is named after John Hadley, George’s brother, who invented the octant, a forerunner of the sextant.
Sources
Hadley’s Principle: Understanding and Misunderstanding the Trade Winds by Anders Persson, in History of Meteorology 3 (2006)
Hadley cell. (2011). In Encyclopædia Britannica. Retrieved from http://www.britannica.com/EBchecked/topic/251175/Hadley-cell
Sally Anne Lewis - Sally has master's degrees in science and journalism, is fascinated to a degree in most things. Always something to learn.
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