Hadley, George (1685–1768)

GEORGE HADLEY WAS an English lawyer, physicist, and meteorologist who first accurately theorized a hypothesis for the trade winds and the associated north-south circulation pattern now known as the Hadley Cell. This piece of information was particularly important in Hadley s times, as it helped the journeys of English vessels toward North American shores. Hadley was intrigued that winds that should have blown northward had, instead, a western course.

Hadley was born on February 12,1685, in London, England to Katherine Fitzjames and George Hadley. He was part of a family of six children and his father was the high sheriff of Hertfordshire. George was initially overshadowed by the fame of his brother John, the inventor of the octant. He studied law, but soon found out that he preferred physics to legal work. For 7 years he was in charge of the meteorological observations for the Royal Society.

Yet, his fame rests on his work of the 1730s, when Hadley was able to complete Edmond Halley s theory of trade winds. Halley had failed to explain completely the westward component of the trades. In his famous paper “Concerning the Cause of the General Trade Winds,” published in 1735, Hadley related the direction of trade winds to the rotation of the Earth.

The meteorologist started from the well-known fact that air at the equator is heated more strongly than at any other place on Earth. Air above the poles is cooler than at any other location. Therefore, it is speculated that surface air near the equator will ascend into the upper atmosphere and, above the poles, descend from the upper atmosphere to ground level. In order to balance these vertical movements of air, Hadley also assumed that air flows across the Earths surface from each pole back to the equator and, in the upper atmosphere, from above the equator to above the poles.

This movement of air is called a convection cell, where convection describes the transfer of heat carried by a moving fluid. Yet, Hadley knew that his model was too simplified to explain the motion of the winds, which obviously do not blow from north to south in the Northern Hemisphere and from south to north in the Southern Hemisphere. He then explained that winds actually tend to blow from the east or west because of Earths rotation.

The movement of the planet causes airflows that would otherwise be from the north or south to be diverted to the east or west. Because of his fame, Hadley was elected a Royal Fellow in 1745. He died on June 28,1768.

A century after Hadley first proposed his theory, the French physicist Gaspard Gustave de Coriolis [Page 488]devised a mathematical description detailing how an object, in motion on a rotating body, follows a curved path in relation to any other body on the same rotating body.

Hadley Circulation provides westward windflow at the Earth's surface and eastward jet streams at higher altitudes. The circulating air patterns create convection currents in four global locations—each current is called a Hadley cell.