Hadley Describes Atmospheric Circulation

Date 1735

George Hadley, an amateur scientist, described global atmospheric circulation as driven by solar heating and the rotation of the Earth. He was the first person to provide a working explanation for the atmospheric circulation patterns observed in the tropics and subtropics, including the trade winds.

Locale England

Key Figures

• George Hadley (1685-1768), English amateur scientist and a lawyer
• Gaspard-Gustave de Coriolis (1792-1843), French mathematician and physicist
• Edmond Halley (1656-1742), English astronomer

Summary of Event

Many astute fifteenth century European navigators were familiar with the overall westerly winds of the midlatitudes, the easterly winds of the lower latitudes, and the so-called doldrums that lay in equatorial regions. It was Christopher Columbus who first demonstrated the importance of these zonal winds in transoceanic travel. Instead of sailing west from the Iberian Peninsula, Columbus first sailed the three Spanish ships under his command south to the Canary Islands before crossing the Atlantic Ocean in 1492. He used a brisk tailwind to sail from the Canary Islands to the Bahamas in thirty-six days, a southerly route that allowed Columbus to cross the Atlantic sailing within the low-latitude easterlies. Thus, he is frequently credited with having discovered the trade winds. Columbus returned to Europe sailing at a higher latitude, stopping at the Azores as he sailed in the favorable westerlies.

In the sixteenth century, the easterly trade winds in the low latitudes north and south of the equator became the preferred route between Europe and the Western Hemisphere. There was a consensus that these winds arose as the Earth rotated from west to east, but many mathematicians and astronomers argued that the Earth’s rotation was insufficient to power the trade winds. British scientists, including astronomerEdmond Halley, became interested in providing a solid scientific explanation for these winds. In 1686, he published a study of the trade winds in the Philosophical Transactions of the Royal Society, in which he argued that solar heating caused atmospheric circulation. With the article came publication of the first weather map, which showed average winds over the oceans.

A barrister (lawyer) by training and brother of the astronomer John Hadley, George Hadley became interested in weather phenomena and wanted to provide a scientific explanation for the midlatitude westerlies and the easterly low-latitude trade winds. He realized that the trade winds could be explained only by using a rotating coordinate system under the influence of solar heating. Hadley used Halley’s work on trade winds as a starting point and concluded that solar heating of the atmosphere is at its maximum at the equator, causing warm equatorial air to rise at low latitudes and move aloft toward the poles. Upward air movement occurs in the doldrums, and cooler surface air is constantly moved eastward toward the equator to be warmed. (This explanation only roughly conserves angular momentum.) Hadley envisioned this atmospheric circulation system as zonally symmetric, with the Northern and Southern Hemispheres having mirror-image latitudinal wind systems.

Hadley’s explanation for the trade winds phenomenon was generally accepted when he presented his work to the Royal Society in London in 1735. Within fifty years, however, his explanation had been forgotten, and English meteorologist and physicistJohn Dalton and German philosopher Immanuel Kant independently proposed explanations similar to Hadley’s. Eventually, meteorologists determined that Hadley’s assumption of conservation of velocity instead of conservation of angular momentum was incorrect.

A century after Hadley advanced his explanation for the trade winds, Gaspard-Gustave de Coriolis explained mathematically the apparent deflection (commonly referred to as the Coriolis effect) of winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The amount of deflection is a function of wind speed and latitude; the Coriolis effect is zero at the equator.

Significance

Sailing ships gave way to steam-powered vessels, lessening the importance of the trade winds to commerce. The study of meteorology and Earth’s atmosphere remained important in other contexts, however. By the mid-1850’s, two American meteorologists and a British mathematician had independently proposed a three-cell meridional circulation structure in each hemisphere. This concept maintained the low-latitude Hadley cell, with the midlatitude cell being called a Ferrel cell. The Ferrel cell was envisioned to have a rising motion at about 60 degrees latitude. The Hadley cell still was credited with supplying a westerly momentum to the midlatitudes. This structure (even when known to be scientifically inaccurate) continued to be used as a simplistic illustration of the global atmospheric circulation through most of the twentieth century.

With the proliferation of meteorological observations in the centuries following Hadley’s work, atmospheric circulation was determined not to be zonally symmetric. Even though it was obvious that the trade winds were adequately described by zonal averages, observations had clearly established that large departures from zonal means were common. These departures were called eddies (as in coastal eddies) by scientists.

With the growth of observational meteorology in the late nineteenth century and with satellite meteorology beginning in the 1960’s, scientific understanding of atmospheric circulation patterns grew. Satellite imagery clearly defines the region where the trade winds converge. Viewed from space over the oceans, this convergence is visible as a band of clouds caused by thunderstorm activity. This band of clouds arises in the region where Hadley concluded that warm, moist air ascends; the region is now known as the Intertropical Convergence Zone, or ITCZ. By the late twentieth century, zonally averaged atmospheric circulation was accepted as a convenient subset of the total atmospheric circulation. The term Hadley cell continues to be commonly used to refer to the zonally averaged low-latitude winds.

Bibliography

Glickman, Todd S., ed. Glossary of Meteorology. 2d ed. Boston: American Meteorological Society, 2000. A definitive scientific reference for English-language meteorological terms.

Hadley, George. “Concerning the Cause of the General Trade-Winds.” Philosophical Transactions 29 (1735): 58-62. Hadley’s important paper presented to the Royal Society.

Lindzen, Richard S. Dynamics in Atmospheric Physics. New York: Cambridge University Press, 1990. A monograph that discusses the problems of observed atmospheric structures and atmospheric circulation. A knowledge of differential equations is needed to appreciate this book. The laminated cover shows Hadley’s concept of the general circulation patterns of the atmosphere. No index.

Monmonier, Mark. Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather. Chicago: University of Chicago Press, 2000. Written for the general reader, this book presents a nonmathematical approach to when and how scientists learned about the general circulation of the atmosphere.

Palmén, Erik, and Chester W. Newton. Atmospheric Circulation Systems: Their Structure and Physical Interpretation. New York: Academic Press, 1969. This college-level textbook puts the problems of angular momentum of the atmosphere into perspective. A knowledge of differential equations is helpful.