Hadley cell
Encyclopedia
George Hadley
George Hadley was an English lawyer and amateur meteorologist who proposed the atmospheric mechanism by which the Trade Winds are sustained. As a key factor in ensuring that European sailing vessels reached North American shores, understanding the Trade Winds was becoming a matter of great...
, is a circulation pattern that dominates the tropical atmosphere, with rising motion near the equator, poleward flow 10–15 kilometers above the surface, descending motion in the subtropics, and equatorward flow near the surface. This circulation is intimately related to the trade wind
Trade wind
The trade winds are the prevailing pattern of easterly surface winds found in the tropics, within the lower portion of the Earth's atmosphere, in the lower section of the troposphere near the Earth's equator...
s, tropical rainbelts, subtropical deserts and the jet stream
Jet stream
Jet streams are fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. The main jet streams are located near the tropopause, the transition between the troposphere and the stratosphere . The major jet streams on Earth are westerly winds...
s.
There are three primary circulation cells. They are known as the Hadley cell, Ferrel cell, and Polar cell
Polar vortex
A polar vortex is a persistent, large-scale cyclone located near one or both of a planet's geographical poles. On Earth, the polar vortices are located in the middle and upper troposphere and the stratosphere...
.
Mechanism
The major driving force of atmospheric circulationAtmospheric circulation
Atmospheric circulation is the large-scale movement of air, and the means by which thermal energy is distributed on the surface of the Earth....
is solar heating, which on average is largest near the equator and smallest at the poles. The atmospheric circulation transports energy polewards, thus reducing the resulting equator-to-pole temperature gradient. The mechanisms by which this is accomplished differ in tropical and extratropical latitudes.
Between 30°N and 30°S latitude, this energy transport is accomplished by a relatively simple overturning circulation, with rising motion near the equator, poleward motion near the tropopause
Tropopause
The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...
, sinking motion in the subtropics, and an equatorward return flow near the surface. In higher latitudes, the energy transport is instead accomplished by cyclones and anticyclones that cause relatively warm air to move polewards and cold air to move equator wards in the same horizontal plane. The tropical overturning cell is referred to as the Hadley cell. Why it extends only to 30 degrees latitude and what determines its strength are questions addressed by modern dynamical meteorology
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
.
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...
, which turns winds to the right in the Northern hemisphere and to the left in the Southern Hemisphere, creating the subtropical jet stream
Jet stream
Jet streams are fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. The main jet streams are located near the tropopause, the transition between the troposphere and the stratosphere . The major jet streams on Earth are westerly winds...
s that flow from west to east. Analogously, near the surface, the equatorward return flow is turned to the west by the Coriolis effect
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...
. These resulting surface winds, with both an equatorward and a westward component, are referred to as the trade winds.
History of discovery
In the early 18th century, George HadleyGeorge Hadley
George Hadley was an English lawyer and amateur meteorologist who proposed the atmospheric mechanism by which the Trade Winds are sustained. As a key factor in ensuring that European sailing vessels reached North American shores, understanding the Trade Winds was becoming a matter of great...
, an English lawyer and amateur meteorologist, was dissatisfied with the theory that the astronomer Edmond Halley
Edmond Halley
Edmond Halley FRS was an English astronomer, geophysicist, mathematician, meteorologist, and physicist who is best known for computing the orbit of the eponymous Halley's Comet. He was the second Astronomer Royal in Britain, following in the footsteps of John Flamsteed.-Biography and career:Halley...
had proposed for explaining the trade winds. What was no doubt correct in Halley's theory was that solar heating creates upward motion of equatorial air, and air mass
Air mass
In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of square miles, and adopt the characteristics of the surface below them. They are classified according to latitude and their continental or maritime...
from neighboring latitudes must flow in to replace the risen air mass. But for the westward component of the trade winds Halley had proposed that in traveling across the sky the Sun heats the air mass differently over the course of the day. Hadley was not satisfied with that part of Halley's theory and rightly so. Hadley recognized that Earth's rotation plays a role in the direction taken by air mass that moves relative to the Earth, and he was the first to do so. Hadley's theory, published in 1735, remained unknown, but it was rediscovered independently several times. Among the rediscoverers was John Dalton
John Dalton
John Dalton FRS was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and his research into colour blindness .-Early life:John Dalton was born into a Quaker family at Eaglesfield, near Cockermouth, Cumberland,...
, who later learned of Hadley's priority. Over time the mechanism proposed by Hadley became accepted, and over time his name was increasingly attached to it. By the end of the 19th century it was shown that Hadley's theory was deficient in several respects. One of the first who accounted for the dynamics correctly was William Ferrel
William Ferrel
William Ferrel , an American meteorologist, developed theories which explained the mid-latitude atmospheric circulation cell in detail, and it is after him that the Ferrel cell is named. He was born in southern Pennsylvania. His family moved to what would become West Virginia in 1829...
. It took many decades for the correct theory to become accepted, and even today Hadley's theory can still be encountered occasionally, particularly in popular books and websites. Hadley's theory was the generally accepted theory long enough to make his name become universally attached to the circulation pattern in the tropical atmosphere. In 1980 Isaac Held
Isaac Held
Isaac Held is an American meteorologist. He is currently head of the Weatherand Atmospheric Dynamics Group at the Geophysical Fluid Dynamics Laboratory.Held was elected to the United States National Academy of Sciences in 2003.-Biography:...
and Arthur Hou developed the Held-Hou Model
Held-Hou Model
The Held-Hou Model is a model for the Hadley circulation of the atmosphere that would exist in the absence of atmospheric turbulence. The model was developed by Isaac Held and Arthur Hou in 1980. By comparing a motionless atmosphere where the air is radiatively cooled with one where motion is...
to describe the Hadley circulation.
Horse latitudes
Horse Latitudes or Subtropical High are subtropical latitudes between 30 and 35 degrees both north and south. This region, under a ridge of high pressure called the subtropical high, is an area which receives little precipitation and has variable winds mixed with calm.The consistently warm, dry...
".
Major impacts on precipitation by latitude
The region in which the equatorward moving surface flows converge and rise is known as the intertropical convergence zoneIntertropical Convergence Zone
The Intertropical Convergence Zone , known by sailors as The Doldrums, is the area encircling the earth near the equator where winds originating in the northern and southern hemispheres come together....
, or ITCZ, a high-precipitation band of thunderstorms.
Having lost most of its water vapor to condensation and rain in the upward branch of the circulation, the descending air is dry. Low relative humidities are produced as the air is adiabatically warmed due to compression as it descends into a region of higher pressure. The subtropics are relatively free of the convection, or thunderstorms, that are common in the equatorial belt of rising motion. Many of the world's deserts are located in these subtropical latitudes.
Hadley cell expansion
There is some evidence that the expansion of the Hadley cells is related to climate change. The majority of earth's driest and arid regions are located in the areas underneath the descending branches of the Hadley circulation around 30 degrees latitude. Both idealised and more realistic climate model experiments show that the Hadley cell expands with increased global mean temperature (perhaps by 2 degrees latitude over the 21st century ); this can lead to large changes in precipitation in the latitudes at the edge of the cells. Scientists fear that the ongoing presence of global warming might bring drastic changes to the ecosystems in the deep tropics and that the deserts will become drier and expand. As the areas around 30 degrees latitude become drier, those inhabiting in that region will see less rainfall than traditionally expected, which could cause major problems with food supplies and habitability.Hadley Cell expansion is a clear indicator of climate change, as it can affect earth's mean temperature in drastic ways. A poleward expansion of the Hadley cell could have a dramatic impact on locations such as southwestern North America, the Mediterranean, southern South America, South Asia and Australia.