Dictionary of Meaning
<<Back
Please select a letter:
A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
L |
M |
N |
O |
P |
Q |
R |
S |
T |
U |
V |
W |
X |
Y |
Z |
0-9
Click here for Shopping
Hadley Cell
*** Shopping-Tip: Hadley Cell
The major driving force of
atmospheric circulation in the
tropical regions is
solar heating. Because of the Earth's 23.5 °
axial tilt, the sun is never more than a few tens of degrees from directly overhead at noon in the tropics; as a consequence, incident
solar radiation provides maximum
energy at the equator. This heat is largely transported into the atmosphere as
latent heat via
convection in daily
thunderstorms that form in this weather belt.
In the early 1700s,
George Hadley, an English lawyer and amateur
meteorologist, set out to determine what it was that caused the Northern-hemisphere
trade winds to blow west instead of straight south.
Edmond Halley, the astronomer noted for the comet bearing his name, had some years earlier proposed a theory of circulation which would result in trade winds, but which could not take into account their change in direction. It was Hadley who realized that a rotating coordinate system would be required to describe the motions of the winds, and hence, that
Gaspard-Gustave Coriolis’ work in producing the necessary equations would be germane to his own work. And so it was that the necessary critical link was added which resulted in Hadley’s description of the equatorial zone circulation cell, which is known today as the '''Hadley cell'''.
image:HadleyCross-sec.jpg thumb|300px|right|The Hadley cell carries heat and moisture from the tropics to the northern and southern mid-latitudes.
The Hadley cell is a closed
atmospheric circulation circulation cell. At (1) in the diagram to the right, warm, moisture-bearing air converges at the equator. Because the
Coriolis effect Coriolis force is negligible within about five degrees either side of the geographic equator, winds are light and capricious in direction. The primary movement of air is upward, and thunderstorms are prime transporters of moisture and heat to the upper
troposphere. These air masses eventually reach the
tropopause, the boundary between the
troposphere and the
stratosphere, at a height of between 12 and 15 km, and cannot proceed upward. Nor, because of the upward-welling air below them, can they sink. As a consequence, they are forced either north (a) or south (b) of the
equator (2a, 2b).
It is at this point the Coriolis force evidences itself, leading to a clockwise rotation in the northern hemisphere and a counterclockwise rotation on the southern hemisphere.
The airflow loses heat as it travels, and at about 30° north/south of the equator, it begins to descend (3). As it descends it is compressed, increases in temperature from
adiabatic lapse rate adiabatic heating and thus the
relative humidity decreases, so skies in this high-pressure weather regime tend to be cloud-free, and windless days are common.
This region marks the zone of separation between the Hadley cell and the
temperate zone
Ferrel cell, and is known as the "
horse latitudes". According to the story, in times when ship's captains relied upon the wind to reach their destinations, finding themselves becalmed was usually bad news for any horses aboard, which were thrown overboard in order to conserve precious water.
Image:Omega-500-july-era40-1979.png thumb|300px|right|Vertical velocity at 500 hPa, July average. Ascent (negative values) is concentrated close to the solar equator; descent (positive values) is more diffuse
At about 2 km above the surface of the Earth, the subsiding air enters the
boundary layer, a region of the atmosphere where winds are subject to the influence of surface topography. While the Coriolis force twists the downfalling air, the effect of the friction layer is to contribute to this rotational effect. As well, the speed of the wind is affected by the terrain over which it passes - winds blow more strongly over grasslands and ocean than they do over forests and foothills. It is likewise in this layer that such localized and regionally peculiar atmospheric phenomena as lenticular (lens-shaped)
clouds and
chinook winds occur.
The airmass spreads out over the surface of the Earth, moving in a westerly direction. It begins its journey back toward the equator, (4) closing the Hadley cell, and appears as the
Trade Winds.
As with any idealized system, it is important to realize that it is precisely that. A Hadley cell is bound as much by the laws of chaos as by the laws of thermodynamics, and what is here today may be there, or somewhere else, tomorrow.
See also
*
Atmospheric circulation
*
Intertropical convergence zone
Category:Tropical meteorology
zh-min-nan:Hadley sûn-khoân
de:Hadley-Zelle
es:Célula de Hadley
ja:�ドレー循環
pl:Komórka Hadleya
fi:Hadleyn solu
see
Hadley cell
*** Shopping-Tip: Hadley Cell
