Geoscience Reference
In-Depth Information
The first attempt to explain the global
atmospheric circulation was based on a simple
convectional concept. In 1686 Halley associated
the easterly Trade Winds with low-level conver-
gence on the equatorial belt of greatest heating
(i.e., the thermal equator). These flows are
compensated at high levels by return flows aloft.
Poleward of these convectional regions, the air
cools and subsides to feed the northeasterly and
southeasterly Trade Winds at the surface. This
simple mechanism, however, presented two
significant problems: what mechanism produced
the observed high pressure in the subtropics and
what was responsible for the belts of dominantly
westerly winds poleward of this high pressure
zone? It is interesting to note that it was not until
1883 that Teisserenc de Bort produced the first
global mean sea-level map showing the main
zones of high and low pressure. The climatic
significance of Halley's work rests also in his
thermal convectional theory for the origin of the
Asiatic monsoon which was based on the
differential thermal behavior of land and sea; i.e.,
the land reflects more and stores less of the
incoming solar radiation and therefore heats
and cools faster. This heating causes continental
surface pressures to be generally lower than
oceanic ones in summer and higher in winter,
causing seasonal wind reversals. The role of
seasonal movements of the thermal equator
in monsoon systems was only recognized much
later. Some of the difficulties faced by Halley's
simplistic large-scale circulation theory began
to be addressed by Hadley in 1735, who was
particularly concerned with the deflection of
winds on a rotating globe, to the right (left) in the
Northern (Southern) Hemisphere. Like Halley,
he advocated a thermal circulatory mechanism,
but was perplexed by the existence of the
westerlies. Following the mathematical analysis
of moving bodies on a rotating earth by Coriolis
(1831), Ferrel (1856) developed a three-cell model
of hemispherical atmospheric circulation by
suggesting a mechanism for the production of
high pressure in the subtropics (i.e., 35
latitude). The tendency for cold upper air to
subside in the subtropics, together with the
latitudinal increase in the deflective force (the
Coriolis force, the product of wind speed and the
the Coriolis parameter which increases with
latitude) applied by terrestrial rotation to upper
air moving poleward above the Trade Wind Belt,
would cause a buildup of air (and therefore of
pressure) in the subtropics. Equatorward of these
subtropical highs the thermally direct Hadley cells
dominate the Trade Wind Belt but poleward of
them air tends to flow towards higher latitudes at
the surface. This airflow, increasingly deflected
with latitude, constitutes the westerly winds in
both hemispheres. In the Northern Hemisphere,
the highly variable northern margin of the
westerlies is situated where the westerlies are
undercut by polar air moving equatorward. This
margin was compared with a battlefield front
by Bergeron who, in 1922, termed it the Polar
Front. Thus, Ferrel's three cells consisted of two
thermally direct Hadley cells (where warm air rises
and cool air sinks), separated by a weak, indirect
Ferrel cell in mid-latitudes. The relation between
pressure distribution and wind speed and
direction was demonstrated by Buys-Ballot in
1860.
D CLIMATOLOGY
During the nineteenth century it became possible
to assemble a large body of global climatic data
and to use it to make useful regional generaliza-
tions. In 1817 Alexander von Humboldt produced
his valuable treatise on global temperatures con-
taining a map of mean annual isotherms (lines of
equal temperature) for the Northern Hemisphere
but it was not until 1848 that Dove published the
first world maps of monthly mean temperature.
An early world map of precipitation was produced
by Berghaus in 1845; in 1882 Loomis produced
the first world map of precipitation employing
mean annual isohyets (lines of equal precipita-
tion); and in 1886 de Bort published the first
world maps of annual and monthly cloudiness.
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