Geoscience Reference
In-Depth Information
was only accurately determined by satellites in the
1990s.
build-up of air (and therefore of pressure) in the sub-
tropics. 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 demon-
strated by Buys-Ballot in 1860.
C GLOBAL CIRCULATION
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 convergence 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
trades at the surface. This simple mechanism, however,
presented two significant problems - what mechanism
produced 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 not until 1883 did Teisserenc de Bort produce
the first global mean sea-level map showing the main
zones of anticyclones and cyclones (i.e. 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 behaviour 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 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. He
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 math-
ematical analysis of moving bodies on a rotating earth
by Coriolis (1831), Ferrel (1856) developed the first
three-cell model of hemispherical atmospheric circula-
tion by suggesting a mechanism for the production of
high pressure in the subtropics (i.e. 35°N and S latitude).
The tendency for cold upper air to subside in the
subtropics, together with the increase in the deflective
force applied by terrestrial rotation to upper air moving
poleward above the Trade Wind Belt, would cause a
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 generalizations. In 1817
Alexander von Humboldt produced his valuable treatise
on global temperatures containing a map of mean annual
isotherms 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 precip-
itation employing mean annual isohyets; and in 1886
de Bort published the first world maps of annual and
monthly cloudiness. These generalizations allowed,
in the later decades of the century, attempts to be
made to classify climates regionally. In the 1870s
Wladimir Koeppen, a St Petersburg-trained biologist,
began producing maps of climate based on plant
geography, as did de Candolle (1875) and Drude (1887).
In 1883 Hann's massive three-volume
Handbook of
Climatology
appeared, which remained a standard until
1930-40 when the five-volume work of the same title by
Koeppen and Geiger replaced it. At the end of the First
World War Koeppen (1918) produced the first detailed
classification of world climates based on terrestrial
vegetation cover. This was followed by Thornthwaite's
(1931-33) classification of climates employing evapo-
ration and precipitation amounts, which he made more
widely applicable in 1948 by the use of the theoretical
