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characterized by light winds and fine, clear weather. These highs were some-
times known in times past as the horse latitudes or the Calms of Cancer and
Capricorn. According to the Oxford English Dictionary, the origin of the name
horse latitudes is uncertain. However, it has been suggested that sailing ships
carrying horses to the West Indies, if becalmed unduly in the Sargasso Sea,
occasionally had to jettison their live cargo as fodder ran out.
With an east-west oriented surface pressure system, the STHP systems
dominate the circulation patterns over the oceans. The location of these systems
varies throughout the year, being closer to the equator during the winter season
and most distant during the summer. The intensities of the STHP systems, as
represented by sea level pressures, are also variable, and may be declining in
some portions while strengthening in others (Jones 1991 ; Inoue and Bigg 1995 ).
They are also asymmetric, with the highest surface pressures in their eastern
portions. It is in these eastern areas that subsidence occurs; in the western
portions rising air is more common and it is here that precipitation is more likely
to occur. This is explained in part by the vertical structure of the STHP systems,
which is illustrated in Figure 3.4 . Cores of maximum high pressure are at the
surface in the east, but at higher elevations on the western sides of the systems.
Figure 3.4 Isobaric
patterns of subtropical high
pressure cells at upper levels
(top) and close to the surface
(center). The lower figure
shows a cross section of
orbital movements. (After
McGregor and Nieuwolt
1998 )
High
W
E
Upper air isobars
High
W
E
Surface isobars
Upper
Surface
W
E
Vertical cross section
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