Global Positioning System Reference
In-Depth Information
pressure repeats, resulting in the
polar front
at 60\ latitude—regions of low
pressure—and
polar caps
, where the pressure is high. These zonal patterns
are displaced north and south seasonally, because of the changing amount
of solar heat that reaches a given area. Such seasonal winds include the
monsoon
of India and Southeast Asia.
Cyclones
are seasonal; they are cir-
culating areas of low pressure that give rise to
hurricanes
in the North
Atlantic (fig. 1.15). The same phenomena are called
typhoons
in the west-
ern Pacific. The
roaring forties
are regions between 40\ and 50\ south, with
strong prevailing westerlies that are unimpeded by land masses.
Ocean currents are driven by solar heating, winds, gravity, and the
ubiquitous Coriolis force. Water in equatorial regions is heated and so
expands, raising the sea level some 8 cm over vast areas of ocean. The water
falls down this slope, giving rise to currents that are modified by winds
and land masses. A map of the major ocean surface currents is shown in
figure 1.16.
Winds and ocean currents have influenced navigation in two ways. First,
they drove sailing ships before them, whether or not the pilots wished it;
and secondly, they exacerbated the problem of estimating the speed of a
sailing ship and so made dead reckoning more di≈cult. Dead reckoning is
discussed in chapter 6, and so I will say no more about it here.
