Geoscience Reference
In-Depth Information
Mechanisms of
Climate Variability
Climatic hazards originate with the processes that
move air across the Earth's surface due to differential
heating and cooling. Surprisingly, examination of these
processes has focused upon heating at the tropics and
downplayed the role of cold air masses moving out of
polar regions due to deficits in the radiation balance in
these latter regions. Fluctuations - in the intensity of
pulses of cold air moving out of polar regions or
of heating at the equator - and the location of the
interaction between these cold and warm air masses,
are crucial factors in determining the magnitude, fre-
quency, and location of mid-latitude storm systems.
While most of these factors are dictated by internal
factors in the Earth-atmosphere system, modulation
by 11-year geomagnetic cycles linked to solar activity
and by the 18.6 year M N lunar tide also occurs. This
chapter examines these processes and mechanisms.
The responses in terms of centers of storm activity will
be examined in the following chapter.
(Bryson & Murray, 1977; Lamb, 1982)
How air moves
Barometric pressure represents the weight of air above
a location on the Earth's surface. When the weight of air
over an area is greater than over adjacent areas, it
is termed 'high pressure'. When the weight of air is
lower, it is termed 'low pressure'. Points of equal
pressure across the Earth's surface can be contoured
using isobars , and on weather maps contouring is
generally performed at intervals of 4 hectopascals (hPa)
or millibars (mb). Mean pressure for the Earth is
1013.6 hPa. Wind is generated by air moving from
high to low pressure simply because a pressure gradient
exists owing to the difference in air density (Figure 2.1):
the stronger the pressure gradient, the stronger the
wind. This is represented graphically on weather maps
by relatively closely spaced isobars. In reality, wind does
not flow down pressure gradients but blows almost
parallel to isobars at the surface of the Earth because of
Coriolis force (Figure 2.1). This force exists because
of the rotation of the Earth and is thus illusionary. For
example, a person standing perfectly still at the pole
would appear to an observer viewing the Earth from
the Moon to turn around in a complete circle every
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