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Southern hemisphere
Northern hemisphere
wind deflected to the left
wind deflected to the right
Wind direction
Wind strength
Potential wind
Actual wind
Wind movements relative to isobars, with and without Coriolis force (i.e. potential and actual).
Fig. 2.1
24 hours as the Earth rotates. However, a person
standing perfectly still at the equator would always
appear to be facing the same direction and not turning
around. Because of Coriolis force, wind tends to be
deflected to the left in the southern hemisphere and to
the right in the northern hemisphere. Coriolis force can
be expressed mathematically by the following equation:
rotate in opposite directions in each hemisphere. For
example, tropical cyclones cannot cross the equator
because they rotate clockwise in the southern hemi-
sphere and anti-clockwise in the northern hemisphere.
Palmén-Newton model of global
circu lation
Upward movement of air is a prerequisite for vortex
development, with about 10 per cent of all air
movement taking place vertically. As air rises, it spirals
because of Coriolis force, and draws in adjacent air at
the surface. Rising air is unstable if it is warmer than
adjacent air, and instability is favored if latent heat can
be released through condensation. The faster air rises,
the greater the velocity of surface winds spiraling into
the center of the vortex. Of course, in the opposite
manner, descending air spirals outward at the Earth's
surface. These concepts can be combined to account for
air movement in the troposphere . The Palmén-Newton
general air circulation model is one of the more
thorough models in this regard (Figure 2.2). Intense
heating by the sun, at the equator, causes air to rise and
spread out poleward in the upper troposphere. As this
air moves towards the poles, it cools through long wave
Coriolis force = 2
= rate of spin of the Earth
= latitude
= wind speed
Clearly, the stronger the pressure gradient, the
stronger the wind and the more wind will tend to be
deflected. This deflection forms a vortex . Also, the
stronger the wind, the smaller and more intense is
the resulting vortex. Very strong vortices are known as
hurricanes, typhoons, cyclones, and tornadoes. Coriolis
force varies across the surface of the globe because of
latitude and wind velocity. Coriolis force is zero at the
equator (sin 0° = 0), and maximum at the pole
(sin 90° = 1). The equator is a barrier to inter-
hemispheric movement of storms because vortices
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