Geoscience Reference
In-Depth Information
after the French mathematician who formalized the
concept. The value of the Coriolis force changes with the
angle of latitude and the speed of the air; mathematically
for a unit mass of air it equals - 2
in Figure 6.8 . As one proceeds towards the equator, so
Earth's surface eventually becomes parallel to the axis of
rotation. Its effect can be demonstrated by pendulum
experiments. Using the Foucault pendulum, which por-
trays free motion in space as closely as can be achieved at
Earth's surface, a disc will rotate under the freely swinging
pendulum in one day at the poles. At latitude 30
is
the rate at which Earth rotates, V is air velocity and is
the angle of latitude). This term (2
V sin
(where
) is often
referred to as the Coriolis parameter. It is greatest at the
poles, where Earth's surface is at right-angles to the axis
of rotation, but it gets progressively less towards the
equator, where it reaches zero. The reason for this is shown
V sin
it will
take two days to rotate (sin 30
= 0·5) and at the equator
it does not turn at all.
Geostrophic wind
Let us now return to our parcel of air experiencing a
pressure gradient force on the rotating Earth. Initially the
parcel will move down the pressure gradient, but as soon
as it begins to move it will start to be affected by the
Coriolis force, which pulls at 90
N Pole
100%
87%
60°N
to the flow, so that it will
be deflected towards the right in the northern hemisphere
and towards the left in the southern hemisphere ( Figure
6.9a ). As the wind accelerates, its speed will increase and,
because the Coriolis force is related to speed (2
Deflection to
right
50%
30°N
0
Equator
No deflection
),
the two forces pulling together eventually produce an
equilibrium flow. This will occur when the two forces are
equal and opposite, the resultant wind blowing parallel
with the isobars; it is known as the geostrophic wind .Its
velocity will be determined primarily by the pressure
gradient, though, because the value of the Coriolis force
varies with latitude, the geostrophic wind for the same
pressure gradient will decrease towards the poles.
V sin
Deflection to
left
30°S
60°S
S Pole
Figure 6.8 The changing magnitude of the Coriolis force
with latitude.
(a)
Northern hemisphere
Southern hemisphere
992 hPa
1004 hPa
p.g.f.
p.g.f.
Equilibrium flow or
geostrophic wind
p.g.f.
1000 hPa
996 hPa
C.f.
C.f.
996 hPa
1000 hPa
p.g.f.
p.g.f.
C.f. = 0 if V = 0
1004 hPa
992 hPa
(b)
Low
High
p.g.f.
Gradient
wind
C.f.
Gradient
wind
Centripetal
acceleration
Centripetal
acceleration
C.f.
p.g.f.
Figure 6.9 Balance of forces for a westerly geostrophic wind (a) when isobars are straight and (b) for the gradient wind when
curvature of the isobars is included. C.f.Coriolis force; p.g.f.pressure gradient force.
 
 
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