Environmental Engineering Reference
In-Depth Information
curvature of the isobars is included. C.f., Coriolis force;
p.g.f., pressure gradient force.
be deflected towards the right in the northern hemisphere (Figure 6.9a). As the wind
accelerates, its speed will increase and, because the Coriolis force is related to speed
(2ω
V
sin ), 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 to 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.
Although we have considered only two of the forces acting upon the air parcel, the
geostrophic wind is nevertheless a useful approximation. Strictly, it operates only when
the isobars are straight - a rare event. Normally isobars are curved and winds are subject
to another force termed
centripetal acceleration
which acts towards the centre of rotation.
When this rotational component is included, the resultant wind is called the
gradient
wind
, which is closer to observed flow in the upper atmosphere (Figure 6.9b).
FRICTION
Inspection of a surface weather map will show that, at ground level, the wind does not
blow parallel to the isobars. It blows across the isobars towards the area of lower
pressure. The more observant may notice that this angle between the wind flow and the
isobars is greater over land areas than over oceans. This may give a clue to the reasons
for the change. Land surfaces are rougher than
Figure 6.10
The effect of friction on the geostrophic wind.
The Coriolis force is always at right-angles to the actual
wind. It is smaller than the pressure gradient force because
friction has reduced the speed of the wind.
seas; they tend to slow the wind down through friction more effectively. Friction acts as a
force pulling against the direction of flow. We can now rearrange our 'balance of forces'
to include friction. To achieve balance, the flow will be across the isobars because the
Coriolis pull to the right decreases as the air velocity falls (Figure 6.10). From these
