Environmental Engineering Reference
In-Depth Information
If this rate is kept up for one hour (3,600 seconds) we would have a value of 4·65 m s
−1
after one hour. As pressure gradients of this size can last for days, we might expect very
high wind speeds to develop unless other forces interfered. There are two main forces
which prevent this happening. One is friction and the other is Earth's rotation.
If we look at the wind field on a weather map, it will be immediately apparent that air
does not flow down the pressure gradient towards areas of low pressure. If it did, the low-
pressure areas would fill and the wind movement would stop. Instead we find that the
wind is blowing parallel (or almost) to the isobars rather than across them. This is due to
the effect of Earth's rotation.
CORIOLIS FORCE
Although we are not aware of it, Earth is rotating from west to east at 15° longitude per
hour. Reference back to Newton's laws shows that if we have a parcel of air moving
southwards and there are no forces acting upon it, it will continue to move in the same
absolute direction (i.e in a straight line as viewed from space). However, Earth is
gradually turning, and so, relative to the ground surface, the parcel will appear to follow a
curved track towards the right in the northern hemisphere and to the left in the southern
hemisphere (Figure 6.7). To explain this apparent deflection in Newtonian terms, we
have to introduce a force to account for the movement as observed from the ground. This
force is called the Coriolis force, 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 - 2ω
V
sin (where ω is the rate
at which Earth rotates,
V
is air velocity and is the angle of latitude). This term (2ω
V
sin
) 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
Figure 6.7
The effect of Earth's rotation on air movement.
