Geoscience Reference
In-Depth Information
6000 m
14
10
F = - -—- --------
= 0·00103 m s
-1
Low
Low
1·29 300
100
If this rate is kept up for one hour (3,600 seconds) we
would have a value of 3·72 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.
3000 m
Low
High
Surface
Warm Cold Warm
Warm Cold Warm
6000 m
High
High
3000 m
Low
High
Surface
Cold Warm Cold
Cold Warm Cold
pressure surfaces.
Coriolis force
992 hPa
996 hPa
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
,
1000 hPa
1004 hPa
Horizontal pressure
gradient force
Isobars -
lines of equal pressure
angles to the isobars.
so the speed of the wind is inversely proportional to the
distance between the isobars. Thus the closer the isobars
are together, and the more rapidly pressure falls with
distance, the stronger is the wind.
Mathematically, this relationship can be written as:
Earth's
rotation
Path relative to
Earth's surface
1
p
2
-
p
1
F = - -- --------
Apparent movement
viewed from space
n
where pressure values at points 2 and 1 are
p
2
and
p
1
,
n
is
the distance separating points 2 and 1;
is air density and
F
is the resulting acceleration. We can use this formula to
indicate how quickly the parcel ought to accelerate. The
standard isobaric interval on pressure charts is 4 hPa and
air density is 1·29 kg m
-3
. Suppose the isobars are 300 km
apart on a sea-level chart. What will be the acceleration
down the pressure gradient? In uniform units, the formula
will become:
