In the
last post I mentioned the Coriolis Force and its role in adding ‘spin’ to
weather. There are also several other forces that shape the weather, some more
important than others. Perhaps the most important force, maybe even more
important than the Coriolis, is the pressure gradient force (I’ll be calling it
PGF). This force results from the differences in pressure from one place to
another. It is somewhat analogous to water in hilly terrain; the tops of hills
are like high pressure centers and water flows down them to the valleys and
lakes below, which are like troughs and low pressure centers, respectively.
Thus, air is forced toward areas of low pressure and away from high pressure.
One important detail about PGF is that unlike the Coriolis Force, it can affect
the speed of air, since in the absence of all other forces air would accelerate
toward low pressure centers. When these two forces become balanced the air flow
is said to be in Geostrophic Balance. The best way to describe what this
condition means and how it comes about is with a series of diagrams.
The simple
hypothetical initial state is some level in the mid to upper troposphere where
pressure contours run right-left (east-west) with low pressure at the top
(north) and high pressure at the bottom (south). These diagrams are assumed to
be in the northern hemisphere, as are all my other discussions and diagrams
unless otherwise specified.
Now, a
small piece of air (typically called an air parcel, here represented by a black
dot) is placed in this pressure field. It is being affected by the PGF
(symbolized by the arrow with PGF next to it) which is exerting a force toward
the low pressure in the north. Note that since the parcel has yet to begin
moving, it is unaffected by the Coriolis Force.
Now, the
parcel has begun to accelerate thanks to the PGF and has some forward speed
(symbolized by the arrow with a V next to it). Since it is now moving, the
Coriolis Force (symbolized by the arrow with a C next to it) will begin
deflecting it to the right (east), even though the force is minimal since the
parcel has not gained much speed. It is important to note that the Coriolis
Force always exerts a force 90 degrees to the right of the direction of motion.
Here, as
the parcel continues to pick up speed it has been deflected to the right. Note
that the angle between the PGF and the direction of the parcel’s motion has
increased. Therefore, the amount of the PGF that is affecting the speed of the
parcel has decreased.
The
parcel is now highly deflected to the right due to the Coriolis Force and the
amount of the PGF that influences the parcel’s speed is very small.
Finally,
the parcel is at a right angle to the PGF, so it no longer causes the parcel to
accelerate. Now, all of the force is now being exerted to parcel’s left
(north), and the Coriolis Force is exerting a force on the parcel to its right
(south). At this point, the forces are in geostrophic balance. Therefore, in
the absence of other forces, the parcel will continue to move to the east
parallel to the pressure lines, and at a constant speed.
This
balance explains why air travels clockwise around a high and counter-clockwise
around a low, since those are really like the diagrams above, just curled up.
Geostrophic balance is a great way to estimate roughly the direction of wind
based solely on pressure. Of course, I use to term ‘roughly’ since there are
many other forces on air flow, especially near the ground where one must
consider the effects of terrain and friction with the ground. Regardless, the
concept of Geostrophic Balance is a great rule of thumb when reading weather
maps, whether at the surface or upper levels, just check out the image below.
This IR satellite image has height contours and wind barbs from 500mb, the flow
up there was clearly close to being geostrophically balanced.
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