Atmospheric Forces
|
|
|
|
Principle
|
Wind speed and direction respond to
pressure gradient forces that exist between high and low
pressure areas. In the Northern Hemisphere and because of
the rotation of the earth, winds circulate in a clockwise
fashion around areas of high pressure and in a
counter-clockwise manner around regions of lower pressure.
Air pressure decreases relatively slowly with height in
regions dominated by warm air and relatively rapidly with
height in areas where cold air prevails. As a result, wind
patterns in the upper atmosphere tend to flow in an
oscillating manner around major pockets of warm and cold
air.
|
|
|
|
Forces That Create or Act Upon the Wind
|
- Wind results from physical
forces that act on the air.
A force is an influence on a
body which causes the body to accelerate (change speed or
direction). Newton's First Law of Motion states that a
body at rest will remain at rest, and a body in motion
will remain in motion unless acted upon by an unbalanced
force. If forces balance (no net force), then we have
either no motion or uniform motion in a straight
line.
- Differences in air pressure
(called a pressure gradient) lead to air motion.
Air "parcels" will try to move
from areas of high pressure to areas of low pressure. In
addition, colder temperatures near the poles generally
are associated with higher pressures than warmer
temperatures near the equator. Thus, unequal solar
heating of the earth directly causes large-scale winds,
called the jet stream.
- The larger the difference in
air pressure, the stronger the winds.
Newton's 2nd Law of Motion
states that the acceleration (rate of change of velocity)
of a body is directly proportional to the net force upon
the body, or F = ma, where F = force, m = mass, and a =
acceleration.
- The primary forces that cause
large-scale motion in the atmosphere are as
follows:
- Gravitational force- keeps
the molecules in the atmosphere from moving into
space. Gravity's influence is stronger near the
earth's surface and weaker aloft.
- Vertical pressure gradient
force- closely balances gravity so that all the
molecules in the atmosphere are not forced into the
lowest meter above the ground. The vertical pressure
gradient force results from molecules in the high
pressure near the earth's surface trying to move
upward where the pressure is lower.
- Horizontal pressure gradient
force- results from the high and low pressure systems
(highs, lows, troughs and ridges) in the atmosphere.
Air will tend to move from high pressure to low
pressure.
- Coriolis force- the force
that results from Earth's rotation.
- Friction- the drag exerted
on the air by the earth's surface (e.g., plants,
trees, buildings, mountains, etc.).
- Centrifugal force- the
tendency for a body to resist a change in
direction.
|
|
|
|
Pressure Gradient Force
|
- Horizontal pressure gradient
force- results from the high and low pressure systems
(highs, lows, troughs and ridges) in the atmosphere. Air
tends to move air from regions of high pressure to
regions of low pressure.
"Gradient" refers to how rapidly
a quantity (such as pressure or temperature) changes in a
given distance. It can be thought of as measure of
"steepness", like the topography on a contour
plot.
- The larger the gradient, the
stronger the wind.
Strong winds are found in areas
of tightly packed isobars. In general, the closer the
isobars are to one another on a weather map, the greater
is the pressure gradient force (be careful to look at the
intervals!).
|
|
|
|
Friction
|
|
|
|
|
Coriolis Force
|
- Coriolis force- the force that
results from Earth's rotation.
The Coriolis force solely
results from living on a rotating object -- Earth. It
acts only on objects moving with respect to the earth's
surface (e.g., the air, planes, birds, missiles, etc.).
It is only significant over long distances (e.g.,
hundreds or thousands of miles) and long time spans
(e.g., 12 hours or longer). Hence, tornadoes are not
influenced by the Coriolis force. Neither is the water
draining in your sink.
Example of the Coriolis force:
Suppose Mark and Jane are on a merry-go-round rotating
counterclockwise (when viewed from above). Mark throws a
ball directly to Jane. Mark misses. Why? But Jane rotated
away from the straight-line path of the ball while the
ball was in the air. However, to Mark, it looks like the
ball curved to the right.
Think of the earth as the
merry-go-round when looked at from above the North Pole,
or below the South Pole. From those vantage points, Earth
is rotating counterclockwise in the Northern Hemisphere
and clockwise in the Southern Hemisphere. To someone on
the earth, air blowing in a straight line seems to blow
to the right in the Northern Hemisphere and to the left
in the Southern Hemisphere.
- The Coriolis force always will
deflect objects to the right in the Northern Hemisphere
and to the left in the Southern Hemisphere.
- The Coriolis force will never
change the speed of an object, only its direction.
- Air currents, which exist as a
response to pressure forces, are "deflected" by the
rotation of the earth.
If Earth did not rotate, air
currents would flow directly from areas of high pressure
to areas of low pressure. However, because Earth does
rotate, air currents ultimately become involved in a
tug-of-war between pressure gradient forces and the
Coriolis force. The result is that air circulates counterclockwise around
areas of low pressure in the Northern Hemisphere and
clockwise around areas of high pressure.
|
|
|
|
Centrifugal Force
|
- Centrifugal force- the tendency
for a body to resist a change in direction.
If we have a ball tied to a
string and throw it around in a circle at a constant
speed, we feel a "force" pulling the ball outward. This
is the centrifugal force. It is the same force that makes
you lean to the right when a car makes a left turn, or
the force that you feel riding a roller coaster.
- The centrifugal force will try
to pull air parcels outward if they are moving in a
curved path around a ridge or a trough.
- The magnitude of the
centrifugal force is related to both the speed of the air
parcel and the radius of curvature (how tightly it goes
around the curve).
|
|
|
|
End
|
|
