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.
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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!).
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Friction
- Friction- the drag on the
air by the earth's surface (e.g., plants, trees, buildings,
mountains, etc.).
Friction always acts opposite
to air motion and, hence, reduces wind speed. Its greatest
effect is near the earth's surface and rapidly decreases
with height (within lowest 1 km).
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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? Because
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.
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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).
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