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Text 2. Synoptic winds
Synoptic winds are winds associated with large-scale events such as warm and cold
fronts, and are part of what makes up everyday weather. These include the geostrophic
wind, the gradient wind, and the cyclostrophic wind.
As a result of the Coriolis force, winds always flow clockwise around a high pressure
area and counterclockwise around a low pressure area (the reverse in the southern
hemisphere). At the same time, winds always flow from areas of high pressure to areas of
low pressure. These two forces are opposite but not equal, and the path that results when the
two forces cancel each other out runs parallel to the isobars. Wind following this path is
known as geostrophic wind. It is rare, however, to find things quite so tidy. Winds are said
to be truly geostrophic only when other forces (e.g. friction) acting on the air are negligible,
a situation which is often a good approximation to the large-scale flow away from the
tropics.
In nature, isobars are almost always curved. The result is that a wind moving parallel to
the isobars encounters a third force, the centripetal force. This is the force which tends to
keep a body in motion moving in the same direction. The effect of this force, though not a
force in itself, is called the centrifugal force, and acts to counteract the Coriolis force
(coincidentally also the effect of a force rather than a force in itself) and decrease the wind
speed. This much more common situation results in what is known as a gradient wind.
In certain circumstances, the Coriolis force acting on moving air may be almost or
entirely overwhelmed by the centripetal force. Such a wind is said to be cyclostrophic, and
is characterized by rapid rotation over a relatively small area. Hurricanes, tornadoes, and
typhoons are examples of this type of wind.
Special winds are winds which blow under only certain circumstances. These may
result from differential heating, from barrie rs to airflow, or fro m gravitational effects.
Differential heating is the motive force behind land breezes and sea breezes (or, in the
case of larger bodies, lake breezes), also known as on- or off-shore winds. Water is a rapid
abs orber/radiator of heat, whereas land not only absorbs heat more s lowly but releases it
over a greater period of time. The result is that, in locations where sea and land meet, heat
absorbed over the day will be released more quickly by the water. Air contacting water
cools. Over the land, heat is s till being releas ed into the air, which rises . This convective
motion draws the cool sea air in to replace the rising air, resulting in a sea breeze. During
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