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Pressure Gradients

The atmosphere is composed of a mixture of gases. Air has weight which is directed towards the centre of the earth. The force (weight) of air per unit area acting on object on the earth is called the atmospheric pressure (Diane, 11). The atmospheric pressure is measured by an instrument called a barometer. On a map, regions of equal pressure are linked by lines called isobars. These lines are drawn in such a way that they never cross each other. The drawing of these lines is based on the average sea level pressure reports. It is true that majority of the weather stations are situated some heath above sea level.

Therefore the pressure measured at a particular place has to be converted into sea level pressure before the isobars are drawn. This is due to the fact that pressure decreases with increase in height and the pressures on maps should be as a result of weather conditions and not due to changes in height(Integrated Publishing). According to Diane, Pressure differences occur due to differences in the heat absorbed by the land or water. When air is heated, it expands reducing its density and rises. A partial vacuum is therefore created. Cold denser air flows to take up the space left by the rising air.

Basically wind flows from a region of high pressure to a region of low pressure. Therefore, as the pressure changes from place to place, the speed of wind also changes. Diane defines pressure gradient as the rate of change of pressure at a particular time in a particular place. The force that pushes air from a region of high pressures to the region of low pressure is called the pressure gradient force. The force is greater when the pressure gradient is high. Eady notes that the speed of wind depends on the pressure gradient, being higher with greater pressure gradient. It could be expected that the wind will flow perpendicular to the isobars.

However the pressure gradient force is somewhat circular as it depends not only on pressure gradient but also on Coriolis Effect, centrifugal force and frictional force (Eady, 142). Coriolis force is brought about as a result of the rotation of the earth. All particles at the surface of the earth rotate once every day. Since the radious is greater at the equator than towards the poles, an object rotates faster at the equator than at higher latitudes. If air moves northwards due to pressure gradient force, it maintains it horizontal motion; in accordance to Newton’s first law of motion (Ferrel, 98).

The air therefore moves in a North Easterly direction. In actual sense, wind flows in a circular path (integrated Publishing). For a body to maintain a circular path, centripetal force must be provided. In accordance to Newton’s third law of motion an equal and opposites force called centrifugal force acts on the body (Eady, 139). This force tends to push winds out of their paths and it increases with speed of wind. As wind flows over objects over the earth’s surface it experiences fractional drag. This drag results to wind flowing at an angle to the isobars near the earth’s surface.

At higher altitudes air flows parallel to the isobars (Joseph). Diane Fisher contends that in some cases, air cools at a particular place in the troposphere. The cooling results into contraction of air which in turn leads to increase in mass per unit volume of air. This leads to increased pressure directed downwards. This air then attempts to flow outwards towards regions of low pressure. But due to Coriolis force, the air is made to rotate clockwise in the northern hemisphere and anticlockwise in the southern hemisphere. This is referred to as an anticyclone.

On the other hand, Diane tells us that a region of rising air of low pressure is called a cyclone. This occurs at the boundary of cold and warm air. It is caused by warm air rising and cools air descending which creates of reduced pressure. An inward pressure gradient is created and in addition to Coriolis Effect causes air to circulate in anticlockwise direction in the Northern Hemisphere and Clockwise in the Southern Hemisphere. The cyclone usually moves upwards and it contributes to the circulation of air in the atmosphere. Works Cited Diane Fisher.

“Become a weather wizard”. In The Technology Teacher, Vol. 61, 2002:11 – 15. Eady, E. T. “The general circulation of the atmosphere and oceans. ” In: The Earth and Its Atmosphere, ed. D. R. Bates. New York, Basic Books, (1957):130-151. Ferrel, W. The influence of the Earth’s rotation upon the relative motion of bodies near its surface. Astron. J. , Vol V. No. 109, (1858):97-100. Integrated Publishing . <http://www. tpub. com/content/aerographer/14312/css/14312_16. htm> Joseph E Van Riper. Man’s Physical World. New York: McGraw-Hill Book Company, Inc. 1962.

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