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FORCES GOVERNING AIR MOVEMENT


Published on: 12/15/2020 5:46:13 AM

The Existence of pressure differentials in the atmosphere is the immediate primary force causing air movement. The pressure gradient force always acts down the pressure gradient, attempting to cause the general movement of air away from high pressure towards low-pressure areas. The force exerted is proportional to the steepness of the gradient. This should mean that winds ought to blow at right angles to the isobars, but in practice wind hardly ever flows solely under the influence of the PGF: in fact, it tends to flow more nearly parallel to the isobars. This is because there are other important controls on air motion.

Coriolis Force

The effect of the rotation of the earth about its axis is to cause an apparent deflection of moving air from its original path. The deflection is always to the right of the direction of motion in the northern hemisphere, and to the left in the southern, whatever the original bearing of the wind. The phenomenon affects all freely moving objects, including air, ocean currents, rockets and projectiles. The degree of the deflecting force varies with the speed of the moving air and with latitude. The faster the wind, the more ground it covers in a given time, and the greater the effect of rotation can be. Near the equator, where the earth's surface is spinning round in a plane almost parallel to the axis of rotation, the coriolis force is very slight. In higher latitudes, however, it has marked effects.

Geostrophic Wind

In reality wind directions adopt a condition of equilibrium or balance between various forces, the most important of which is the strophic balance which exists between the pressure gradient and coriolis force. In the free atmosphere, above the level of air flow affected by surface topography, winds generally blow at right angles to the pressure gradient; this indicates that the pressure gradient force is exactly balanced by the coriolis force acting in a diametrically opposite direction this sort of air motion is k/a the geostrophic wind. A qualitative expression of the geostrophic situation is Buy Ballot's law. Not all winds are geostrophic. As pressure pattern change, the balance is upset, but wind always strives to readjust itself until it obtains the new geostrophic speed.

 

Frictional Forces

In the lowest parts of the atmosphere, normally below 750m, the frictional drag exerted by the ground on the airflow above has an effect on the balance of the other wind forces. Friction lessens the speed of the wind, and in doing so weakens the coriolis force. This allows the pressure gradient to assert its greater strength by causing the air to flow more towards low pressure. Thus, the usual situation is that surface winds flow at a slight angle to the isobars.

Centripetal Force

This force applies when the isobaric pattern is markedly curved, as within cyclonic systems or around high-pressure centres. The fact that air is following a curved path means that in addition to the pressure gradient and the Coriolis force, a third force is acting centripetally, pulling the air inwards. Wind which is in balance with these three forces is k/a the gradient wind.

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