Perhaps the simplest example of the relationship of wind to the pressure gradient force is a common phenomenon of coasts-the sea breeze and land breeze. During the daytime more rapid heating of the lower air layer over the land than over the ocean causes a pressure gradient from sea to land. Air moving land ward in response to this gradient from higher to lower pressure constitutes the sea breeze. At higher levels a reverse flow sets in. Together with weak rising and sinking air motions, a complete flow circuit is formed. During the night, when radiational cooling of the land is rapid, the lower air becomes colder over land than over the water. Higher pressure now develops over land and the barometric gradient is reversed. Air now moves from land to sea as a land breeze.
The illustration above shows that a pressure gradient can be developed through unequal heating or cooling of a layer of the atmosphere. Air that is warmed also expands and becomes less dense. Air that is cooled contracts and becomes denser. The upward change in barometric pressure is then more rapid within the cooler air layer than within the warmer layer. Heat energy drives the circulation system by changing air densities and setting up barometric pressure gradients. The entire mechanism is often described as a heat engine, since kinetic energy of air motion is derived from the input of heat. This type of air circulation is also important on a large scale in the atmosphere.
The Coriolis force and its effect on winds - If the earth did not rotate upon its axis, winds would follow the direction of pressure gradient. Instead, earth rotation produces another force, the Coriolis force, which tends to turn the flow of air. The direction of action of the Coriolis force is stated in Ferrel's law: Any object or fluid moving horizontally in the northern hemisphere tends to be deflected to the right of its path of motion, regardless of the compass direction of the path.
In the southern hemisphere a similar deflection is toward the left of the path of motion. The Coriolis force is absent at the equator but increases progressively poleward.
Ocean current patterns are greatly affected by it, and streams occasionally will show a tendency to undercut their right-hand banks in the northern hemisphere. Driftwood floating in rivers at high northerly latitudes concentrates along the right-hand edge of the stream. Rifle bullets are slightly deflected over long ranges.
Applying these principles to the relation of winds to pressure the gradient force (acting in the direction of the pressure gradient) and the Coriolis force (acting to the right of the path of flow) quickly reach a balance, or equilibrium, when the wind has been turned to the point that it flows in a direction at right angles to the pressure gradient, that is, parallel with the isobars. The ideal wind in this state of balance with respect to the two forces is termed the geostrophic wind for cases in which the isobars are straight. Where isobars are curved, centrifugal force must also be taken into account, but, in general, airflow at high altitudes parallels the isobars. The rule for the relation of wind to pressure in the northern hemisphere is known as Ballot's law; it states: Stand with your back to the wind and the low pressure will be on your left, the high on your right.
Near the earth's surface, at levels from the ground upward to about 2000 or 3000 ft (600 or 900 in), yet another force modifies the wind direction. This is the force of friction of the air with the ground. It acts in such a way as to counteract in part of the Coriolis force and to prevent the wind from being deflected until parallel with the isobars. Instead, the wind blows obliquely across the isobars, the angle being from 20° to 45°. The angle is large for rugged terrain, small for smooth surfaces, such as water or a flat plain. Wind speed is reduced in proportion to ground friction.
Wind speeds are, in general, proportional to steepness of the pressure gradient. Where the isobars are closely spaced, winds are strong; where isobars are far apart, winds are weak.
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