Pressure Gradient Force
The variation of heating (and consequently the variations of pressure) from one locality to another is the initial factor that produces move-ment of air or wind. The most direct path from high to low pressure is the path along which the pressure is changing most rapidly. The rate of change is called the pressure gradient. Pressure gradient force is the force that moves air from an area of high pressure to an area of low pressure. The velocity of the wind depends upon the pressure gradient. If the pressure gradient is strong, the wind speed is high. If the pressure gra-dient is weak, the wind speed is light. (See fig. 3-1-7.)
Figure 3-1-9.—Examples of circulation about high and low
Figure 3-1-9.—Examples of circulation about high and lowpressure systems.
Figure 3-1-9 shows that the flow of air is from the area of, high pressure to the area of low pressure, but it does not flow straight across the isobars. Instead the flow is circular around the pressure systems. Pressure gradient force (PGF) causes the air to begin moving from the high-- pressure to the low-pressure system. Coriolis (deflective) force and centrifugal force then begin acting on the flow in varying degrees. In this ex-ample, frictional force is not a factor.
If pressure gradient force were the only force affecting windflow, the wind would blow at right angles across isobars (lines connecting points of equal barometric pressure) from high to low pressure. The wind actually blows parallel to isobars above any frictional level. Therefore, other factors must be affecting the windflow; one of these factors is the rotation of Earth. A particle at rest on Earth’s surface is in equilibrium. If the particle starts to move because of a pressure gradient force, its relative motion is affected by the rotation of Earth. If a mass of air from the equator moves northward, it is deflected to the right, so that a south wind tends to become a southwesterly wind.
In the Northern Hemisphere, the result of the Coriolis effect is that moving air is deflected to the right of its path of motion. This deflection to the right is directly proportional to the speed of the wind; the faster the wind speed, the greater the deflection to the right, and conversely, the slower the wind speed, the less the deflection to the right. Finally, this apparent deflective force is stronger at the polar regions than at the equator.
According to Newton’s first law of motion,a body in motion continues in the same direction in a straight line and with the same speed unless acted upon by some external force. Therefore, for a body to move in a curved path, some force must be continually applied. The force restraining bodies that move in a curved path is called the centripetal force; it is always directed toward the center of rotation. When a rock is whirled around on a string, the centripetal force is afforded by the tension of the string.
Newton’s third law states that for every ac-tion there is an equal and opposite reaction.Centrifugal force is the reacting force that is equal to and opposite in direction to the centripetal force. Centrifugal force, then, is a force directed outward from the center of rotation.
As you know, a bucket of water can be swung over your head at a rate of speed that allows the water to remain in the bucket. This is an exam-ple of both centrifugal and centripetal force. The water is held in the bucket by centrifugal force tending to pull it outward. The centripetal force, the force holding the bucket and water to the center, is your arm swinging the bucket. As soon as you cease swinging the bucket, the forces cease and the water falls out of the bucket. Figure 3-1-10 is a simplified illustration of centripetal and cen-trifugal force.
Figure 3-1-10.—Simplified illustration of centripetal and centrifugal force.
High- and low-pressure systems can be compared to rotating discs. Centrifugal effect tends to fling air out from the center of rotation of these systems. This force is directly propor-tional to the wind speed; the faster the wind, the stronger the outward force. Therefore, when winds tend to blow in a circular path, centrifugal effect (in addition to pressure gradient and Coriolis effects) influences these winds.
The actual drag or slowing of air particles in contact with a solid surface is called friction. Friction tends to retard air movement. Since Coriolis force varies with the speed of the wind, a reduction in the wind speed by friction means a reduction of the Coriolis force. This results in a momentary disruption of the balance. When the new balance (including friction) is reached, the air flows at an angle across the isobars from high pressure to low pressure. (Pressure gradient force is the dominant force at the surface.) This angle varies from 10 degrees over the ocean to more than 45 degrees over rugged terrain. Frictional effects on the air are greatest near the ground, but the effects are also carried aloft by turbulence. Surface friction is effective in slowing the wind to an average altitude of 2,000 feet (about 600 meters) above the ground. Above this level, called the gradient wind level or the second standard level, the effect of friction decreases rapidly and may be considered negligible. Air above 2,000 feet normally flows parallel to the isobars.