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a direction perpendicular, or across the isobars. Wind speed is directly proportional to the pressure gradient. This means the strongest winds are in the areas where the pressure gradient is the greatest. Since pressure applied to a fluid is exerted equally in all directions throughout the fluid, a pressure gradient exists in the horizontal (along the surface), as well as in the vertical (with altitude) plane in the atmosphere. [Figure 4-11] 

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The horizontal pressure gradient is steep or strong when the isobars determining the pressure gradient are close together. It is flat or weak when the isobars are far apart. If isobars are considered as depicting atmospheric topography, a high pressure system represents a hill of air, and a low pressure system represents a valley of air. The vertical pressure gradient always indicates a decrease in pressure with altitude, but the rate of pressure decrease (gradient) varies directly 

with changes in air density with altitude. The vertical cross section through a high and a low depicts the surface pressure gradient. 

The pressure gradient force is a force that tries to equalize pressure differences. This is the force that causes high pressure to push air toward low pressure. Thus, air would flow from high to low pressure if the pressure gradient force was the only force acting on it. 

## Surface Friction 

Friction is the third component that determines the flow of wind. Because the surface of the Earth is rough, it not only slows the wind down, it also causes the diverging winds from highs and converging winds near lows. Since the 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. This angle varies from 10° over the ocean to more than 45° 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 up to an average altitude of 2,000 feet above the ground. Above this level, the effect of friction decreases rapidly and may be considered negligible. Air above 2,000 feet above the ground normally flows parallel to the isobars. [Figures 4-12 and  4-13] 

## Wind Patterns 

Since air always seeks out lower pressure, it flows from areas of high pressure into those of low pressure. In the Northern Hemisphere, this flow of air from areas of high to low pressure is deflected to the right and produces a clockwise circulation around an area of high pressure known as anti- cyclonic circulation. The opposite is true of low pressure 

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