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. 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]  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] 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  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  Wind Patterns Surface Friction Figure 4-11.  Principles of pressure gradients. 1020 1016 1012 I s o b a r s • The wider the pressure gradient,  the weaker the wind. Weak or Flat Pressure Gradient • The closer the spacing of isobars,  the stronger the pressure gradient. • The stronger the pressure gradient,  the stronger the wind. Strong or Steep Pressure Gradient 1020 1016 HIGH LOW Surfac e  W in d Gradient Wind 1,000 Fe e t Gradient  W $_{ind }$2,000 Fe e t Gradient Wind 3,000 Feet Figure 4-12.  Examples of variations of wind direction with height. 4-12