Horizontal pressure gradients arise in the atmosphere primarily 

 because of density differences, which in turn are generated primarily by 

 temperature differences. Wind results from nature's efforts to eliminate 

 the pressure gradients, but is modified by many other factors. 



The pressure gradient is nearly always in approximate equilibrium with 

 the acceleration produced by the rotation of the earth. The geostrophia 

 wind is defined by assuming that exact equilibrium exists, and is given by 



1 dp 

 U = -T / . (3-19) 



* pt an 



where U^ is the wind speed, p^ the density of the air, f the coriolis 

 parameter, f = 2w sine}), where to = 7.292 x 10"^ radians/second and (f) 

 is the latitude, and dp/dn is the horizontal gradient of atmospheric 

 pressure. A graphic solution of this equation is given in Figure 3-11, 

 Section 3.41, Estimating the Wind Characteristics. The geostrophic wind 

 blows parallel to the isobars with low pressure to the left, when looking 

 in the direction toward which the wind is blowing, in the Northern 

 Hemisphere, and low pressure to the right in the Southern Hemisphere. 

 Geostrophic wind is usually the best simple estimate of the true wind in 

 the free atmosphere. 



When the trajectories of air particles are curved, equilibriiim wind 

 speed is called gradient wind. Gradient wind is stronger than geostrophic 

 wind for flow around a high pressure area, and weaker than geostrophic wind 

 for flow around low pressure. The magnitude of the difference between 

 geostrophic and gradient winds is determined by the curvature of the tra- 

 jectories. If the pressure pattern does not change with time and friction 

 is neglected, trajectories are parallel with the isobars. The isobar curva- 

 ture can be measured from a single weather map, but at least two maps must 

 be used to estimate trajectory curvature. There is a tendency by some 

 analysts to equate the isobars and trajectories at all times, and to 

 compute the gradient wind correction from the isobar curvature. When the 

 curvature is small, but the pressure is changing, this tendency may lead 

 to incorrect adjustments. Corrections to the geostrophic wind that can- 

 not be determined from a single weather map are usually neglected, even 

 though they may be more important than the isobaric curvature effect. 



The equilibrium state is further disturbed near the surface of the 

 earth by friction. Friction causes the wind to cross the isobars toward 

 low pressure at a speed lower than the wind speed in the free air. Over 

 water, the average surface wind speed is generally about 60 to 75 percent 

 of the free air value, and wind crosses the isobars at an angle of 10 to 

 20 degrees. In individual situations, the magnitude of the ratio between 

 the surface wind speed and the computed free air speed may vary from 20 to 

 more than 100 percent, and the crossing angle may vary from 0° to more than 

 90°. The magnitude of these changes is determined by the vertical tempera- 

 ture profile and the turbulent viscosity in the atmosphere. 



3-21 



