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 geostvophie wind is 

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



U = -^ 4^ (3-30) 



g p^f dn 



a 



where 



Mg = windspeed 



p = air density 



f = coriolis parameter = 2a) sin cj) 



oj = 7.292 X 10"^rad/s 



^ = latitude 



-r^ = horizontal gradient of atmospheric pressure 



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, equilibrium windspeed 

 is called gradient wind. In the Northern Hemisphere, 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 trajectories. If the pressure pattern does not change with 

 time and friction is neglected, trajectories are parallel with the isobars. 

 The isobar curvature 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 and the pressure is changing, this tendency may lead to 

 incorrect adjustments. Corrections to the geostrophic wind that cannot be 

 determined from a single weather map are usually neglected, even though they 

 may be more important than the isobaric curvature effect. 



When forecasting for oceans or other large bodies of water, the most 

 common form of meteorological data used is the synoptic surface weather 

 chart. {Synoptic means that the charts are drawn by analysis of many 

 individual items of meteorological data obtained simultaneously over a wide 

 area.) These charts depict lines of equal atmospheric pressure, called 

 isobars. Wind estimates at sea based on an analysis of the sea level 

 atmospheric pressure are generally more reliable than wind observations 

 because pressure, unlike wind, can be measured accurately on a moving ship. 

 Pressures are recorded in millibars, 1,000 dynes per square centimeter; 1,000 



3-34 



