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2. Variation of Air Pressure with Altitude

3. Mandatory reporting levels Radiosondes record temperature, relative humidity, and location angles vs pressure during their ascent. From these, dew point, elevation, and wind speed are derived. Significant changes with elevation are reported as well as conditions at certain pressure levels, e.g., 1000, 850, 700, 500, and 300 millibars. Maps can then be made of conditions at these levels. http://virga.sfsu.edu

4. A half psi pressure difference over 500 miles accelerates wind to 80 mph in 3 hours. Increase the distance to 1000 miles and the wind would only be 40 mph in 3 hours.

5. The force exerted by a pressure difference is called the Pressure Gradient Force. It is a vector quantity, with magnitude and direction and is perpendicular to isobars, directed from high pressure to low pressure. The pressure gradient force is inversely proportional to the spacing of isobars (SLP) or height contours on a constant pressure surface ;(e.g., 500 mb or 300 mb maps. Thus the strongest winds will occur in these areas. Interestingly, however, the winds produced are not in the same direction as the pressure gradient force due to other forces.

7. Pressure Gradient Force The pressure gradient force is the ONLY force that can drive the horizontal winds in the atmosphere. The other forces, such as Coriolis, drag, centrifugal, and even advection disappear for zero wind speed. They can only change the direction and speed of an existing wind.