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LESSON 4
Pressure Gradient ForceSpray paint cans exhibit warning labels that the contents are under pressure. When the nozzle is squeezed, or the sides punctured, the large pressure difference between the air and the inside of the can forces the contents out of the can. Pressure differences exert a force and when not balanced by other forces cause movement. The force that results from pressure differences in a fluid such as our atmosphere is called the pressure force or the pressure gradient force. Air moves because of a pressure gradient force. The existence of a pressure gradient force is essential for sustaining winds. The pressure gradient force (PG) always acts from high pressure towards low pressure. Its magnitude is equal to the pressure gradient, or the rate of change in pressure (D P) with distance (D) at a specific time divided by the air density (r ).
In the above formula, density is expressed in units of kilograms per cubic meter, distance is measured in meters, and pressure is expressed in pascals or newtons per square meter. To convert from millibars to pascals, multiple the number of millibars by 100 (i.e., 500 millibars equals 50,000 pascals). When pressure changes rapidly over a small distance, the pressure gradient force is large. Strong winds result from large pressure gradients. Return to Lesson 4 Page. Return to Weather For Pilots Main Page
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