Newtons first two laws of motion indicate that motion tends to be in straight lines and only deviates from such lines when acted upon by another force or by a combination of forces. Air tends to move in a straight line from a high-- pressure area to a low-pressure area. However, there are forces that prevent the air from moving in a straight line.

There are four basic forces that affect the directional movement of air in our atmosphere: pressure gradient force (PGF), the Coriolis effect, centrifugal force, and frictional force. These forces, working together, affect air movement. The direction that the air moves is deter-mined by the forces that are affecting it at that particular time. Also, the different names given to the movement of the air (geostrophic wind, gra-dient wind, etc.) depends on what forces are af-fecting it.

The rate of change in pressure in a direction perpendicular to the isobars is called pressure gradient. Pressure applied to a fluid is exerted equally in all directions throughout the fluid; e.g., if a pressure of 1013.2 millibars is exerted downward by the atmosphere at the surface, this same pressure is also exerted horizontally outward at the surface. Therefore, a pressure gradient exists in the horizontal (along the surface) as well as the vertical plane (with altitude) in the atmosphere.