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Newton’s 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.

Wind Forces

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.

Pressure Gradient

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.

HORIZONTAL PRESSURE GRADIENT.— The horizontal pressure gradient is steep or flat or when the isobars are far apart.

VERTICAL PRESSURE GRADIENT.— If isobars are considered as depicting atmospheric topography, a high pressure system represents a hill of air, and a low pressure system represents a depression or valley of air. The vertical pressure gradient always indicates a decrease in pressure with altitude, but the rate of pressure decrease (gradient) varies directly with changes in air

Figure 3-1-7.—Horizontal pressure gradient.

density with altitude. Below 10,000 feet altitude, pressure decreases approximately 1 inch of mer-cury per 1,000 feet in the standard atmosphere. The vertical cross section through a high and low (view A in fig. 3-1-8) depicts the vertical pressure gradient. A surface weather map view of the horizontal pressure gradient in the same high and low is illustrated in view B of the figure 3-1-8.

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