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Since there is a direct relationship between pressure gradient and wind speed and direction, we have a variety of wind types to deal with. We discuss below the relationship of winds and circulations, the forces involved, and the effect of these factors on the general circulation.

Geostrophic and Gradient Wind

On analyzed surface weather charts, points of equal pressure are connected by drawn lines referred to as isobars, while in upper air analysis, points of equal heights are connected and called isoheights.

The variation of these heights and pressures from one locality to another is the initial factor that produces movement of air, or wind. Assume that at three stations the pressure is lower at each successive point. This means that there is a horizontal pressure gradient (a decrease in pressure in this case) for each unit distance. With this situation, the air moves from the area of greater pressure to the area of lesser pressure. If the force of the pressure were the only factor acting on the wind, the wind would flow from high to low pressure, perpendicular to the isobars. Since experience shows the wind does not flow perpendicular to isobars, but at a slight angle across them and towards the lower pressure, it is evident that other factors are involved. These other factors are the Coriolis effect, frictional force, and centrifugal effect. When a unit of air moves with no frictional force involved, the movement of air is parallel to the isobars. This wind is called a gradient wind. When the isobars are straight, so only Coriolis and pressure gradient forces are involved, it is termed a geostrophic wind.

Letís consider a parcel of air from the time it begins to move until it develops into a geostrophic wind.

As soon as a parcel of air starts to move due to the pressure gradient force, the Coriolis force begins to deflect it from the direction of the pressure gradient force. (See views A and B of fig. 3-1-11). The Coriolis force is the apparent force exerted upon the parcel of air due to the rotation of Earth. This force acts to the right of the path of motion of the parcel of air in the Northern Hemisphere (to the left in the Southern Hemisphere). It always acts at right angles to the direction of motion. In the absence of friction, the Coriolis force changes the direction of motion of the parcel until the Coriolis force and the pressure gradient force are in balance. When the two forces are equal and opposite, the wind blows parallel to the straight isobars (view C in fig. 3-1-11). The Coriolis force only affects the direction, not the speed of the motion of the air. Normally, Coriolis force is not greater than the pressure gradient force. In the case of super-gradient winds, Coriolis force maybe greater than the pressure gradient force. This causes the wind to deflect more to the right in the Northern Hemisphere, or toward higher pressure.

Under actual conditions, air moves around high and low pressure centers toward lower pressure. Turn back to figure 3-1-9. Here, the flow of air is from the area of high pressure to the area of low pressure, but, as we mentioned previously, it does not flow straight across the isobars (or isoheights). Instead, the flow is circular around the pressure systems.

Figure 3-1-11.óDevelopment cycle of a geostrophic wind.

The Coriolis force commences deflecting the path of movement to the right (Northern Hemisphere) or left (Southern Hemisphere) until it reaches a point where a balance exists between the Coriolis and the pressure gradient force. At this point the air is no longer deflected and moves forward around the systems. Once circular motion around the systems is established, then centrifugal force must be considered.

Centrifugal force acts outward from the center of both the highs and the lows with a force dependent upon the velocity of the wind and the degree of curvature of the isobars. However, the pressure gradient force is acting towards the low; therefore, the flow in that direction persists. When the flow is parallel to the curved portion of the analysis in figure 3-1-9, it is a GRADIENT WIND. When it is moving parallel to that portion of the analysis showing straight flow, it is a GEOSTROPHIC WIND.

We defined pressure gradient as being a change of pressure with distance. This means that if the isobars are closely spaced, then the pressure change is greater over a given distance; it is smaller if they are widely spaced. Therefore, the closer the isobars, the faster the flow.

Geostrophic and gradient winds are also dependent, to a certain extent, upon the density of the atmosphere and the latitude. If the density and the pressure gradient remain constant and the latitude increases, the wind speed decreases. On the other hand, if the latitude decreases, the wind speed increases. If the density and the latitude remain constant and the pressure gradient decreases, the wind speed decreases. If the pressure gradient and the latitude remain constant and the density decreases, the wind speed increases. If the density increases, the wind speed decreases.

True geostrophic wind is seldom observed in nature, but the conditions are closely approxi-mated on upper-level charts.

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