OVERVIEW Define convergence and divergence and describe the importance of each in the science of meteorology.

Convergence is the accumulation of air in a region or layer of the atmosphere, while levels. Coincidently, this is also the layer of maximum winds in the atmosphere; cores of jet streams are usually found here. These high-speed winds are directly related to convergence and divergence. The combined effect of wind direction and speed (velocity) is what produces convergent and divergent air flow.

Learning Objective: Define convergence and divergence, and describe the im-portance of each in the science of meteor-ology.

IMPORTANCE OF CONVERGENCE AND DIVERGENCE

The importance of convergence and di-vergence is related to pressure changes at the surface and height changes of the constant-pressure levels. As the air accumulates in the 300-200-mb stratum over a region, greater pressure is exerted throughout the atmosphere. Its like inflating a tire.

As the air flows into the stratum, the pressure increases. Barometric pressure at the surface rises, as do the heights of the constant-pressure levels. Put another way, we say that upper-level con-vergence causes pressure and height rises. The exact opposite effect takes place when air is depleted from the 300-200-mb stratum. This is where we deflate the tire. As the air flows out of this stratum, pressure is lost. Barometric pressure at the surface falls, as do the heights of the constant-pressure levels. We say that upper-level divergence causes pressure and height falls.

Convergence and divergence arent the only processes at work in the atmosphere that can cause pressure and height changes, but youre going to hear these terms with increasing regularity as you progress up the AG rate ladder. They are primarily used by forecasters to explain why systems are expected to fill or deepen during the forecast period. Since the upper winds are the producer of convergence and divergence, you should be able to recognize the flow patterns associated with their production. The flow patterns range from simple to complex.

In order for convergence to take place, the winds must be such as to result in a net inflow of air into a layer or region. At the surface, low-pressure systems are associated with con-vergent flow. The winds cross isobars toward the center of the low and push the air in the center upward into the atmosphere. The currents are illustrated in figure 8-4-1. The upward vertical motion is a prime contributor to the occurrence of precipitation. In meteorology, convergence is classified as horizontal or vertical, because there are horizontal and vertical currents occurring in the atmosphere.

In order for divergence to take place, the winds must be such as to result in a net outflow of air from a layer or region. High-pressure systems are associated with divergent flow. The winds cross isobars, flowing out from the highs center and depleting the air within the high. The air above the high sinks to replace the outflow of air at the surface. This downward vertical motion (subsidence) is associated with dry air. Divergence can also be classified as horizontal or vertical, depending on the winds axis. See figure 8-4-1.

The simplest form of convergence and di-vergence is the type that results from wind direction alone. Two flows of air brought together, no matter what the angle, result in convergence. Where the air flow splits and winds go in different directions, divergence is occurring. Figure 8-4-2 illustrates these types of convergent and divergent air flow. 

Wind speed in relation to wind direction is also a contributor to convergence and divergence. If the wind speed decreases downstream, theres a net inflow of air into the region, and convergence takes place. If wind speeds increase downstream, theres a net outflow of air from the region, and divergence occurs. In an area of uniform wind speeds, if the winds fan out (split), divergence occurs. If these same winds are brought together, convergence occurs. See figure 8-4-2, view B.

The fact that contours converge or diverge doesnt necessarily indicate convergence or divergence, because wind speeds must also be considered. If wind speeds increase downstream and the contours spread apart, supergradient winds are said to be occurring. This com-bination of wind direction and speed produces divergence. On the other hand, if wind speeds decrease downstream and the contours converge,

Figure 8-4-1.Convergence and divergence. (A) Vertical perspective; (B) Horizontal perspective (Northern Hemisphere).


Figure 8-4-2.Convergence and divergence; (A) directional, (B) speed.

Figure 8-4-3.Convergence and divergence in supergradient and subgradient air flow.

subgradient winds are occurring. This combina-tion produces convergence. See figure 8-4-3.

There are other cases where it is difficult to tell whether divergence or convergence is taking place. When wind speed decreases downstream and the contours spread apart, both convergence and divergence are indicated. The wind speed suggests convergence, but the spreading contours suggest divergence. A similar situation arises when wind speeds decrease downstream and the contours converge. Here, were looking at speed divergence and directional convergence. These are complex motions, however, and a special evaluation is required to determine the net inflow or outflow.

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