The simplest forms of convergence and divergence are the types that result from wind direction alone. Two flows of air need not be moving in opposite directions to induce divergence, nor moving toward the same point to induce convergence, but maybe at any angle to each other to create a net inflow of air for convergence or a net outflow for divergence.

Convergence is occurring when wind speeds are decreasing downstream; that is, mass is accumulating upstream. Conversely, divergence is occurring when wind speeds are increasing downstream; that is, mass is being depleted upstream.

Wind speed in relation to the wind direction is also a valuable indicator. For example, on a streamline analysis chart we can analyze both wind direction and wind speed, variations in wind speed along the streamlines, or the convergence or divergence of the streamlines.

The following are some of the combinations or variations of wind speed and direction:

. In a field of parallel streamlines (wind flow), if the wind speed is decreasing downstream (producing a net inflow of air for the layer), convergence is taking place. If the flow is increasing downstream (a net outflow of air from the layer), divergence is occurring.

. In an area of uniform wind speed along the streamlines, if the streamlines diverge (fan out), divergence is occurring; if the streamlines converge (come together), convergence is taking place.

. Normally, the convergence and divergence components are combined. The fact that streamlines converge or diverge does not necessarily indicate convergence or divergence. We must also consider the wind speedswhether they are increasing or decreasing downstream in relation to whether the streamlines are spreading out or coming together.

. If, when looking downstream on the streamlines, the wind speed increases and the streamlines diverge, divergence is taking place. On the other hand, if the wind speed decreases downstream and the streamlines come together, convergence is taking place.

There are other situations where it is more difficult to determine whether divergence or convergence is occurring, such as when the wind speed decreases downstream and the wind flow diverges, as well as when wind speed increases downstream and the wind flow converges. A special evaluation then must be made to determine the net inflow or outflow.

DIVERGENCE AND CONVERGENCE (COMPLEX MOTIONS)

In this section we will be discussing high-level convergence and divergence in relation to downstream contour patterns and the associated advection patterns. Low tropospheric advection (and also stratospheric advection) certainly play a large role in pressure change mechanisms.

Since the term divergence is meant to denote depletion of mass, while convergence is meant to denote accumulation of mass, the forecaster is concerned with the mass divergence or mass convergence in estimating pressure or height changes. Mass divergence in the entire column of air produces pressure or height falls, while mass convergence in the entire column of air produces pressure or height rises at the base of the column.

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