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The 300-mb chart is the primary chart used to determine areas of horizontal convergence and divergence; however, if a sparsity of reports limits its use, the 500-mb chart may be substituted. Looking at an analyzed 300-mb chart, you have to examine the winds in relation to height contours to locate areas of divergence. Look for areas where high-speed winds are approaching weak cyclonically curved contour gradients. A jet maximum moving south on the west side of a major trough is a good example of this type of wind-contour relationship. See figure 8-4-4. The jet winds push air at excessive speeds, and when

Figure 8-4-4.—Divergence illustrated.

Figure 8-4-5.—Convergence illustrated,

the air is forced to negotiate the curve in the contour (much like a car trying to negotiate a bend in the road while moving at high speed), it fails. The air continues in a straight line and crosses the contours to the right of its intended path. Based on resultant forces, where the contours weaken and curve cyclonically, the Coriolis force and centrifugal force combine to overpower the pressure gradient force; and the air flows out of the system, across contours, into an area of higher heights. Divergence occurs downstream to the left of the path of the outflowing air. If these high-speed winds continue for a sustained period of time, large height falls occur in this area. Areas of convergence are associated with two wind-contour relationships. The first is where low-speed winds approach strong cyclonically curved contour gradients (fig. 8-4-5, view A). A good example of this is where a jet maximum is located on the east side of a major trough and weak winds are on the west side. As the air on the west side moves slowly toward the base of the trough, it encounters the stronger cyclonically curved contour gradient associated with the trailing edge of jet stream winds. The pressure gradient becomes very intense in this area and the Coriolis and centrifugal forces are unable to balance it. In order to reestablish the balance of forces, the slow-moving air is forced to move to the left of its intended path. As the air moves to the left, it crosses contours, and there’s a net inflow of air into the trough. The convergence occurs where the air flows across the contours.

The second wind-contour relationship that creates convergence occurs when high-speed winds approach winds of lower speeds in weak anti-cyclonically curved contour gradients (fig. 8-4-5, view B). An example you might look for would have a jet maximum approaching the top of a sharply curved ridge. As the air is pushed into the ridge at excessive speeds, it is unable to make the sharp turn necessary to follow the contours. Here the centrifugal force and pressure gradient force combine to overpower Coriolis force. The air is forced to cross the contours to the left of its intended path and flows into the downstream trough. The convergence occurs to the right of the path of inflowing air.

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