The jet stream varies in width and speed, as well as in direction. These variations cause air to be pushed together, or compressed, in some places causing an increase in pressure. In other places, the air is pulled apart, and the pressure goes down. Either of these situations can result from changes in the width of the jet stream, its speed, or both. When air is pushed together, it is said to be converging, and when it is pulled apart, it is said to be diverging.
Fig. 2-6 illustrates convergence and divergence. Directional convergence and directional divergence are shown at A and B, respectively. Speed convergence and speed divergence are shown at C and D, respectively.
Fig. 2-6. At A, directional convergence of air. At B, directional divergence. At C, speed convergence. At D, speed divergence.
Directional effects take place only at bends in the jet stream. But speed effects can occur whether there is a bend or not. Divergence gives rise to low-pressure systems, because divergence creates a drop in pressure. If the air in a particular part of the jet stream diverges to a great enough extent, clouds form and the weather becomes foul. The system is carried along, from west to east, by the jet stream. Warm tropical air moves toward the pole ahead of the system, and cold polar air flows in behind the low-pressure center. The result is a frontal cyclone, also known as a low. This type of system can form anywhere along the jet stream, provided there is enough divergence of the air.
The most intense frontal cyclones are generated when there is not only strong divergence in the air, but also a cyclonic bend in the jet stream. If the jet stream is turning in a cyclonic direction (toward the left in the northern hemisphere, or toward the right in the southern) when a frontal cyclone forms, the circulation is given an extra push. If an existing frontal cyclone encounters a cyclonic bend in the jet stream, intensification is likely.
Frontal cyclones are always accompanied by clouds. As a low-pressure system approaches, the first sign is an increase in high-altitude, thin clouds. The clouds thicken and become lower, until the sky is a dull overcast and rain or snow falls. If the center of a frontal cyclone in the northern hemisphere passes to the south, the weather becomes chillier, and the winds shift from south to east, then to the north and northwest. If the center of the system passes to the north, the rain or snow may abate, and the temperature usually rises. The wind shifts to the southwest. A day or two later, the sky becomes cloudy again. In the summer, fair-weather clouds give way to towering cumulonimbus, and heavy thunderstorms are common. In the winter, freezing rain or snow falls, driven by strong westerly or northwesterly winds.
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