The sharpness of surface air mass differences fuels middle latitude cyclones so
occlusion represents the dying out of a cyclone. The expected weather with an
occluded frontal passage is cool air followed by cold air with cloudy, showery
weather. The occluded front lengthens by “zippering closed” the surface cold
and warm fronts.
A dryline may be present in selected areas of the middle latitudes such as the
southern Great Plains of the United States. The dryline is a front-like feature rep-
resenting the boundary between hot, dry continental tropical (cT) air and warm,
moist maritime tropical (mT) air. In some cases, the dryline tends to move in one
direction during the day, and in the opposite direction at night over a several
hundred kilometer span. In fact, in certain times of the year at some locations,
the passage of the same dryline might occur for several days in a row. Wind con-
verges from both sides of the dryline. The typical directions are from the southeast
in the mT air side and from the southwest on the cT air side. The strongly converg-
ing air streams are forced to rise in the dryline. The mT air is so unstable that there
are typically huge and sometimes tornadic thunderstorms in front of the daytime
advance of the dryline. The cT air is much warmer and without clouds.
Fronts 137