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e. Cold fronts move into an area rapidly and make a complete change in the weather
within the period of a few hours, and then move on to another area. The squall line that is
formed is generally very narrow--50 to 100 miles in width--but it is likely to extend for
hundreds of miles in length. Altostratus clouds sometimes form slightly ahead of the front,
but they are seldom more than a hundred miles in advance. After the front drier air, and is
passed, the weather clears rapidly, there is usually unlimited ceilings and visibility.

4. Occluded Front

a. An occluded front occurs when a cold front overtakes a slower moving warm front
and forces it aloft so that the warm air is no longer in contact with the ground. This is a
condition in which a warm air mass is caught between two colder air masses. It is forced aloft
to higher and higher altitudes until it finally spreads out and loses its identity as a particular
air mass.

b. Meteorologists subdivide occlusions into two types: cold front occlusions, depending
on the relative tem-perature of the air mass that is advancing.