Figure 3-11 Cold Front
Cold front weather can vary greatly depending on the speed of the front and the characteristics of
the air masses. Usually, though, as the cold front approaches, the southwesterly winds in the
warm air mass ahead of the front begin to increase in speed. Meanwhile, the barometric pressure
decreases, and altocumulus clouds appear on the horizon. Next, the cloud bases lower, and rain
increases in intensity and may persist as the front nears the station. As the front passes, the
pressure rises sharply and the wind shifts approximately 90° from SW to NW. The postfrontal
weather includes rapidly clearing skies, fair weather cumulus clouds, and decreasing temperature
and dew point. The extent of postfrontal cloudiness depends on the degree of stability and
moisture content of the cold air mass. In some cases, the sequence of events described here may
be considerably different, depending on the specific atmospheric conditions (Figure 3-12).
Weather with fast-moving cold fronts occurs in a narrow band, is usually severe, and clears
rapidly behind the front. Cumuliform clouds, showers, or thunderstorms may form near the front
position. Lines of fast-moving thunderstorms, or squall lines, can form well ahead of the front.
Weather with slow-moving cold fronts (usually from late fall through early spring) occurs over a
large area, is less severe, but may persist for hours, even after the front has passed.
3-12 Mechanics of Frontal Systems