Cold Fronts

A cold front is the transition zone where a colder, drier air mass is replacing a warm, moist air mass. Across the frontal zone, there is a sharp temperature decrease, a sharp humidity decrease, and a shift in wind direction. Often, there are clouds and precipitation. Cold fronts often bring very severe weather. This is due to the fact that warm, moist air is less dense than cold, dry air. The warm, moist air is forced upward as the cold front moves in, causing thunderstorms to form. Notice that the leading edge of the cold front has a high angle, which can rapidly force warm, moist air aloft.

There are specific weather conditions associated with all fronts, and cold fronts are no exception. The table below (from Ahrens' "Meteorology Today", a superb meteorology textbook), shows the "typical" conditions associated with cold fronts:

Weather Element	Before passing	While passing	After passing
Winds	South or southwest	Gusty, shifting	West or northwest
Temperature	Warm	Sudden drop	Steadily dropping
Pressure	Falling steadily	Minimum, then sharp rise	Rising steadily
Clouds	Increasing cirrus,cirrostratus,then either towering cumulus or cumulonimbus	Towering cumulus or cumulonimbus	Often cumulus
Precipitation	Short period of showers	Heavy showers of rain or snow, sometimes with hail, thunder, and lightning	Decreasing intensity of showers, then clearing
Visibility	Fair to poor in haze	Poor, followed by improving	Good except in showers
Dew point	High; remains steady	Sharp drop	Lowering