Cold fronts contribute to the formulation of cumulonimbus, or thunderstorm clouds, which are greater, more extreme versions of cumulus clouds. These dense clouds can tower up to 50,000 feet and have a fluffy appearance; they are commonly characterized by their mushroom-like tops, flattened by heavy winds. Cumulonimbus clouds scatter ahead of, behind or within a cold front and usually foretell signs of severe weather. Such clouds are often accompanied by thunder, lightning and quick, harsh precipitation.
Thunderstorms and heavy downpours are the most regular occurrences seen with cold fronts. In colder weather, however, freezing rain and snow may also occur. During more severe conditions violent winds and tornadoes can develop. Although tornadoes are seen less often, they are not uncommon outcomes -- but because a cold front is generally the quickest-moving type of front, this type of severe weather is usually short-lived.
A squall line is a large group of individual severe thunderstorms that move in a horizontal line and appears at the edge of an advancing cold front. Squall lines stretch for hundreds of miles and form due to unstable atmospheric conditions and when a large amount of wind shear is present. (Wind shear is a term that refers to inconsistency in wind patterns.) Squall lines are accompanied by heavy rains, high-speed winds, thunder, lightning and tornadoes. Passing squall lines are usually followed by lighter rain showers.
The weather that follows a cold front is often noticeably cooler and drier, as a passing cold front will leave a decrease in both temperature and humidity. Because the air that rose during the front will begin to descend, air pressure will rise once more. Since high air pressure inhibits the presence of moisture, heavy cloud cover after a cold front is unlikely, and the cumulonimbus clouds will have disappeared altogether. Stratus or stratocumulous clouds may appear scattered throughout otherwise gentle skies.