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Cold Fronts

Cold fronts are usually associated with depressions. A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. At a cold front cold air following warm air undercuts the warm air, heaving it upwards with a more violent thrust compared to the steady rise of air at a warm front. The air associated with a cold front is usually unstable and conducive to cumulonimbus cloud formation. Because the up thrust is delivered along a boundary between the two air masses, the cumulonimbus form a well-defined line in contrast to the well-spaced clouds forming during thermal convection. Usually, rainfall associated with cold fronts is in the form of heavy deluge. More rain may fall in a few minutes as the cold front passes than during the whole passage of a warm front. As the cold front passes, the clouds roll by and the air temperature may become noticeably cooler, with temperatures dropping by 5°C or more within the first hour.