A cold front is when a cool air mass is replacing a warm air mass. The air behind a cold front is usually colder and dryer than the air in front of it. This front usually causes cumulus clouds to form. Before a cold front passes there is a short period of showers. While a cold front is passing it usually produces heavy rains, sometimes with hail, and thunder and lightning. And after a cold front passes there is still showers but then starts clearing.