A warm air mass is overtaking a cold air mass. Since warm air is less dense than cold air, it rises above the retreating cold air. Warm fronts normally have a gradual increase in cloudiness. First, high clouds move in from the southwest and then begin to lower and thicken. Then, light precipitation begins to fall and it gets steadier and heavier. In the warmer months of the year, thunder and lightning is possible. During the winter months in some parts of the country, snow may fall as a warm front approaches but it usually turns to sleet and then rain as the front gets closer and the temperatures increase. Freezing rain is also a possibility. After the warm front passes by, the weather usually clears and becomes warm.
Cold air is overtaking warm air. Since cold air is denser than warm air, cold air undercuts a warm air mass. This produces a more concentrated line of heavy precipitation as the push of the cold air literally lifts the warm air just ahead of the front. This lifting of the air many times causes thunderstorms or very heavy showers. The increase of cloudiness preceding a cold front is usually rapid. The clearing behind the front is sometimes rapid, but there are times when the clearing is very slow after the front passes. This is when the cooler air mass is shallow behind the front. Also, there are times when a cold front and a warm front do not have any precipitation associated with them.