A warm front is a boundary between an advancing warm air mass and a retreating cold air mass. Shown by a red line and half-circles, winds behind the front will shift to the south in the Northern Hemisphere. When the two air masses meet, they do not mix. Since warm air is less dense, it rises slowly over the cold, denser air. Clouds will form and steady rain will fall over a larger area. Sometimes this rain can be extremely heavy. In the winter, snow can be observed ahead of the front, but sleet and freezing rain can be seen closer to the warm front.