Air masses sit over their source region and pick up their characteristics. However, after a time, they begin to move. A front is a boundary between two different air masses. When the air masses move, they bring entirely different air into a new region. This is what causes most major weather changes.

Four different kinds of fronts influence the weather. These four fronts are warm front, cold front, stationary front, and occluded front.

A warm front is formed when warm air moves in to replace cold air. A warm front brings warmer temperatures. It is characterized by a forewarning cloud sequence, gentle slope out ahead of the front at the surface, lowering and thickening clouds with the approach, slow and steady precipitation that stops soon after the passage of the front, and rise in temperatures after the passage of the front. On a weather map, the warm front is represented by a line with semi-circles.

A cold front is formed when cold air moves in to replace warm air. A cold front is characterized by a steep, blunt nose, so the warmer and moist air ahead of the front rises rapidly, cools quickly, and results in a shorter period of heavy precipitation. This often creates thunderstorms. The air falls both before and after the passage of the front at the surface. With the frontal passage, the temperatures fall and when the air aloft is not longer rising significantly, the precipitation ceases. On a weather map, the cold front is represented by a line with triangles.

A stationary front resembles a warm front, but the boundary between the warm and the cold air is not moving appreciably. On one side, the air is cold, while on the other it is warm. As the warm, moist air rises over the colder, denser air, it cools, and condensation and precipitation may result. A stationary front may sit over a location for a period of several days with one area continually receiving rain while across the front on the other side the weather remains pleasantly warm and dry. On a weather map, the stationary front is represented by a line with triangles and semi-circles on opposite sides.

An occluded front is the result of the faster moving cold front overtaking a warm front in a middle latitude cyclone. The type of occlusion will depend on whether the colder air is in the front or in the back. Occlusion results in more warm and moist air being lifted and cooled, so greater condensation and precipitation results. Eventually, this warm, moist air will have been lifted and dried out, so that the system will dissipate or die out. Since this is the area of greatest lifting, this is the center of the lowest pressure. On a weather map, the occluded front is represented by a line with triangles and semi-circles on the same side.

In a middle latitude cyclone, the polar front develops a kink where cold northern air pushes southward making a cold front and warm southern air pushes northward making a warm front. The cold front moves faster than the warm front, so eventually, the cold front overtakes the warm front creating an occluded front. The cold front lifts the warm air off of the surface. In rising, the warm air cools, condenses, and dries out, eventually causing the dissipation of the entire system.

Since fronts cause the lifting of air, they are associated with falling pressures and cyclones. In the northern hemisphere, winds blow clockwise and inward around lows or cyclones and counterclockwise and out around high pressures. This explains, the typical circulation pattern or wind patterns around a middle latitude cyclone and the following anticyclone.