What Are Stationary Fronts?

Air Masses

• Air masses are large bodies of air that have uniform temperatures and moisture content. These air masses form over source regions, such as arctic regions, northern oceans or the tropics. These air masses can be stable or unstable, cold or warm, and dry or moist. Air masses form the building blocks of the weather systems we encounter every day.

Fronts

• When an air mass leaves its source region, it comes into contact with other air masses with different properties. The zone between these interacting air masses is called a frontal zone or front. These zones have both a vertical and horizontal component, affecting the types of weather created. Fronts are characterized by changes in temperature, humidity and wind. Narrow zones exhibit rapid changes, while broad or diffuse fronts exhibit gradual changes.

Cold Fronts

• Cold fronts are formed on the leading edge of a cold air mass as it overtakes and replaces a warm air mass. Cold air masses are denser than the warm air masses they are forcing out of the way. This allows them to move more rapidly and exert more force upon the warm air mass, resulting in a sharp, upward rise of the warm air. This causes cold fronts to exhibit frontal boundaries that are narrow and well defined, producing sharp lines of stormy weather.

Warm Fronts

• Warm fronts are formed on the leading edge of a warm air mass as it overtakes and replaces a cold air mass. Because they are less dense than the cold air they are replacing, warm fonts move more slowly and result in a gradual rise of the warmer air. This creates a frontal boundary that is broad and less well defined. Precipitation forms farther in advance of a warm front than a cold front and is more constant.

Stationary Fronts

• Stationary fronts are formed when cold and warm air masses collide, but neither air mass is able to move, or replace, the other. This type of frontal boundary has little or no movement and the surface winds will blow parallel to the boundary, or frontal, line. Stationary fronts are generally associated with stratiform cloud formation and steady precipitation. On a weather map, stationary fronts are represented by alternating blue and red lines, with blue triangles pointing toward the warmer air and red semicircles pointing toward the colder air. Once the boundary zone begins to move, it will once again become a cold front or warm front, depending upon which air mass is able to replace the other.