Fronts

Polar Front

Often, we can observe regions of strong temperature contrasts on weather maps. These regions of contrast are often very deep and extend through the troposphere.  Such zones of contrast are marked by lines on the surface weather map called fronts.

The polar front is the general name given to the boundary separating the polar air masses from tropical air masses that extends around the world. Symbols are used on surface weather maps to indicate the characteristics or type of front. But the polar front exists aloft too....the polar jet stream (storm track) is the mid and upper tropospheric manifestation of the polar front.

Frontal Types

A stationary front line is indicated by blue triangles on one side of the line alternating with red semi-circles on the opposite side of the line. The triangles point away from the colder air while the semi-circles point away from the warmest air. Stationary fronts are found where the cold and warm air masses are moving parallel to the front (not moving with respect to one another) or if the cold and warm air masses are meeting directly with comparable wind speeds.

A cold front is a front that is moving in the direction of the warmer air. In this case, cold air is colliding with the warm air (cold air is "doing" the colliding). A cold front is indicated on the line by blue triangles pointing away from the colder air and in the direction towards which the cold air is moving.

A warm front is a front that is moving in the direction of the colder air. In this case, warm air is colliding with the cold air (warm air is "doing" the colliding). A warm front is indicated on the line by red semi-circles pointing away from the warmer air and pointing in the direction towards which the warm air is moving.

Wave Cyclones

Along sections of the polar front, cold and warm air masses may move differentially with respect to one another. One way in which this occurs is when divergence aloft overspreads the polar front. A low pressure area then develops on the polar front and induces a counterclockwise motion locally in the cold and warm air masses on either side of the polar front. This is the birth of the frontal or wave cyclone. A wave cyclone is a frontal cyclone in which air moving counterclockwise and inward induces a wave like configuration in the frontal pattern, which then evolves as depicted.

Observations show that wave cyclones undergo an evolution. Generally they get stronger and larger with time, initially. At some point, relative to the ground, it appears that the cold front "catches up" with the warm front. The resulting "merged" front is known as an occluded front, even though, technically, there is cold air on both sides of the front. An occluded front is indicated on the line by alternating purple triangles and semicircles on the same side of the line and pointing in the direction of the cold front symbols on the cold front.

Over time, more and more of the wave occludes, and the portion of the frontal system dominated by the occluded front increases. Eventually, the storm completely occludes. Since, by defintion, to stay existant, a front must have a temperature contrast across it, the clouds and precipitation associated with occluded fronts dissipate...over a period of a day or so. Occluded fronts are still drawn on weather maps until all the clouds and precipitation associated with them have dissipated.