Fronts

Some of the weather conditions most adverse to fire control, such as strong, gusty winds, turbulence, and lightning storms, occur in frontal zones. Sometimes there is insufficient moisture in the warm air mass, or inadequate lifting of this mass, so that no precipitation occurs with the front. Strong, gusty, and shifting winds are typical of a dry frontal zone, adding greatly to the difficulty of fire control.

Types of fronts are distinguished by the way they move relative to the air masses involved. If a front is moving so that cold air is replacing warm air, it is a cold front. If the warm air is advancing and replacing cold air ahead, the front is a warm front. If a front is not moving, it is a stationary front. Cold fronts are indicated on weather maps by pointed cusps, and warm fronts by semicircles, on the side toward which they are moving. A stationary front is indicated by a combination of both.

In a frontal zone, the warmer air mass, being lighter, will be forced over the colder air mass. The rotation of the earth deflects the movement of both the cold and the warm air masses as one tries to overrun or underride the other, and prevents the formation of a horizontal discontinuity surface. Instead, the frontal surface slopes up over the colder air. The slope varies from about 1/50 to 1/300. A 1/50 slope means that for every 50 miles horizontally, the front is 1 mile higher in the vertical. The amount of slope is dependent upon the temperature contrast between the two air masses, the difference in wind speed across the front, and the relative movements of the air masses involved; that is, whether cold air is replacing warm air at the surface or warm air is replacing cold air. On a surface weather map, only the intersection of the frontal surface with the earth is indicated. The contrast between the air masses is strongest near the earths surface, and decreases upward in the atmosphere.

The central portions of air masses are usually associated with areas of high pressure, but fronts are formed in troughs of low pressure. From a position on a front, we find that the pressure rises both toward the warmer air and toward the colder air. Because the gradient wind in the Northern Hemisphere always blows with high pressure on the right, as one faces downstream, this means that the wind blows in one direction in the cold air and a different direction in the warm air. At a given location the wind shifts in a clockwise direction as a front passes--for example, from southeast to southwest or from southwest to northwest.

The wind-shift line and pressure trough line provide good clues to the weatherman for the location of fronts, but there are other indications to consider. A temperature discontinuity exists across a front. As a rule, the greater and more abrupt the temperature contrast, the more intense the front. Weak fronts are characterized by gradual and minor changes in temperature. The moisture contrast between air masses on different sides of a front may be indicated by the dew-point temperatures. Usually the cold air mass will be drier than the warm air mass. Other indications of front location are cloud types, pressure changes, and visibility changes.