Cold Fronts

The leading edge of an advancing cold air mass is a cold front. It forms a wedge, which pushes under a warm air mass forcing the warm air to rise. Because of surface friction, the lowest layers of the cold air are slowed down. This increases the steepness of the frontal surface and causes a cold front to have a blunted appearance when viewed in cross-section. The slopes of cold fronts usually vary from 1/50 to 1/150.

There are wide variations in the orientation and speed of cold fronts. Usually, they are oriented in a northeast-southwest direction, and they move to the east and southeast, at speeds varying from about 10 to 40 m.p.h. and faster in the winter.

As a cold front approaches, the southerly winds increase in the warm air ahead of the front. Clouds appear in the direction from which the front is approaching. The barometric pressure usually falls, reaches its lowest point as the front passes, then rises sharply. Winds become strong and gusty and shift sharply to westerly or northwesterly as the cold front passes. Temperature and dew point are lower after the cold front passes. In frontal zones with precipitation, the heaviest precipitation usually occurs with the passage of the front. Then it may end quickly and be followed by clearing weather.

There are many exceptions to the foregoing general pattern of cold-front passages. The severity of the weather associated with cold fronts depends upon the moisture and stability of the warm air, the steepness of the front, and the speed of the front. Since cold fronts are usually steeper and move faster than warm fronts, the accompanying band of weather is narrower, more severe, and usually of shorter duration than with warm fronts.

With slow-moving cold fronts and stable warm air, rain clouds of the stratus type form in a wide band over the frontal surface and extend for some distance behind the front. If the warm air is moist and conditionally unstable, thunderstorms may form, with the heaviest rainfall near the frontal zone and immediately following. If the warm air is fairly dry and the temperature contrast across the front is small, there may be little or no precipitation and few or no clouds.

With rapidly moving cold fronts, the weather is more severe and occupies a narrower band. The disturbance is also of shorter duration than that caused by a slow-moving front. If the warm air is relatively stable, overcast skies and precipitation may occur for some distance ahead of the front, and the heaviest precipitation may occur ahead of the surface cold front. If the warm air is moist and conditionally unstable, scattered showers and thunderstorms form just ahead of the cold front. The weather usually clears rapidly behind a fast-moving cold front, with colder temperatures and gusty, turbulent surface winds following the frontal passage.

Under some conditions, a line of showers and thunderstorms is formed from 50 to 300 miles ahead of, and roughly parallel to, a cold front. This is called a squall line. The weather associated with squall lines is often more severe than that associated with the subsequent cold front. After the passage of the squall line, the temperature, wind, and pressure usually revert to conditions similar to those present before the squall line approached. Occasionally, the showers and thunderstorms are scattered along the squall line so that some areas experience strong, gusty winds without any precipitation.

Dry cold fronts often cause very severe fire weather in many sections. Dry cold-front passages may occur in any region, but they are a major problem in the Southeast. Cold fronts tend to be drier farther away from the low-pressure center with which they are associated. Thus, a cold front associated with a Low passing eastward across Southern Canada or the Northern States may be very dry as it passes through the Southeast. In addition, the polar air mass following the cold front may become quite unstable because of surface heating by the time it reaches the Southeast.

The combination of strong, gusty winds and dry, unstable air creates serious fire weather. The second of two cold fronts passing through the Southeast in rapid succession also tends to be dry. The warm air mass ahead of the first cold front may be moist and produce precipitation, but the air mass between the first and second fronts usually will not have had time to acquire much moisture. Therefore, the second cold-front passage may be dry and will be the more serious from the fire-control standpoint.

The dry, trailing ends of cold fronts cause serious fire weather wherever they occur. Along the Pacific coast, the winds behind such cold fronts are, at times, from a northeasterly direction. This offshore direction means that the air flows from high elevations to low elevations and has foehn characteristics. The strong, shifting, gusty winds of the cold-front passage combine with the dry foehn wind to the rear of the front to produce a short-lived but extremely critical fire-weather condition.