The fast-moving cold front is a very steep front that has warm air near the surface being forced vigorously upward. At high levels, the warm air is descending downward along the frontal surface. This front has a slope of 1:40 to 1:80 miles and usually moves rapidly; 25 to 30 knots may be considered an average speed of movement, They move with 80 to 90 percent of the wind component normal to the front. As a result of these factors, there is a relatively narrow but often violent band of weather.

Figure 4-3-2 shows a vertical cross section of a fast-moving cold front with resultant weather. Also indicated in the lower half of the diagram is the surface weather in advance of the front and the upper airflow above the front.

If the warm air is moist and unstable, a line of thunderstorms frequently develops along this front. Sometimes, under these conditions, a line of strong convective activity is pro-jected 50 to 200 miles ahead of the front and parallel to it. This may develop into a line of thunderstorms called a squall line. On the other hand, when the warm air is stable, an overcast layer of altostratus clouds and rain may extend over a large area ahead of the front. If the warm air is very dry, little or no cloudiness is associated with the front. The front depicted is a typical front with typical characteristics.

The fast-moving cold front is considered an INACTIVE front because lifting occurs only at and ahead of the front. The lifting is caused by descending air ahead of the front and only in part by the frontal surface.

Pressure tendencies fall ahead of the front with sudden and strong rises after frontal passage.

Figure 4-3-2.Typical vertical structure of a fast-moving cold front with upper windflow across the front.

If a squall line lies some distance ahead of the front, there may be a strong rise asso-ciated with its passage and a shift in the wind. However, after the influence of the squall line has passed, winds back to southerly and pressures level off. The temperature falls in the warm air just ahead of the front. This is caused by the evaporation of falling precipi-tation. Rapid clearing and adiabatic warming just behind the front tend to keep the cold air temperature near that of the warm air. An abrupt temperature change usually occurs far behind the front near the center of the high-pressure center associated with the cold air mass. The dew point and wind direction are a good indication of the passage of a fast-moving cold front. The wind veers with frontal passage and is strong, gusty, and tur-bulent for a considerable period of time after passage. The dew point decreases sharply after frontal passage.

Weather

Cumulonimbus clouds are observed along and just ahead of the surface front. Stratus, nimbostratus, and altostratus may extend ahead of the front in advance of the cumulonimbus and may extend as much as 150 miles ahead of the front. These clouds are all found in the warm air. Generally, unless the cold air is unstable and descending currents are weak, there are few clouds in the cold air behind the front. Showers and thunderstorms occur along and just ahead of the front. The ceiling is low only in the vicinity of the front. Visi-bility is poor during precipitation but improves rapidly after the frontal passage.

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