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Slow-moving cold fronts (active cold front)

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SLOW-MOVING COLD FRONTS (ACTIVE COLD FRONT)

With the slow-moving cold front, there is a general upward motion of warm air along the entire frontal surface and pronounced lifting along the lower portion of the front. The average slope of the front is approximately 1:100 miles. Near the ground, the slope is often much steeper because of surface friction.

Figure 4-3-1 illustrates the typical characteristics in the vertical structure of a slow-moving cold front. The lower half shows the typical upper airflow behind the front, and the upper half shows the accompanying surface weather. This is only one typical case. Many varia-tions to this model can and do occur in nature. The slow-moving cold front is an ACTIVE front because it has widespread frontal cloudiness and precipitation at and behind the front caused by actual frontal lifting.



Figure 4-3-1.—Typical vertical structure of a slow-moving cold front with upper windflow in back of the front.

Surface Characteristics

The pressure tendency associated with this type of frontal passage is indicated by either an unsteady or steady fall prior to frontal passage and then weak rises behind. Temperature and dew point drop sharply with the passage of a slow-moving cold front. The wind veers with the cold frontal passage and reaches its highest speed at the time of frontal passage. Isobars are usually curved anticyclonically in the cold air. This type of front usually moves at an average speed between 10 and 15 knots. Slow-moving cold fronts move with 100% of the wind component normal to the front.

Weather

The type of weather experienced with a sslow-moving cold front is dependent upon the stability of the warm air mass. When the warm air mass is stable, a rather broad zone of altostratus and nimbostratus clouds accompany the front and ex-tend several hundred miles behind the front. If the warm air is unstable (or conditionally unstable), thunderstorms and cumulonimbus clouds may develop within the cloud bank and may stretch for some 50 miles behind the surface front. Thes  cumulonimbus clouds form within the warm air mass. In the cold air there may be some stratus or nimbo-stratus clouds formed by the evaporation of falling rain; but, generally, outside of the rain areas, there are relatively few low clouds. This is because of the descending motion of the cold air that sometimes produces a sub-sidence inversion some distance behind the front.

The ceiling is generally low with the fron-tal passage, and gradual lifting is observed after passage. Visibility is poor in precipita-tion and may continue to be reduced for many hours after frontal passage as long as the precipitation occurs. When the cold air behind the front is moist and stable, a deck of stratus clouds and/or fog may persist for a number of hours after frontal passage. The type of precipitation observed is also dependent upon the stability and moisture conditions of the air masses.

Upper Air Characteristics

Upper air contours show a cyclonic flow and are usually parallel to the front as are the isotherms. The weather usually extends as far in back of the front as these features are parallel to it. When the orientation changes, this usually indicates the position of the asso-ciated upper air trough. (A trough is an elon-gated area of relatively low pressure.)

The temperature inversion on this type of front is usually well marked. In the precipita-tion area the relative humidity is high in both air masses. Farther behind the front, subsidence may occur, giving a second inversion closer to the ground.

The wind usually backs rapidly with height (on the order of some 60 to 70 degrees between 950 and 400 mb), and at 500 mb the wind direction is inclined at about 15 degrees to the front. The wind component normal to the front decreases slightly with height, and the component parallel to the front increases rapidly.

On upper air charts, slow-moving cold fronts are characterized by a packing (concentration) of isotherms behind them. The more closely packed the isotherms and the more nearly they parallel the fronts, the stronger the front.

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