OVERVIEW Describe the characteristics of the stable and unstable quasi-stationary front.

OUTLINE

A quasi-stationary front, or stationary front as it is often called, is a front along which one air mass is not appreciably replacing another air mass. A stationary front may develop from the slowing down or stopping of a warm or a cold front. When this front forms, the slope of the warm or cold front is initially very shallow. The dense cold air stays on the ground, and the warm air is displaced slowly upward. The front slows or stops moving because the winds behind and ahead of the front become parallel to the stationary front. It is quite unusual for two masses of different properties to be side by side without some movement, so the term stationary is a misnomer. Actually the front, or dividing line between the air masses, is most likely made up of small waves undulating back and forth; hence the term quasi-stationary. The important thing is that the front is not making any appreciable head-way in anyone direction. A front moving less than 5 knots is usually classified as a stationary front.

Learning Objective: Describe the characteristics of stable and unstable quasi-stationary fronts.

When a front is stationary, the whole cold air mass does not move either toward or away from the front. In terms of wind direction, this means that the wind above the friction layer blows neither toward nor away from the front, but PARALLEL to it. The wind shift across the front is usually near 180 degrees. It follows that the isobars, too, are nearly parallel to a stationary front. This characteristic makes it easy to recognize a stationary front on a weather map.

There is frictional inflow of warm air toward a stationary front causing a slow upglide of air on the frontal surface. As the air is lifted to and beyond saturation, clouds form in the warm air above the front.

If the warm air in a stationary front is stable and the slope is shallow, the clouds are stratiform. Drizzle may then fall; and as the air is lifted beyond the freezing level, icing conditions develop and light rain or snow may fall. At very high levels above the front, ice clouds are present. (See fig. 4-6-1.)

If, however, the slope is steep and significant warm air is being advected up the frontal slope, stratiform clouds with embedded showers result (view B of fig. 4-6-1). Slight undulation or move-ment of the quasi-stationary front toward the warm air mass adds to the amount of weather and shower activity associated with the front.

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