AIR MASSES AND FRONTS

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 headway in any one
direction. A front moving less than 5 knots is usually
classified as a stationary front.
CHARACTERISTICS
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.
STABLE STATIONARY FRONT
There is frictional inflow of warm air toward a
stationary front causing a slow upglide of air on the
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COLD AIR
COLD
AIR
STRATUS
SCUD
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A. SHALLOW STATIONARY FRONT
STRATUS OR
STRATOCUMULUS
SCUD
4-44
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. 4-42).
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.
4-42). Slight undulation or movement of the
quasi-stationary front toward the warm air mass adds to
the amount of weather and shower activity associated
with the front.
UNSTABLE STATIONARY FRONT
If the warm air is conditionally unstable, the slope
is shallow, and sufficient lifting occurs, the clouds are
then cumuliform or stratiform with embedded towering
cumulus. If the energy release is great (warm, moist,
unstable air), thunderstorms result. Within the cold air
mass, extensive fog and low ceiling may result if the
cold air is saturated by warm rain or drizzle falling
NIMBOSTRATUS
B. STEEP STATIONARY FRONT
Figure 4-42.—Types of stable stationary fronts.
WARM AIR
WARM
AIR
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