Clearing usually occurs after the passage of a warm
front, but under some conditions drizzle and fog may
occur within the warm sector. Warm fronts usually
move in the direction of the isobars of the warm sector;
in the Northern Hemisphere this is usually east to
The amount and type of clouds and precipitation
vary with the characteristics of the air masses involved
and depending on whether the front is active or inactive.
Generally, with warm fronts, an increase of the wind
component with height perpendicular to the front gives
an active front. This produces strong overrunning and
pronounced prefrontal clouds and precipitation.
Inactive fronts, characterized by broken cirrus and
altocumulus, are produced by a decrease with height of
the wind component perpendicular to the front.
When the overrunning warm air is moist and stable,
nimbostratus clouds with continuous light to moderate
precipitation are found approximately 300 miles ahead
of the front. The bases of the clouds lower rapidly as
additional clouds form in the cold air under the frontal
surface. These clouds are caused by evaporation of the
falling rain. These clouds are stratiform when the cold
mass is stable and stratocumulus when the cold air is
When the overrunning air is moist and unstable,
cumulus and cumulonimbus clouds are frequently
imbedded in the nimbostratus and altostratus clouds. In
such cases, thunderstorms occur along with continuous
precipitation. When the overrunning warm air is dry, it
must ascend to relatively high altitudes before
condensation can occur. In these cases only high and
middle clouds are observed. Visibility is usually good
under the cirrus and altostratus clouds. It decreases
rapidly in the precipitation area. When the cold air is
stable and extensive, fog areas may develop ahead of
the front, and visibility is extremely reduced in this
UPPER AIR CHARACTERISTICS
Warm fronts are usually not as well defined as cold
fronts on upper air soundings. When the front is strong
and little mixing has occurred, the front may show a
well-marked inversion aloft. However, mixing usually
occurs and the front may appear as a rather broad zone
with only a slight change in temperature. Quite
frequently there may be two inversionsone caused by
the front and the other caused by turbulence. Isotherms
are parallel to the front and show some form of packing
ahead of the front. The stronger the packing, the more
active the front. The packing is not as pronounced as
with the cold front.
WARM FRONTS ALOFT
Warm fronts aloft seldom occur, but generally
follow the same principles as cold fronts aloft. One case
when they do occur is when the very cold air
underneath a warm front is resistant to displacement
and may force the warm air to move over a thinning
wedge with a wave forming on the upper surface. This
gives the effect of secondary upper warm fronts and
may cause parallel bands of precipitation at unusual
distances ahead of the surface warm front. Warm air
advection is more rapid and precipitation is heaviest
where the steeper slope is encountered. Pressure falls
rapidly in advance of the upper warm front and levels
off underneath the horizontal portion of the front. When
a warm front crosses a mountain range, colder air may
occur to the east and may move along as a warm front
aloft above the layer of cold air. This is common when a
warm front crosses the Appalachian Mountains in
What is the average speed of a warm front?
What cloud types, and in what order usually
form in advance of a warm front?
THE OCCLUDED FRONTS
LEARNING OBJECTIVE: Describe the
formation, structure, and characteristics of cold
and warm air occluded fronts.
An occluded front is a composite of two fronts.
They form when a cold front overtakes a warm front
and one of these two fronts is lifted aloft. As a result, the
warm air between the cold and warm front is shut off.
An occluded front is often referred to simply as an
occlusion. Occlusions may be either of the cold type or
warm type. The type of occlusion is determined by the
temperature difference between the cold air in advance
of the warm front and the cold air behind the cold front.
A cold occlusion forms when the cold air in
advance of a warm front is warmer than the cold air to
the rear of the cold front. The overtaking cold air
undercuts the cool air in advance of the warm front.
This results in a section of the warm front being forced
aloft. A warm occlusion forms when the air in advance
of the warm front is colder than the air to the rear of the
cold front. When the cold air of the cold front overtakes