Active warm fronts are generally located in pressure troughs on surface
charts. See figure 10-8. The troughs are not as pronounced as those
observed with cold fronts; therefore, other meteorological elements are
used as follows in locating warm fronts accurately:
1. Pressure tendencies. Pressure usually falls for an appreciable length
of time before the front passes. Normally it is steady after passage.
The tendencies in advance of the front are therefore a steady or unsteady
fall. A warm frontal passage is usually indicated by a tendency.
2. Wind. The wind in advance of a warm front in the Northern
Hemisphere is usually from the southeast, shifting to southwest after
passage. The wind speed normally increases as the front approaches.
The wind shift accompanying a warm front is seldom as abrupt as with
a cold front.
3. Cloud forms. Warm fronts are nearly always well defined by
tropical stratified clouds. They are generally cirrus, cirrostratus,
altostratus, nimbostratus, and stratus with the cirrus appearing as much
as 1,000 miles before the actual surface passage. The cloud types that
form after passage of the warm front are typical of the warm air mass.
4. Precipitation. The precipitation area of warm fronts extends about
300 miles in advance of the surface front. Precipitation occurs mainly in
the form of continuous or intermittent rain, snow, or drizzle. However,
when the warm air is connectively unstable, showers and thunderstorms
may occur in addition to the steady precipitation.
5. Temperature and dew-point chances. Abrupt temperature changes,
like those characteristic of cold fronts, do not accompany the warm
frontal passage. Instead, the temperature change is gradual. It starts
increasing slowly with the approach of the front and increases slightly
more rapidly with the passage. The dew point is normally observed to
rise as the front approaches, and a further increase follows the frontal
passage when the air in the warm sector is of maritime origin.
6. Visibility and ceiling. The visibility and ceiling are normally good
until the precipitation begins. Then they decrease rapidly. Dense fog
frequently occurs in advance of a warm front. These conditions improve
after the front passes.