Fronts are boundary surfaces that separate air masses of different densities. Warm fronts occur when warmer air overrides cooler air, causing the warmer air to lift up along the wedge of cold air. This lifting causes clouds and precipitation to form ahead of the surface position of the warm front. As the warm front passes, air temperatures gradually rise and skies typically clear.

1. Weather Patterns

2. Weather Patterns
• Fronts- Are boundary surfaces that separate
air masses of different densities.
• One air mass is usually warmer and often
contains more moisture than the other air
mass.
• However, fronts can form between any two
contrasting air masses.

3. Weather Patterns
• Above the ground, the frontal surface slopes
at a low angle.
• Warmer air overlies cooler air.
• In perfect conditions, air masses on both sides
of the front move in the same direction and
speed.
• In this situation, the fronts act as a barrier
traveling along with the air masses.

4. Weather Patterns
• But in reality, the distribution of pressure
across a front is such that one air mass moves
faster than the other.
• So…… one air mass actively collides with the
other.
• Norwegian meteorologists visualized these
conditions as resembling a battle front, and so
named them fronts.

5. Weather Patterns
Weather Patterns 5

6. Weather Patterns
• Warm Fronts
• When the surface (ground) position of a front moves in
such a way that warm air occupies territory formerly
covered by cooler air, denser air acts as a wedge on
which lifting takes place.
• Overrunning is the term applied to warm air gliding up
along a cold air mass.
• On a weather map, the surface position of a warm front
is shown by a red line with red semi-circles protruding
(sticking) into the cooler air.

7. Warm Fronts
• East of the Rockies (Rocky Mountains)
maritime tropical (mT) air often enters the
U.S. from the Gulf of Mexico.
• It often overruns receding cooler air.
• Less dense warmer air has a more difficult
displacing heavier, cooler air.
• As a result, the boundary separating these air
masses acquires a very gradual slope.

8. Warm Fronts
• The slope of a warm front is very gradual.
• As warm air rises over the retreating wedge of
cooler air, it expands and cools adiabatically.
• This causes moisture to condense into clouds
that often produce precipitation.
• The following cloud sequence (Figure 9-4a)
typically precedes a warm front.

9. Warm Fronts
Weather Patterns 9

10. Warm Fronts
• The first sign of an approaching warm front is
cirrus clouds.
• These high clouds form where the overrunning
warm air has ascended high up The wedge of cold
air (about 600 miles – 1000 kilometers) ahead of
the surface front.
• Another indication of an approaching warm front
is the appearance of contrails.
• On a clear day when these contrails last for a long
time, it’s a sign that comparatively warm, moist
air is ascending overhead.

11. Warm Fronts
• As the front nears, cirrus clouds grade into
cirrostratus clouds that blend into denser
sheets of altostratus.
• About 180 miles (300 kilometers) ahead of the
front, thicker stratus and nimbostratus clouds
appear.
• Precipitation begins.

12. Warm Fronts
• Because of their relatively slow
movement, warm fronts tend to produce light
to moderate precipitation.
• But not always.
• When the overriding air mass is relatively dry
(low dew point temperatures) there may be
minimal cloud development and no
precipitation.

13. Warm Fronts
• However, during the hot summer
months, very moist air is often associated with
an approaching warm front.
• If this unstable air is lifted sufficiently, it will
freely rise on its own producing
cumulonimbus clouds and thunderstorms.
• (Figure 9-4) Precipitation associated with a
warm front occurs ahead of the surface
position of the front.

14. Warm Fronts
• Some of the rain that falls through the cool air
below the clouds evaporates.
• When this happens, the air directly beneath
the cloud base become saturated and a
stratus cloud deck develops.
• These clouds occasionally grow rapidly
downward and causes problems for pilots of
small aircraft that require visual landings. (No
radar).

15. Warm Fronts
• Occasionally during the winter time, a
relatively warm air mass is forced over a body
of subfreezing air.
• When this happens, hazardous driving
conditions may result.
• Raindrops become supercooled at they fall
through the subfreezing air.
• Upon colliding with the road surface, the flash
freeze to produce an icy layer called glaze.

16. Warm Fronts
• When a warm front passes, air temperatures
gradually rise.
• The increase is most apparent when there is a
large contrast between adjacent air masses.
• Moreover, a windshift from east to southwest is
generally noticeable. (We’ll learn more about this
later.
• Moisture content and stability of the encroaching
warm air mass largely determines the time period
required for clear skies to return.

17. Warm Fronts
• During the summer, cumulus and occasionally
cumulonimbus clouds are embedded in the
warm unstable air mass that follows the front.
• These clouds may produce precipitation which
can be heavy but is usually scattered and of
short duration.