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.
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.
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.
