Weather Patterns

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.

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.

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.

Weather Patterns

Weather Patterns 5

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.

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.

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.

Warm Fronts

Weather Patterns 9

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.

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.

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.

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.

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).

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.

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.

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.