Air Masses and Fronts – II

Brief review

•An air mass is a large body of air whose properties of temperature and humidity are fairly similar in any horizontal direction

•Source regions for air masses tend to be generally flat, of uniform composition, and in an area of light winds, dominated by surface high pressure

•Continental air masses form over land. Maritime air masses form over water. Polar air masses originate in cold, polar regions, and extremely cold air masses form over arctic regions. Tropical air masses originate in warm, tropical regions.

•Continental polar (cP) air masses are cold and dry; continental arctic (cA) air masses are extremely cold and dry; continental tropical (cT) air masses are hot and dry; maritime tropical (mT) air masses are warm and moist; maritime polar (mP) air masses are cold and moist

Air mass source regions and their paths

What is an atmospheric front?• A front is a transition zone between two air masses of different

densities.

• The density contrast results from:♦ Difference in temperature;♦ Difference in humidity.

• The frontal zone (surface) is the upward extension of the front.

• Sometimes the frontal zones can be very sharp.

• The intensity of the weather along the front depends on the contrast of the air mass properties.

• The type of front depends on both the direction in which the air mass is moving and the characteristics of the air mass.

Types of Fronts

• Cold front: cold, dry stable air is replacing warm, moist unstable air. • Warm front: warm, moist unstable

air is replacing cold dry stable air.

• Stationary front: boundary between the two air masses is not moving.

• The symbols on a map are in the direction of the air mass motion.

• Occluded front: when a cold front catches up with a warm front

Weather Map•Shown: surface-pressure systems, air masses, fronts,

isobars, winds and air flow (large arrows)

•Green-shaded area: precipitation

What are the Signs of a Passing Front?

•Signs♦ Sharp temperature changes over a relatively short

distance.♦ Changes in the air’s moisture content (as indicated

by changes in the dew point).♦ Shifts in wind direction♦ Pressure and pressure changes. ♦ Clouds and precipitation patterns.

•The location of the front is not always very obvious! Even meteorologists sometimes disagree…

Cold Fronts

• Cold front- a front in which cold air is replacing warm air at the surface.

• Notice the difference in♦ Temperature♦ Dew point♦ Wind direction♦ Pressure

• Associated with low pressure centers (low pressure troughs): follow the dashed line

• The pressure is minimum as the front passes (first decreases as the front approaches and then increases behind the front)

Characteristics of a Cold Front

• Steep leading edge: the vertical slope of a cold front surface is 1:50 - 1:100 (ratio of vertical rise to horizontal distance). For comparison: warm fronts have ratios 1:200 – 1:300.

• The steeper the edge, the faster the front (the effect of surface friction).

• Cold fronts tend to move faster than all other types of fronts.

• Cold fronts tend to be associated with the most violent weather among all types of fronts.

• Cold fronts tend to move the farthest while maintaining their intensity.

Cold Front: cloud and precipitation patterns

• The warm, moist air ahead of the front is forced upward and condenses♦ Cirrus clouds well ahead of

the front ♦ Strong thunderstorms with

heavy showers and gusty winds along and ahead of the front: squall lines

♦ Broad area of cumulus clouds immediately behind the front (although fast moving fronts may be mostly clear behind the front).

Warm Fronts

• Warm front - in which warm air replaces cooler air at the surface

• Notice the difference in♦ Temperature♦ Dew point♦ Wind direction♦ Pressure

• Notice the presence of precipitation well ahead of the front

Characteristics of a Warm Front• The slope of a typical warm front is 1:200 (more gentle than cold

fronts) -> warm fronts tend to advance more slowly.

• Warm fronts are typically less violent than cold fronts.

• Overrunning: warmer, less-dense air rides up and over the colder, more-dense surface air.

• Frontal inversion: temperature inversion at the front -> stable atmosphere

Warm Fronts: cloud and precipitation patterns

•Although they can trigger thunderstorms, warm fronts are more likely to be associated with large regions of stratus clouds and light to moderate continuous rain.

•Warm fronts are usually preceded by cirrus first, then altostratus or altocumulus, then stratus and possibly fog.

•At the warm front, gradual transition.

•Behind the warm front, skies are relatively clear.

Occluded fronts.

• Cold fronts move faster than warm fronts. They can catch up and overtake their related warm front. When they do, an occluded front is formed.  

• Cold occlusion: very cold air behind, not so cold air ahead of, the warm front

• The upper warm front follows the surface occluded front

Cold occlusion

Warm Occlusion

• Very cold air ahead of, not so cold air behind, the warm front

• The cooler air from the cold front cannot lift the very cold air ahead, rides “piggyback”

• The warm front aloft precedes the surface occluded front

Stationary Front

• Stationary front- a front which does not move or barely moves.

• Stationary fronts behave like warm fronts, but are more quiescent.

• Many times the winds on both sides of a stationary front are parallel to the front and have opposite direction.

• Typically stationary fronts form when polar air masses are modified significantly so as to lose their character (e.g., cold fronts which stall).

• Typically there is no strong precipitation associated with stationary fronts (why? – no big contrast in the air mass properties, no air uplifting and condensation).

Weakening/Strengthening of the Front

• Frontogenesis:♦ The front intensifies.♦ Why? – The temperature

(humidity) contrast across the front is increasing.

♦ Example: cP air mass moves over warm ocean water.

• Frontolysis:♦ The front weakens and dissipates♦ Why?-the air masses start losing

their identities.♦ The temperature (humidity)

contrast across the front is decreasing.

♦ Typical for slow moving fronts