Class #20: Friday, October 15

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Class #20: October 15, 2010 1

Air MassesFronts

Air Masses

• An air mass is an extremely large body of air whose properties of temperature and moisture content (humidity) are similar in any horizontal direction.

• In a typical year, air mass weather kills more people in the U.S. than all other weather phenomena combined.– Heat waves, most dangerous weather type– Cold air outbreaks are also dangerous

Air mass types by temperature

• Polar (P): formed poleward of 60º– Cold or cool

• Arctic (A): formed over the arctic– Very cold

• Tropical (T): formed within 30º of the equator– Hot or warm

Air mass types by moisture amount

• Continental (c): formed over large land masses– Dry

• Maritime (m): formed over the oceans– Moist

Fronts

• Air masses are important in themselves, and at their boundaries, fronts occur.

• A front is the transition zone between two different air masses.

• Fronts were named around the time of World War I (1910s) because they had disruptive weather and looked like the boundaries on military maps separating armies.

The generic front

• Is the boundary between 2 (3 for the occluded front) air masses of differing temperature.

• Slopes in the vertical up from the surface toward the colder air mass.

• Always has the warmer air mass above the colder air mass (never the reverse).

• Is the scene of frontal lifting if winds blow in part across the front.

The generic front (continued)

• Always has a temperature contrast at the surface between the two air masses.

• Is of synoptic scale along the front and mesoscale across the front.

• Has a cyclonic (counterclockwise in NH) wind shift, a minimum (trough) in surface pressure, and usually a change in humidity across the front.

The generic front (continued)

• Looks like a line on a surface weather map.

• Is called a frontal zone where it meets the ground on the surface weather map.

• Is an area where weather conditions change rapidly over short distances (maybe even a few miles) from one air mass to another.

Different types of fronts

• Stationary front: – Remains in roughly the same location– Surface winds in both air masses blow along

the front– Precedes the development of an extratropical

cyclone– Common in the location of the polar front– Separates T and P– More on stationary fronts later

Cold and warm fronts

• Form together when a stationary front starts to move

• Form when the surface winds along a stationary front start to blow across the front

• Form when a stationary front deforms into a comma or wavelike shape

• Form when a surface low center develops on the stationary front

Cold and warm fronts

• Are named by the temperature changes that result after an air mass passes

• Are enhanced by convergence that intensifies contrasts in temperature, pressure, wind, and humidity

• Air is colder after a cold front passes• Air is warmer after a warm front passes

Cold fronts

• Have a slope up from the surface that is closer to vertical than warm fronts.

• Have the colder air mass replacing the warmer air mass at the surface.

• Have some of the most dramatic frontal passages at the surface—greatest weather changes in the shortest amount of time.

Cold fronts

• Move fairly rapidly• May have thunderstorms in the warm

moist unstable air ahead of the front (mT) or along the front

• Usually have fairly narrow rainbands along and across the front

• Frequently lines of thunderstorms called squall lines form ahead of and parallel to cold fronts.

Real cold fronts

• Don’t always look exactly like the idealized fronts in the textbook

• The meteogram shows a frontal passage at about 2200 UTC

• May be dry, with no clouds or precipitation• May have blowing dust• Can cause precipitation even at night

Warm fronts

• Have a slope upward from the ground inclined more towards the horizontal than cold fronts

• Have weaker vertical motions than warm fronts

• Have a special name for the upglide of horizontal and vertical motion called overrunning, warmer air over colder air

Class #20: Friday, October 15

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