Class #20: Friday, October 16

Air Masses Fronts Class #20: October 16, 2009 1

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 Class #20: October 16, 2009 2

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 Class #20: October 16, 2009 3

Air mass types by moisture amount

• Continental (c): formed over large land masses – Dry • Maritime (m): formed over the oceans – Moist Class #20: October 16, 2009 4

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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.

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Class #20: October 16, 2009 9

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 warmer air mass (never the reverse).

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

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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.

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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.

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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 air masses Class #20: October 16, 2009 13

More on stationary fronts

• Surface winds blow along the front, so that it remains stationary • Above the surface, winds may blow across the front and cause frontal lifting, cloudiness, and precipitation with the stationary front Class #20: October 16, 2009 14

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 Class #20: October 16, 2009 15

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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 Class #20: October 16, 2009 17

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.

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Class #20: October 16, 2009 19

Class #20: October 16, 2009 20

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 warm fronts.

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Class #20: October 16, 2009 22

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 Class #20: October 16, 2009 23

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: October 16, 2009 24

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Class #20: October 16, 2009 27

Real warm fronts

• The meteogram shows a warm frontal passage at 1000 UTC • Move at about half the speed of cold fronts • Have a sequence of layer clouds • Have the highest clouds well ahead of the front at the surface • Are very 3-dimensional • Can stall, for example in mountains Class #20: October 16, 2009 28

Real warm fronts

• Can stall when the cold dense air is hard to replace • Can have broad bands of moderate precipitation • Can produce long periods of precipitation when they stall • Can produce frontal fog with evaporation • Are associated with freezing rain and sleet Class #20: October 16, 2009 29

Stationary fronts (continued)

• Weather along a stationary front can resemble a warm front • Although the front is stationary at the surface, strong winds aloft may blow across the front aloft, causing overrunning • Can have extended periods of cloudiness and precipitation on the cold side of the front.

• Can have a jet stream aloft Class #20: October 16, 2009 30

Occluded fronts

• Involve 3 air masses – 2 polar air masses at the surface, usually mP and cP – 1 tropical air mass, mT that has been lifted entirely off the surface, and is occluded or hidden from the surface weather map • Have weather like warm fronts where mT and mP air masses meet, and weather like cold fronts where mT and mP air masses meet Class #20: October 16, 2009 31

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Occluded fronts (continued)

• Are of 2 types, warm occlusions and cold occlusions, named for the change in temperature behind the front.

• Warm occlusions form on the west coast of the U.S. and Europe, when the air mass behind the front is usually from the ocean, mP.

• Cold occlusions frequently form in the eastern half of the U.S., with mP ahead and cP behind the front.

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Class #20: October 16, 2009 34

Class #20: October 16, 2009 35

Drylines

• Are not true fronts because there is no temperature contrast across the front.

• Resemble fronts because there is a boundary between air masses, cT and mT.

• Resemble fronts because there is a wind shift, pressure trough, convergence, and often convective clouds along the dryline.

• Occur in West Texas in spring and early summer, and severe weather can occur.

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Class #20: October 16, 2009 37

Drylines (continued)

• Have a strong contrast in humidity and wind direction across the front • Can persist for several days.

• Can move westward at night (called the dew-point front) • Move from west to east during the day • Are a powerful source of convergence • The moister air is lighter, and rises Class #20: October 16, 2009 38