Class #21: Monday, October 18, 2010

Class #21: October 18, 2010 1

Class #21: Monday, October 18, 2010

Fronts


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





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


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


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

Early stages in the life cycle of the extratropical cyclone


Frontal lifting and cloud types

• Frontal lifting is weaker at warm fronts than cold fronts

• Convergence is weaker at warm fronts than cold fronts

• Convection is rare at warm fronts, common with cold fronts

• Layer clouds are common with fronts


The real extratropical cyclone


The earlier time


The later time


Frontal inversions, freezing rain, and sleet


Frontal inversions, freezing rain, and sleet


Later stages in the life cycle of the extratropical cyclone



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



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.





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.



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

3-D view of extratropical cyclone life cycle


Divergence aloft and surface low pressure


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Class #21: Monday, October 18, 2010

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