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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.
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Class #21: Monday, October 18, 2010 Fronts Class #21: October 18, 2010
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. Class #21: October 18, 2010
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. Class #21: October 18, 2010
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. Class #21: October 18, 2010
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. Class #21: October 18, 2010
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 Class #21: October 18, 2010
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 #21: October 18, 2010
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 #21: October 18, 2010
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. Class #21: October 18, 2010
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. Class #21: October 18, 2010
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 #21: October 18, 2010
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 #21: October 18, 2010
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 #21: October 18, 2010
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 #21: October 18, 2010
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 #21: October 18, 2010
Early stages in the life cycle of the extratropical cyclone Class #21: October 18, 2010
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 Class #21: October 18, 2010
The real extratropical cyclone Class #21: October 18, 2010
The earlier time Class #21: October 18, 2010
The later time Class #21: October 18, 2010
Frontal inversions, freezing rain, and sleet Class #21: October 18, 2010
Frontal inversions, freezing rain, and sleet Class #21: October 18, 2010
Later stages in the life cycle of the extratropical cyclone Class #21: October 18, 2010
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 #21: October 18, 2010
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. Class #21: October 18, 2010
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. Class #21: October 18, 2010
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 #21: October 18, 2010
3-D view of extratropical cyclone life cycle Class #21: October 18, 2010
Divergence aloft and surface low pressure Class #21: October 18, 2010
