Warm and/or moist air encroaching on a mass of cooler or drier air results in a warm front. Warm and moist air is naturally less dense than cold and dry air and will easily "ride" over the top of cool air. Accordingly, a warm front does not represent an abrupt or sudden contrast, as does a cold front (Figure 9.29).
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Warm fronts are represented on weather maps as a red line with "half suns" on the leading edge (Figure 9.30).
The leading edge of a warm front may have a 30,000 foot or greater altitude and be hundreds of miles in advance of the line at ground level separating warm and cold. As the warm air lifts over the cool air, it too cools and at some level condensation begins. The first indication of an approaching warm front may be cirrus clouds (or persistent contrails) followed in time by altostratus and then stratus clouds (Figure 9.31), and possibly widespread and general rain (or snow). Although not considered as violent as weather events associated with a cold front, warm fronts are usually the cause of glaze or freezing rain that can cripple movement of traffic and break trees and power lines. Also, heavy general rains that can occur may result in widespread flooding, as opposed to intense flash flooding often caused by a thunderstorm.
The wind change around a warm front goes from easterly or southeasterly ahead of the front to southerly of southwesterly behind it. Unlike the cold front, advance notice of a warm front is very apparent. Cirrus clouds portend the advance of a warm from 24-36 hours ahead. These clouds are produced by the warm air being lifted to very high levels in the atmosphere. Behind the warm front the dew point and temperature increase as the air mass changes from a cP air mass to a mT air mass. Precipitation can occur, but is more showery type as opposed to the thunderstorm type of the cold front. The extensive cloudiness ahead of the warm front quickly gives way to clear skies. The lift disappears after the front passes. Pressure is usually falling until the front passes, then it may rise slightly as the warm sector of the cyclone passes over.
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Fig. 9.32 A warm front is depicted from southern Canada across the Dakotas. Click image to enlarge.
Usually, the contrast across the warm front is not as striking as across a cold front. But changes can be seen. Ahead of this warm front there is some cloudiness, but not as extensive as usually occurs. This is probably because the air here is very dry after coming off the mountains. The fronts on this map are drawn from 2 hours prior to these observations. Therefore, areas of the eastern Dakotas are probably behind the warm front.