micro-high (small high) may form behind it. Afterpassage of the squall line, the wind backs to southerlybefore the cold frontal passage. When the squall linedissipates, severe weather may develop along thefast-moving cold front.Turbulenceissevereinthesquall-linethunderstormsbecauseofviolentupdraftsanddowndrafts. Above the freezing level, icing may occur.Hailisanotherpossibilityinthesquall-linethunderstorm and can do extensive structural damage toan aircraft. Under the squall line, ceiling and visibilitymay be reduced because of heavy rain showers. Fog is arare occurrence because of the strong wind and gusts,but it may be found in isolated cases. Tornadoesfrequently occur with squall lines when the warm airmass is extremely unstable.Great Plains Squall LinesNot all instability lines that reach the mature orsquall-line stage develops in advance of a fast-movingcold front. The Great Plains region of the United Stateshas a high frequency of these squall lines. The GreatPlains type of squall lines also develop in warm, moist,unstable air masses. The necessary lifting or triggermay be supplied by intense thermal heating, orographiclifting,orconvergentwindsassociatedwithalow-pressure area.FORMATION.—The Great Plains squall lineformswhenanextremelyunstableconditiondevelops—normally in spring and summer. Extremelyunstable conditions exist when moist mP air cools inthe upper levels because of the evaporation of fallingprecipitation. This cooler air aloft then moves overwarm moist mT air (or even warm, moist, highlymodified mP air) at the surface. If a sufficient triggersuch as a steep lapse rate of a lifting mechanism exists,this extremely unstable situation rapidly develops into asquall line.WEATHER.—The weather associated with theGreat Plains squall line is the same as that found withthe prefrontal squall line. Because of the extremeinstability, tornadoes are a common occurrence.REVIEW QUESTIONSQ4-11. What is the pressure tendency with thepassage of a slow moving cold front?Q4-12.What is the normal slope of a fast moving coldfront?Q4-13.Where do prefrontal squall lines normallyform?THE WARM FRONTLEARNING OBJECTIVE: Describe thecharacteristics and weather of warm fronts atthe surface and aloft.A warm front is the line of discontinuity where theforward edge of an advancing mass of relatively warmair is replacing a retreating relatively colder air mass.The slope of the warm front is usually between 1:100and 1:300, with occasional fronts with lesser slopes.Therefore, warm fronts have characteristically shallowslopes caused by the effect of surface friction thatretards the frontal movement near the ground.SURFACE CHARACTERISTICSWarm fronts move slower than cold fronts. Theiraverage speed is usually between 10 and 20 knots. Theymove with a speed of 60 to 80 percent of the componentof the wind normal to the front in the warm air mass.The troughs associated with warm fronts are not aspronounced as those with cold fronts and sometimesmake location difficult on the surface chart. Thepressure tendency ahead of the front is usually a rapidor unsteady fall with a leveling off after frontal passage.A marked decrease in isallobaric gradient is noticed inthe warm sector except when rapid deepening is takingplace. The wind increases in velocity in advance ofwarm fronts because of an increase in pressure gradientand reaches a maximum just prior to frontal passage.The wind veers with frontal passage, usually from a4-37AG5f0435Figure 4-35.—Typical isobaric pattern associated with aprefrontal squall line.
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