EFFECTS CAUSED BY MOVEMENT

Figure 4-45.Orographic effects on a warm front.

Aerographers Mate, Module 05-Basic Meteorology
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EFFECTS CAUSED BY MOVEMENT The weather is greatly affected by the movement of frontal systems. From the time the front develops until it passes out of the weather picture, it is watched closely. The speed of the movement of frontal systems is an important determining factor of weather conditions. Rapidly moving fronts usually cause more severe weather than slower moving fronts. Fast-moving cold fronts often cause severe prefrontal squall lines that are extremely hazardous to flying. The fast-moving front does have the advantage of moving across the area rapidly, permitting the particular locality to enjoy a quick return of good weather. Slow-moving fronts, on the other hand, may cause extended periods of unfavorable weather. A stationary front may bring bad weather and can disrupt flight operations for several days if the frontal weather is sitting over your station. Knowledge of the speed of the frontal system is necessary for accurate forecasting. If the front has a somewhat constant speed, it makes your job and the forecaster’s job comparatively easy. However, if the speed is erratic or unpredictable, you may err as far as time and severity are concerned. If a front was ultimately forecast to pass through your station and instead becomes stationary or dissipates, the station forecast will be a total bust. OROGRAPHIC EFFECTS Mountain ranges affect the speed, slope, and weather associated with a front. The height and horizontal distance of the mountain range along with the angle of the front along the mountain range are the influencing factors. Mountain ranges can affect cold fronts, warm fronts, and occluded fronts differently. Cold Fronts As a cold front approaches a mountain range, the surface portion of the front is retarded and the upper portion pushes up and over the mountain. On the windward side of the mountain, warm air is pushed up along the mountain slope because of the additional lift of a now steeper frontal slope and the mountain itself (view A of fig. 4-44). After the front passes the crest of the mountain, the air behind the front commences to 4-46 COLD A WARM CLEARING B COLD WARM COLDER C EXTREME INSTABILITY STAGNANT WARM AIR AG5f0444 Figure 4-44.—Orographic effects on a cold front.

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