MART Vol. II MO/MP - NESA - Civil Air Patrol

measurements of the height of the front above an airport. The following is amethod of determining the approximate height of the front, considering that windshear is most critical when it occurs close to the ground.• A cold front wind shear occurs just after the front passes the airport andfor a short period thereafter. If the front is moving 30 knots or more, thefrontal surface will usually be 5,000 ft. above the airport about 3 hoursafter the passage.• With a warm front, the most critical period is before the front passes theairport. Warm front shear may exist below 5,000 ft. for approximately 6hours; the problem ceases to exist after the front passes the airport. Datacompiled on wind shear indicate that the amount of shear in warm frontsis much greater than that found in cold fronts.• Turbulence may or may not exist in wind shear conditions. If the surfacewind under the front is strong and gusty there will be some turbulenceassociated with wind shear.The pilot should be alert to the possibilities of low-level wind shear at any timethe conditions stated are present.3.7 ThunderstormsA thunderstorm is any storm accompanied by thunder and lighting. It usuallyincludes some form of precipitation, and can cause trouble for aircraft in manyforms: turbulence, icing, poor visibility, hail, wind shear, micro bursts, lightning,and, in severe cases, tornadoes.Individual thunderstorms may often be very local in nature, although theyoften form along weather fronts and appear to march across the land in long lines.This is the situation when weather forecasters announce that a line ofthunderstorms is approaching, and thunderstorm warnings go into effect.Individual thunderstorms are rarely larger than 10 miles in diameter, and typicallydevelop, mature, and dissipate within an hour and a half at the most. Each isproduced by the growth of a puffy cumulus cloud into a cumulonimbus cloud. Thesevere elements of a thunderstorm result from the vertical air movement, orconvective activity, within the storm.Thunderstorms may be studied by dividing them into three separate growthstages: the cumulus, or building stage, the mature stage, and the dissipatingstage. Figure 3-5 demonstrates the physical appearances of each stage of thedeveloping storm.57