The thunderstorm downdraft may produce the most dangerous shear conditions associated with the outflow of a thunderstorm. The gust front frequently extends 10 to 15 miles away from the thunderstorm. Extreme wind shears of 10 knots per 100 feet of altitude have been measured immediately behind the gust front, while horizon-tal wind shears of 40 knots per mile have been recorded across the gust front. In addition to the tremendous speed shears reached, the most severe thunderstorms produce directional shears of 90 to 180 . In addition, extreme downdrafts may occur beneath the central region of the storm.
Winds can be significantly different in two air masses forming a front.
Low-level wind shear occurs with a cold front after the front passes. Because cold fronts have a greater slope and normally move faster than warm fronts, low-level wind shear usually lasts less than 2 hours. The wind shear associated with a warm front is more dangerous to airport operations. Strong winds aloft, associated with the warm front, may cause a rapid change in wind direction and speed where the warm air overrides the cold, dense air near the surface. Warm-frontal wind shear may persist 6 hours or more ahead of the front because of the front s shallow slope and slow movement.