THUNDERSTORM SURFACE PHENOMENA

The rapid change in wind direction and speed immediately before thunderstorm passage is a significant surface hazard associated with thunderstorm activity. The strong winds that accompany thunderstorm passage are the result of horizontal spreading of downdraft currents from within the storm as it approaches the ground.

Figure 5-2 shows the nature of the wind outflow and indicates how it is formed from the settling dome of cold air that accompanies the rain core during the mature stage of the thunderstorm. The arrival of this outflow results in a radical and abrupt change in the wind speed and direction. It is an important consideration for aircraft that are landing or taking off.

Wind speeds at the leading edge of the thunderstorm are ordinarily far greater than those at the trailing edge. The initial wind surge observed at the surface is known as the first gust. The speed of the first gust is normally the highest recorded during thunderstorm passage, and it may vary as much as 180 degrees in direction from the surface wind direction that previously existed. The mass of cooled air spreads out from downdrafts of neighboring thunderstorms (especially in squall lines), and often becomes organized into a small, high-pressure area called a bubble high, which persists for some time as an entity that can sometimes be seen on the surface chart. These highs may be a mechanism for controlling the direction in which new cells form.

The speed of the thunderstorm winds depends upon a number of factors, but local surface winds reaching 50 to 75 miles per hour for a short time are not uncommon. Because thunderstorm winds can extend several miles in advance of the thunderstorm itself, the thunderstorm wind is a highly important consideration for pilots preparing to land or take off in advance of a storms arrival. Also, many thunderstorm winds are strong enough to do considerable structural damage, and are capable of overturning or otherwise damaging even medium-sized aircraft that are parked and not adequately secured.

The outflow of air ahead of the thunderstorm sets up considerable low-level turbulence. Over relatively level ground, most of the significant turbulence associated with the outrush of air is within a few hundred feet of the ground, but it extends to progressively higher levels as the roughness of the terrain increases.

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