Underneath - P-3030096

CHAPTER FOUR

AVIATION WEATHER

Figure 4-11 Over the Top

Underneath

If you are unable to circumnavigate the thunderstorms in your area and the ceiling capabilities of

your aircraft will not permit an over-the-top flight, you should consider flying below the base of

the cloud. The speed of downdrafts usually decreases closer to the surface (Unless a microburst

is present!). Therefore, an altitude should be selected which will keep you as far away from the

cloud base as possible and still enable you to maintain adequate terrain clearance. Here you can

use the 1/3 rule which specifies selecting an altitude 1/3 the distance from the surface to the base

of the cloud (Figure 4-12). This procedure is not recommended for areas of mountainous terrain.

Below the storm, expect a low ceiling, poor visibility, and moderate turbulence. Perhaps the

most dangerous threat to flight below a thunderstorm is the downburst or microburst, which can

be deadly to the unsuspecting pilot.

Figure 4-12 Under the Thunderstorm

Penetration

Mission urgency or fuel state dictates whether thunderstorm penetration is required when

avoidance is not possible. The lower in the storm the penetration is made, the less the chance of

encountering hail, structural icing, or being struck by lightning. Therefore, another version of

the 1/3 rule applies: penetrate through the lower 1/3 of the storm, since most hazards are more

4-14 Thunderstorms