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e. When air passing over a line of mountains has sufficient water vapor to form clouds,

standing lenticular and rotor clouds form downwind from the mountain and indicate

areas of severe turbulence.

f. Wind shear can exist at the horizontal boundary between layers of air moving in

different directions and/or at different speeds, or it can exist along a vertical boundary

when upward or downward currents are in contact with currents moving in a different
direction and/or at a different speed.

g. As more and more water vapor is pulled into the cloud and condenses, the cloud
builds upward into a towering cumulus, and finally becomes a cumulonimbus cloud

and produces a thunderstorm.

h. The main characteristic of the cumulus or "building" stage is the predominant updraft
which may extend from the earth's surface to several thousand feet above the visible

cloud top.

i. The mature stage begins when drops are thrown out from the updraft, or they become
so large that the updraft can no longer hold them or lift them upward, and the drops

begin to fall.

j. Turbulence should also be expected outside of the area of visible cloud, and in the

case of severe thunderstorms, "severe" to "extreme" turbulence can be encountered

several thousand feet and above and up to twenty miles laterally from the storm.

k. Clear ice accumulation in thunderstorms above the freezing level can be so rapid that

an aircraft may become incapable of keeping its altitude and/or attitude.

1. Usually the approach of a thunderstorm is preceded by the first gust and low level

wind shear, then the roll cloud, and finally the starting of the rain.