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(2) Technically, the funnel or spiral of air must touch the ground to be called a tornado.
When they occur over water, they are called waterspouts. When the characteristic spiraling
clouds extend downward from the cloud base but do not reach the surface, they are called
funnel clouds.

(3) Frequently, cumulonimbus mamma clouds occur in connection with strong
turbulent thunderstorms and tornadoes. Tornadoes occur with individual separate
thunderstorms at times but they more frequently occur with cold fronts and squall lines.

i. Squall Lines

(1) A squall line is a narrow band of active thunderstorms. They are considered
non-frontal because they often develop a few hundred miles ahead of a cold front in moist
unstable air. The line may be several hundred miles long and may vary in width up to 50
miles.
(2) The squall line often contains severe, long-lasting thunderstorms. It is the major
hazard to heavy aircraft that are flying by use of instruments due to restricted visibility.

A squall line usually forms quickly and moves rapidly. It generally completes its life-cycle
within 24 hours and reaches maximum intensity in the late afternoon or early night.