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Squall lines generally form along or ahead of
cold fronts and
drylines and can produce
severe weather in the form of heavy rainfall,
strong winds, large hail,
and frequent lightning.
Squall lines can extend to hundreds of miles in length,
simultaneously affecting several states at a time. They
also can travel quickly -- at speeds up to 60 mph.
Photo by Doswell
Squall lines typically form in unstable
atmospheric environments in which low-level air can rise unaided
after being initially lifted (e.g., by a front) to the point where
condensation of water vapor occurs.
Heat is released during condensation, resulting in the rising air
becoming lighter than nearby air at the same height. This leads to
an increase in the speed of the rising air which sometimes reaches speeds
above 30 mph. In models this initial lifting is specified through
an idealization of the flow associated with the front or other
lifting mechanism or through the use of observational flow information.