They form when you get
humid air that rises for whatever reason,
condenses and forms
clouds, and dumps
precipitation and has strong enough
updrafts and
downdrafts to produce
lightning. The really nasty ones form when you get a bunch of
warm moist air on the ground, and then
cool dry air slides in on top (this particular event is called a
dry line).
This makes the
atmosphere incredibly
unstable since the warm air wants to go up really quickly. As it goes up and forms
clouds, it creates an
updraft that has to suck in more warm moist air from the
ground to replace it. So the
storm now
feeds itself and continues to grow until it tops out at 30,000 feet and gets pushed around by the
upper-level winds; which blow the top of the storm out in front of it to produce the
classic anvil shape. Eventually the droplets of moisture get too big for the updrafts to hold them so they fall, creating downdrafts. Eventually the downdrafts tear the storm apart, but often not before it dumps
baseball-sized hail and produces
tornados if you're un
lucky.