Para - we see these clouds all the time in my part of the world along the Texas Gulf Coast.
They are called cumulus clouds but, in Texas, we call them "thunderheads."
Quote:
Cumulus clouds are puffy clouds that sometimes look like pieces of floating cotton. The base of each cloud is often flat and may be only 1000 m (330 ft) above the ground. The top of the cloud has rounded towers. When the top of the cumulus resembles the head of a cauliflower, it is called cumulus congestus or towering cumulus. These clouds grow upward, and they can develop into a giant cumulonimbus, which is a thunderstorm cloud.
Read more at this link and photos, too: http://eo.ucar.edu/webweather/cumulus.html
And, yes, they can collapse (or disappear in your case) once they've expended all their energy.
Pretty freaky looking, right?
Everytime I see a really huge one, I hear the music tones from Close Encounters and expect to see a UFO land!
