Pictured is a time-lapse video taken last Sunday detailing the formation of one such violent supercell in eastern Wyoming, USA. Starting as part of a large and dark thunderstorm complex, the supercell comes together along with a large rotating updraft of air known as a mesocyclone.
During the middle part of the video, the kilometer-wide supercell can be seen swirling ominously with a nearly flat bottom. Toward the end of the video, another swirling supercell cloud forms but then quickly dissipates.
Click image to see the movie.