Timelapse footage of low cumulus clouds building during a warm summer afternoon, eventually developing into a thunderstorm. This occurs as the sun heats the ground, which heats the air above it, causing it to rise. As it rises it cools and expands, and the water vapour in the rising air condenses, forming a tall cloud. When the growing cloud hits the tropopause it spreads out into a characteristic anvil shape (best seen midway through the clip). Such clouds typically produce heavy rain and lightning.