I saw this cloud formation yesterday around 7:45 PM. Cumulus Mammatus are a pretty amazing sight. They easily identifiable by the drooping pouch type formation of the cloud base. The individual pouches typically only last for about 15 minutes, but the whole Mammatus can last for 2 plus hours. They themselves do not cause severe weather, but are normally a result of severe weather. You normally find them at the base of cumulonimbus clouds or at the top portion of the "anvil" on a towering cumulonimbus. They actually form in descending air and mark the boundary between stable and unstable air. A pretty awesome sight, but pilots beware of possible turbulent conditions.