A cumulonimbus incus (Latin incus, "anvil") also known as an anvil cloud is a cumulonimbus cloud which has reached the level of stratospheric stability and has formed the characteristic flat, anvil-top shape. They can form into supercells resulting in severe storm phenomena such as tornadoes, flash flooding and downbursts. The cumulonimbus incus is formed when the updraft is strong and vigorous. They form from cumulonimbus calvus which have puffy tops. A cumulonimbus incus means that the thunderstorm is in its mature stage and it may bring severe weather. So if a cumulonimbus incus is sighted, people in its path must take shelter immediately to protect themselves from multiple severe weather threats.