Mammatus clouds can also be a sign that tornadoes will develop. Mammatus clouds are created as ice crystals within the anvil sublime into the dry air above them. They are a sign that the anvil of the storm is extremely large and the upcurrents extremely violent. Hail is yet another sign that tornadic activity may occur. Hail is formed when droplets are carried above the freezing-level of the cloud by strong updrafts.
Hailstones grow by repeated freezing and partial melting.
As the hailstones rise and fall they eventually move toward the rear of the cloud and fall in the downdraft. As with mammatus clouds, hail results from strong upcurrents, one of the primary ingredients for tornado formation.