Tornadoes that occur over oceans are called waterspouts, they are usually weaker than tornadoes. The condition for a severe thunderstorm and for a tornado are basically same. Most violent tornadoes are formed from powerful thunderstorms known as supercells. Some sign of a coming tornado are a light rain, then heavier rain and rain mixed with hail. Tornadoes usually occur during the spring and early summer.
Tornadoes usually lasts only a few minutes, but they are very intense.
Each year, the United States ranks number one in tornado incidence with Australia coming in second. Very often, the most devastating ones occur in Bangladesh. The damage from a tornado is determined by the speed of its winds.