How does a tornado form?
Most tornadoes form from thunderstorms. You need warm, moist air and cool dry air . When these two air masses meet, they create instability in the atmosphere. Then the air spirals around. Tornadoes are formed when both high pressure and low pressure air interact. In most tornado's you would probably see cumulonimbus clouds. Tornado speeds can vary from 110mph to 300mph in extreme cases.
Here is a picture from April 1939 tornado outbreak sequence. They went thourgh the Great Plains and Mississippi Valley. One tornado was a F5 that killed 7 people. And it ingured 316 people. Out of all 39 tornado's 25 significant, 3 violent, 11 killer.
May 18-21 2013 Midwest,West South Central States tornado out break. There was 67 tornado's produced. Several tornadoes especially in Oklahoma there was two terrible tornadoes. An EF4 killed two people in the Shawnee area on May 19. One day later a terrible EF5 it killed 24 people. Other strong tornadoes struck elsewhere in Oklahoma, particularly in Carney on May 19, and in Kansas, Illinois and Ontario.
Impact on Human Life
Know we have a warning to know when a tornado is coming we have about 30 minutes notice before we know the tornado will come our way. 75 years ago you knew it was coming your way when you saw it.