Weather Channel—Jim Cantore gives an incredible 3D view into how a tornado forms. 

How to prepare for a Tornado by Ready.gov

• Identify safe rooms built to FEMA criteria or ICC500 storm shelters or other potential protective locations in sturdy buildings near your home, work, and other locations you frequent so you have a plan for where you will go quickly for safety when there is a Warning or an approaching tornado.
• For schools, malls, and other buildings with long-span roofs or open space plans, or many occupants, ask the building manager to identify the best available refuge.  
• Build an emergency kit and make a family communications plan.
• Listen to NOAA Weather Radio or to commercial radio or television newscasts for the latest information. In any emergency, always listen to the instructions given by local emergency management officials.
• Be alert to changing weather conditions. Look for approaching storms.
• Look for the following danger signs:
  • Dark, often greenish sky
  • Large hail
  • A large, dark, low-lying cloud (particularly if rotating)
  • Loud roar, similar to a freight train.
  • If you see approaching storms or any of the danger signs, be prepared to take shelter immediately.