If the upper level of air is disturbed and a weak spot develops, the warm air can rise through the opening. At this point the tornado starts to build. More warm air rushes upward and cold air drops to take its place. As the warm air rises, it begins to cool. It releases heat into the surrounding atmosphere and adds more energy to the storm.
Winds begin to swirl and gain speed.
If the storm's strong enough, a funnel cloud will form. As you know, tornadoes are very dangerous. The National Weather Service is constantly monitoring for the tell-tale sign of possible twister. When meteorologists issue a tornado watch it means that conditions are right in the atmosphere for a tornado to form. However, when a tornado warning is issued it means an actual tornado has been sighted.