Dark, often greenish sky. Look for signs of debris below the visible funnel. Some tornadoes are clearly visible, while others are obscured by rain or nearby low-hanging clouds. Before a tornado hits, the wind may die down, and the air may become very still. An approaching cloud of debris can mark the location of a tornado, even if a funnel is not visible.
Tornadoes generally occur near the trailing edge of a thunderstorm.
It is not uncommon to see clear, sunlit skies behind a tornado. A mobile home can overturn very easily, even if precautions have been taken to tie down the unit. When a tornado warning is issued, take shelter in a building with a strong foundation. If shelter is not available, lie in a ditch or low-lying area a safe distance away from the unit. If you smell gas or hear a blowing or hissing noise, open a window and quickly leave the building.