Radios and weather reports are helpful, but tornadoes are tricky creatures and can appear out of nowhere. Keep an eye on the sky. The first clue a tornado is possible is a sickly green color. Clouds are another good indicator. If they're moving quickly, you should keep an eye on them, and if one starts to rotate in the shape of a funnel, you should take cover immediately. If a tornado watch or warning has been issued, hail is cause for concern. If you see flying debris or hear the sound of a train, head for your shelter immediately.