It varies. With good visibility, some tornadoes may be visible from over 10 miles away. In other cases a tornado may be completely obscured by rain and may not be visible until it is upon you.
If you can see the tornado
Thus Tuscaloosa-Birmingham tornado of 2011 varied in width, but at its maximum the tornado was about a mile and a half wide.
A a radar is better: it can detect a tornado at a distance. A barometer would be of no use unless the tornado came dangerously close.
The greatest distance a tornado is recorded to have traveled is 219 miles.
No. If there is a tornado the sky will be over cast an you won't be able to see the moon.
Tornado Sirens going off
The average distance traveled by a tornado is about 5 miles.
The longest distance a car has been carried in a tornado is believed to be about 1,307 feet (398 meters), which occurred in the 1991 Andover, Kansas tornado. The car was thrown and carried a considerable distance before landing.
If you can see the tornado
A wedge tornado is a tornado that is very large, usually one that is wider than the distance between the clouds and the ground.
Doppler radar can detect the signature of a tornado from a distance.
Yes. There are hundreds of tornado videos on YouTube.
It is safest to stay underground if you hear/see a tornado.
Thus Tuscaloosa-Birmingham tornado of 2011 varied in width, but at its maximum the tornado was about a mile and a half wide.
When you see a tornado, go into a secure building or if there are none near you, huddle in a nearby ditch.
It is difficult to tell for certain. You may be able to eyball the distance if you can see it clearly, but that is not reliable enough to stake your personal safety on. Tornado warning will often call out the location of a tornado relative to one or two communities, which gives an idea of how far away a tornado might be. However, a warning can be several minutes old by the time you read it, which is enough time for a tornado to move a few miles.
The longest a tornado is believed to have listed is about three and a half hours. That same tornado also hold the record for distance traveled: 219 miles.