A tornado is a violent, rotating column of air that extends from a storm cloud to the ground. Tornado winds can reach speeds of over 250 miles an hour, and they are often preceded by hail.
If a tornado forms over or moves to water, it's called a waterspout.
What causes a tornado?
Scientists haven't yet determined the exact criteria necessary for a tornado to form. Tornadoes start when warm, moist air (in the United States, this usually comes up from the Gulf of Mexico) mixes with cool, dry air (down from Canada). But this is also how many thunderstorms begin, most of which don't even come close to spawning tornadoes. Scientists are continually testing theories of tornado formation but have yet to form a complete understanding of the phenomenon.
How long can a tornado last?
Tornadoes can wreak their havoc for less than minute or for over an hour. Extremely long-lasting tornadoes are rare, though, and early-20th-century twisters reported to last for such a long time were probably a series of tornadoes and not a single occurrence. Most tornadoes last for less than 10 minutes.
How are tornadoes measured?
In the early 1970s, Dr. T. Theodore Fujita developed a wind rating system that came to be known as the F-scale. This scale was designed to estimate wind speeds, both tornadic and straight-line, based on the damage the winds caused. The scale ranges from F0, meaning no damage, to a theoretical F12, which represents winds blowing at the speed of sound.
The original F-scale suffered from some deficiencies, most notably its subjectivity. Highly trained engineers and meteorologists could look at the same damage and come up with different F-scores. So the F-scale was replaced in 2007 with the Enhanced F-scale. The EF-scale is more precise because it is calculated by meteorologists and engineers based on 28 different damage indicators that take into account different types of structures and different levels of architectural durability. The EF-scale ranges from EF-0 to EF-5 — no matter how astounding the damage, a tornado will never be rated higher than EF-5.
| EF Number | Estimated 3-second Wind Gust Speed (mph) |
|---|---|
| 0 | 65–85 |
| 1 | 86–110 |
| 2 | 111–135 |
| 3 | 136–165 |
| 4 | 166–200 |
| 5 | Over 200 |