A tornado is a tube of violently spinning air that touches the ground. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a narrower sense, only to name hurricanes or typhoons. Most tornadoes have wind speeds less than 110 miles per hour, are approximately 250 feet across, and travel a few miles before dissipating. The most extreme can attain wind speeds of more than 300 mph, stretch more than two miles across, and stay on the ground for dozens of miles.
Tornadoes often develop from a class of thunderstorms known as supercells. Other tornado-like phenomena that exist in nature include the gustnado (short-lived, low-level rotating cloud), dust devil, fire whirls, and steam devil. Tornadoes have been observed on every continent except Antarctica. However, the vast majority of tornadoes in the world occur in the Tornado Alley region of the United States (the area between the Rocky and Appalachian Mountains).
Occasionally, a single storm will produce more than one tornado, either simultaneously or in succession. Multiple tornadoes produced by the same storm cell are referred to as a ‘tornado family.’ Several tornadoes are sometimes spawned from the same large-scale storm system. If there is no break in activity, this is considered a tornado outbreak.
Tornadoes in the dissipating stage can resemble narrow tubes or ropes, and often curl or twist into complex shapes. These tornadoes are said to be ‘roping out': the length of their funnel increases, which forces the winds within the funnel to weaken due to conservation of angular momentum. Tornadoes normally counterclockwise in the northern hemisphere and clockwise in the southern.
The most extreme tornado in recorded history was the Tri-State Tornado, which roared through parts of Missouri, Illinois, and Indiana in 1925. It was likely an F5, though tornadoes were not ranked on any scale in that era. It holds records for longest path length (219 miles), longest duration (about 3.5 hours), and fastest forward speed for a significant tornado (73 mph) anywhere on Earth. In addition, it is the deadliest single tornado in United States history (695 dead).
There are several different scales for rating the strength of tornadoes. The Fujita scale rates tornadoes by damage caused, and has been replaced in some countries by the updated Enhanced Fujita Scale. An F0 or EF0 tornado, the weakest category, damages trees, but not substantial structures. An F5 or EF5 tornado, the strongest category, rips buildings off their foundations and can deform large skyscrapers.
Folklore often identifies a green sky with tornadoes, and though the phenomenon may be associated with severe weather, there is no evidence linking it specifically with tornadoes. It is often thought that opening windows will lessen the damage caused by the tornado. While there is a large drop in atmospheric pressure inside a strong tornado, it is unlikely that the pressure drop would be enough to cause the house to explode. Some research indicates that opening windows may actually increase the severity of the tornado’s damage.
Another commonly held belief is that highway overpasses provide adequate shelter from tornadoes. On the contrary, a highway overpass is a dangerous place during a tornado. An old belief is that the southwest corner of a basement provides the most protection during a tornado. The safest place is the side or corner of an underground room opposite the tornado’s direction of approach (usually the northeast corner), or the central-most room on the lowest floor. Taking shelter in a basement, under a staircase, or under a sturdy piece of furniture such as a workbench further increases chances of survival.