Landspouts and waterspouts are weak tornadoes, typically EF2 or less, that usually appear while a thunderstorm is still taking shape. The main distinction between them is where they form: landspouts form on dry land and waterspouts, as the name suggests, form over a body of water. Though they often form funnel clouds like supercell tornadoes, landspouts and waterspouts maintain a thin, rope-like appearance instead of swelling according to their moisture intake. Also, unlike supercell tornadoes, the rotating air that becomes a landspout or waterspout begins closer to the ground, without the presence of the powerful updrafts necessary to form a supercell tornado.

Rating Tornadoes

Through the first half of the 20th century, it was difficult to get a precise measurement of how fast and how hard the wind in a tornado blew. In the early 1970s, Dr. Ted Fujita used the limited data available, together with his own experience and insights, to draw connections between the probable wind speed of a tornado and the severity of the damage it had caused.

But as time went on and technology advanced, meteorologists and structural engineers began making new observations. They knew from experience the wind speed necessary to cause a specific type of damage, such as tearing off a roof. They also began taking into account how other structural factors, such as a building’s foundation and any prior damage, may impact the severity of damage to a particular structure. Eventually, even Dr. Fujita himself acknowledged that his scale needed fine-tuning.

In 2007, a research team of meteorologists and engineers published what they called the “Enhanced Fujita scale,” which used the original Fujita scale as a base and gave a range of windspeeds for each category of tornado, instead of a set number. This scale assigned lower wind speeds to tornadoes that, on the original Fujita scale, were rated F-3 and higher. The EF scale bases its ratings on observable physical damages, rather than measured wind speed.

NOAA lists 28 different buildings and other structures, called “damage indicators,” used to calculate a tornado’s EF value. The damage done to each indicator is rated on an eight-point scale, ranging from “just enough to notice” to “complete destruction,” based on the general construction for that type of building. Each point on that scale corresponds to a range of wind speeds that can cause a specific type of damage. For example, a tornado with 60 mph winds could blow the roof off an outbuilding or shed; however, the damage that same wind inflicted on a brick or concrete building would be negligible to nonexistent.

Location

According to NOAA, an actual tornado has been recorded in each of the 50 states. Since tornadoes are so heavily dependent on weather conditions, they are more common in regions where those weather conditions occur more frequently. The areas of greatest risk change depending on the time of year. Tornadoes most frequently occur in the central states between the Rocky Mountains to the west and the Appalachian Mountains to the east.

Although any state can experience a tornado, “Tornado Alley” is a term invented by the media that refers to a range of states in the central and midwestern U.S. that experience tornadoes more frequently than others; it is not a scientific or meteorological term. The exact states in Tornado Alley vary based on what criteria you are using. Generally, though, it begins in central Texas and stretches north to parts of Canada, and it runs from eastern Colorado to Indiana. Though Tornado Alley used to hang mostly over the central and midwestern parts of the country, an uptick in severe storms in recent years has shown that Tornado Alley is slowly shifting eastward.