What makes tornadoes so destructive?      The biggest factor as to why tornadoes are so destructive is wind speed. Wind speeds in a tornado can range from 40 miles per hour (F0) to 318 miles per hour (F5). These winds are capable of pulling apart houses that are not built to withstand them due to what is known as Bernoulli's principle, which "relates the elevation, pressure, and speed of a fluid" (Heckert, Bernoulli's Principle and Storms) and is the reason why planes stay in the air. When high-speed winds go over, say, the roof of a building, a pressure difference is generated due to the fact that the air above the roof is moving fast and the air below the roof (inside the building) is not moving as fast; the fast-moving air will have a lower pressure than the air inside the building, and the pressure gradient force will cause the high pressure air to want to move up to the area of low pressure. Now, pressure is defined as a force per unit area; the greater the pressure of an object, the more force is being applied to it (or, the same amount of force is being applied to a smaller area). In tornadoes ranging from F2 to F5, the wind speeds are very high and thus the pressure difference is large; this results in enough of a pressure gradient force to rip the roof right off a house. The University of Wisconsin has an applet that demonstrates this; in this applet one can change the funnel size and pressure difference of the tornado and then see what happens. It is interesting to note that leaving the pressure difference the same and changing the funnel size has no effect on the amount of damage done by the tornado; only the pressure difference alters the amount of damage done.      Another factor that makes wind speed so dangerous is the materials picked up by tornadoes. In a tornado, the winds are moving so rapidly that any object picked up by them can conceivably turn into a missile. The University of Texas has a research group that does tests involving shooting 2x4's out of a cannon at F5 tornado speeds to see what happens. It turns out that many materials will not stop a wooden board traveling at these speeds. For example, a car door, when hit by one of these 2x4's, can be ripped off its hinges and folds inwards in the direction of the impact. Meanwhile the 2x4 keeps going. They are also capable of going through several inches of bricks without being slowed down at all. Many houses are not built to withstand this kind of pummeling, so when they get hit by objects that have been picked up by a tornado they are very easily damaged and anyone inside is in danger of serious injury or death.