Why are you supposed to open the windows of your house during a tornado?
Or better yet, why aren't you supposed to?
Each year, almost eight hundred tornadoes touch down in the USA and claim around 190 lives. One mid-western tornado claimed 689 lives in three hours' time on March 18, 1925.
Folks in the Midwest get more than their share of tornadoes. Most occur in a wide swath of land known as "Tornado Alley," which runs from Texas to Iowa.
People in the Midwest have been taught since childhood to open the windows and run to the basement during tornadoes. The rationale was that during a tornado a sudden drop in air pressure would cause the house to explode if it was shut up tight. Recently, however, meteorologists have realized that opening windows does not do too much to help equalize pressure, as most houses are fairly well ventilated anyway.
Houses do not really explode during a tornado as was thought. They are blown apart by its 100-mile-per-hour winds. Opening the windows actually can do more damage to your home than good. If a tornado just misses your house, the open windows can contribute to your home's destruction. The strong winds of a tornado blowing on a house can cause a pocket of low pressure on the opposite side, which tugs outward on the leeward wall. If the windows on the windward wall are open, it allows the winds to blow through the house and push on the leeward wall, helping to topple the house.
While the advice about opening your windows may have been wrong, the part about getting into the basement is still valid. The basement is the safest place to seek shelter from a tornado.
Tornadoes form when warm, moist air from the Gulf of Mexico becomes trapped beneath the cooler, heavier air from the Rockies. The warm air tries to rise, and the cool air tries to descend. Severe thunderstorms ensue. Occasionally, the warm air breaks through the cool air cap above and violently rushes upward, causing a tornado. Wind speeds can reach from two hundred to five hundred miles per hour, and can drive a piece of straw (weight to bend: .8 grams) through a tree trunk like a nail!
The average tornado leaves a path of destruction about one thousand feet wide and several miles long. Thankfully, they are short lived, dying out in about twenty minutes. Tornado paths vary in length from a few feet to nearly three hundred miles, but the average is about five miles. The diameter of tornadoes averages about six hundred and fifty feet, but can be anywhere from a few feet to over a mile. The average forward speed is about thirty miles per hour. Seventy-five percent of tornadoes occur from March to July. The month of May has the most, about four per day, but the most violent tornadoes usually strike in April. Tornadoes are most common between 4:00 and 6:00 PM, when the surface air is the most unstable. They are least likely to develop before dawn, when the air is most stable.
While the notion that an entire house can be picked up and carried to some far-off land, such as The Wizard of Oz, is pure fantasy, tornadoes have accomplished some pretty impressive and unusual feats. A railroad car carrying over one hundred people was once picked up and deposited some 80 feet away. Another tornado uprooted a schoolhouse and deposited it (along with the 85 students inside) 200 feet away. The house didn't survive, but all the children did. And while it has never (technically) rained cats and dogs, it has rained frogs and toads... after they had been sucked out of a pond by a tornado!