A violent tornado tore the medium sized branches and bark from this tree. CC 3.0

Some still believe that opening windows ahead of a tornado will reduce the damage from the storm, but this is not true.

One of the oldest pieces of tornado folklore is the idea that tornadoes do most of their damage due to the lower atmospheric pressure at the center of a tornado, which causes the house to explode outward. As the theory goes, opening windows helps to equalize the pressure.

The source of this myth is from the appearance of some destroyed structures after violent tornadoes. When one wall receives the extreme pressure of tornado winds, it will likely collapse inward. This then leads to a considerable outward pressure on the three remaining walls, which fall outwards as the roof falls down, creating the impression of a house which has exploded. Indeed, damage surveys of "exploded" houses usually show at least one wall which has blown inward. Additionally, if the roof is lifted before any walls fail, the walls can fall in any direction. If they fall outward, this structure can also appear to have exploded.

In even the most violent tornadoes, there is only a pressure drop of about 10%, which is about 1.4 pounds per square inch (9.7 kPa). Not only can this difference be equalized in most structures in approximately three seconds, but if a significant pressure differential manages to form, the windows will break first, equalizing the pressure. Additionally, as the windows are the most fragile parts of a house, in a significant tornado flying debris will likely break enough windows to equalize any pressure difference fairly quickly. Regardless of any pressure drop, the direct effects of a tornado's winds are enough to cause damage to a house in all but the weakest tornadoes.

Current advice is that opening windows in advance of a tornado wastes time that could be spent seeking shelter. Also, being near windows is very dangerous during a severe weather event, possibly exposing people to flying glass.

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. A violent tornado can destroy a house whether its windows are open or closed.

See also: Tornado Myths

Source: Wikipedia (All text is available under the terms of the Creative Commons Attribution-ShareAlike License)