Why are tornadoes so much more destructive than hurricanes? OK, so with Helene that just went through Florida we say that storm surge does the most “catastophic” damage but also tornadoes. In this BLOG I speak of why water is more destructive but why are tornadoes? Wind is wind right?
Wind is wind, yes but the loads are different and the location where they are measured is different as well. When we say that Milton made landfall as a Cat 3 storm that is classified as having winds max sustained winds of 111-129 mph. The height of where this is measured at is 33 feet above the ground for a duration of 1 minute.
Let’s compare that to an EF2 tornado which has wind speeds from 111-135 mph, basically the same. BUT this is defined as the :”The EF scale still is a set of wind estimates (not measurements) based on damage. Its uses three-second gusts estimated at the point of damage based on a judgment of 8 levels of damage to the 28 indicators listed below. These estimates vary with height and exposure. Important: The 3 second gust is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in open exposures, using a directly measured, “one minute mile” speed.”
OK, so there are MANY technical reasons why a 130 mph tornado does more damage than a 130 mph hurricane. Frist, the building code recognizes that say a 130 MPH hurricane design load does NOT impart the same load on a structure at 10′ as it does at 30′. The design load is less because the loads are less as one gets closer to the ground where trees and buildings block the wind. So, if your house is designed for a 130 mph hurricane that does NOT make designed for a 130 mph tornado where, for one item, the wind speed is ESIMATED based upon damage observed at the ground level.
Another item is the internal preasure. The faster air moves the lower the preasure of it is. This in Bernouli. In a tornado there is not enough time for the high preasure inside the structure to escape. We see this in the code as the internal pressure coefficient that is increased from 0.18 to 0.55. That’s 3 X the load from this component. BUT OBSERVATIONS HAVE PROVEN THAT ONE SHOULD NOT OPEN WINDOWS TO TRY AND “EQUALIZE” THIS PRESSURE.
And tornadoes also not only move wind horizontal to the ground but also upward. Think of the classic tornado movie with the flying cow. That’s an upward force that (and not the Bernouli force) is trying to lift things up off the ground.
