Jupiter Florida Real Estate Broker

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.

I realized that my answer to this question in THIS BLOG is too long. I just updated this with some more information ahead of Helene.

Here’s the short answer. You’ll need the elevation of the finished floor of your house. This will be on your building plans, survey perhaps or an Elevation Certificate prepared by a land surveyor. What you’re looking for is something like: “FF Elev = 9.1′ (NAVD 88)” or “FF Elev = 9.1 NGVD 1929”.

Now go to the NOAA site and find the tide station closest to your house. Let’s say that your house is near the “East Bay” tide station inside Tampa Bay. On this tide station page click on the “tide/water levels” and select datums. Now set the datum to NAVD 88 and note the Mean High High Water. In this case it’s +0.86′ by the NAVD 88 Datum, the same one used for your house, let’s call that +1′ by the NAVD 88 datum. NOTE THAT THE ACTUAL TIDES HAVE BEEN RUNING ABOUT 1′ HIGHER THAN THIS BUT NOAA PREDICTS STORM SURGE BASED UPON THE HEIGHT ABOVE THE MEAN HIGH HIGH WATER (MHHW) This is the BLUE line on the graph, NOT the red line whish is the measured tide information. So, when NOAA predicts say a 12′ surge it is 12′ above this MHHW level.

So for our example, basically, for this house there is ABOUT 8′ (9.1′-1′) of differance between the calculated mean high high water, at this sensor with no hurricane, and the finished floor elevation of your house. Anything more than a dead flat predicted 8′ surge and your floor is wet.

Now, what about the other number, the 1929 NGVD elevation? According to the SW Florida Water Management to convert the older 1929 datum to the new 1988 DATUM one would SUBTRACT ABOUT 1.1′. So, for our example IF 9.1′ is given on the plan or elevation certificate you found in the older 1929 datum then this would be ABOUT 8.0′ in the now used 1988 datum. So now there’s basically there is ABOUT 7′ of differnence between the high water, at this sensor with no hurricane, and the finished floor elvation of your house. Anything more than a dead flat 7′ surge and your floor is wet.

Please keep in mind we “hide from wind BUT run from water.” This simple example does NOT include localized flooding effects or say wind driven waves or another house floating into your house. Be smart. WHEN IN DOUBT, GET OUT!