The air is the air; what can be done?

You would think when somebody says winds will be 40 miles per hour that would be pretty straightforward. You’d be wrong. In fact there are lots of different ways of measuring and reporting wind, and when trying to calculate the impact of wind, it is vital you use a common reference.  Here’s the first important point: The wind the NHC uses in their reports isn’t the wind measured at the airport! And the winds the TV guys toss out often aren’t consistent either, especially when talking about gusts. To understand this we have to take a deeper dive in to the world of measuring wind …

Unfortunately, even in meteorology there are half a dozen kinds of “wind.” 

For historical reasons, NHC wind is reported as a “one minute sustained wind.” In the old days, a meteorological technician would stare at the mechanical wind gauge for one minute, and the sustained wind was the wind below which the wind did not drop. The offshore buoys report 10 minute averages becuase that is typically used in oceanography to calculate waves. In fact, the World Meteorological standards that define hurricanes use the 10 minute wind. Well, except some buoys report an 8 minute wind. When the automated networks (ASOS, AWOS) were installed at airports in the 1990’s, they were mostly funded by the FAA and designed to support aviation. So they reported a two minute average of three second gusts, and these days we have a dilemma: nothing actually measures the one minute sustained wind that NHC uses in their reports! So there are lots of tables and conversion equations and factors to try to convert between these different standards.

Engineers typically use gusts when designing structures. What kind of gusts? Um, ok, we’re not any better than the meteorologists. Some use 2 second, some 5 second, some 3 second. So, more tables and conversion equations and factors.

The bottom line is that the wind in the NHC reports is about 8 to 10 percent higher than the same wind that will be measured at your local airport. That’s why typically when NHC says “winds of 100mph” and you look at weather reports, the wind at the airport will only read 90 or 92mph. 

But … it’s worse than that. NHC reports “wind at 10 meters over open terrain or water.” Well, now things get really complicated because there isn’t a lot of open terrain inland; there are trees, buildings, etc. And the wind reaching you depends on the terrian several miles upwind from you – so, in a typical location, the wind is now only 80 to 85% of what the wind shown in the NHC reports. You can see some of this complexity in my maps, which take in to account terrain and as you can see above are not smooth except offshore.

Dr. Peter Sparks, Professor of Engineering at Clemson, used to joke that the guys at the hurricane center must be paid based on miles per hour 😛 .. but in fairness, their job is watch and warning, not damage estimation. However it is very confusing to the professional, much less the average person or “journalist”.

Why does all this matter? If you are trying to figure out what the impacts of the storm are going to be, which is my job, you are interested in the dynamic wind pressure. That’s the force the wind pushes on you, trees, buildings, and causes them to fall down or break. Take Downtown Pooler as an example. Let’s say the NHC wind is given as 80mph – minimal hurricane. The dynamic wind pressure is related to the square of the wind speed, so leaving out the other complicated parts of the equation, has a scale of 6400. But … when you account for averaging times that wind is an ASOS (airport wind gauge) value of 74mph, and terrain, likely only 68mph – tropical storm. That means the wind pressure scaling number is only 4624. So the wind pressure is only 72% of what you might get if you just plugged in the numbers, not 92% as you might expect just dividing 74/80. 

The World Meteorological Organization has tried to bring some order to this mess with a report outlining conversion standards, but even there they resort to some hand waving. Even more unfortunately, the vast majority of media/news people don’t understand or appreciate these nuances, much less try to explain them. They often go for the biggest number they can, and do irritating things like say “gusts to hurricane force.” That’s nonsense – hurricane winds are measured by sustained or average winds. A 75mph sustained wind will produce gusts to maybe 85 or 90mph – but a 75mph gust is NOT a hurricane force wind. To say so is both unscientific and scare mongering.

So what’s the bottom line? All these debates over winds, etc. are sort of beside the point. The difference between a 73 mph “tropical storm” and a 74mph “hurricane” isn’t really that important (assuming you’ve done your math properly!). These are complex storms with complex behavior that isn’t captured by one number. But it can be scary when you hear 60mph winds, but the source was using gusts not sustained, and so you think things are going to be a lot worse that they are. That’s why i put effects on my maps like trees sway, or limbs breaking – that makes sense without resorting to differential equations 😛