Winds of Mt. Washington vs Hurricanes
2017-09-13 10:34:23.000 – Adam Gill, Weather Observer/IT Specialist
Working up here I have always been curious as to how our winds compare to that of hurricanes in regard to force. The summit and sea level have different atmospheric densities. So, if the summit and a place at sea level have the same velocity, the wind at sea level would feel stronger due to a greater density. In hurricanes, the highest winds will occur in the eye wall so the pressure is fairly low compared to normal sea level pressure. The temperature is also very warm along with high humidity which helps lower the density even more. On the summit, since we are at elevation, we are above 20% of the atmosphere but with most of our high winds in winter, it is extremely cold and that can increase the density.
Checking out the surface data from Hurricane Irma, I found that in the eyewall at landfall the pressure was around 938 millibars (mb) and the temperature was around 81°F with a humidity of ~95% the density was 1.074 kg/m^3. On the summit, our typical pressure in winter is around 800 mb and the temperature averages are in the lower teens so let’s say 12 degrees with 100% humidity since we are in the fog most of the time. The density I got was 1.062 kg/m^3, which is not too much of a difference from the eyewall of Hurricane Irma.
Our pitot tube that we use to measure wind velocity is hooked up to a pressure transducer that measures differential pressure. We are basically measuring the force of the wind rather than using an anemometer that rotates because it is more accurate in harsh conditions. I can use the force measurement from the pitot to be able to calculate the equivalent velocity in hurricane Irma by correcting the wind speed for the different density that was calculated earlier (1.074 kg/m^3).
The highest wind speed that I have experienced on the summit so far was 138 mph. At the time of the gust, the temperature was 5°F, the pressure was 778.48 mb, and the humidity was 100% leading to a density of 1.046 kg/m^3. The pitot measured a differential pressure of 8.05 inches of water so I can use the 8.05 inches of water and change the density to 1.074 kg/m^3 to get a new velocity 136 mph in Irma’s eye wall. So the 142 mph recorded in Naples felt stronger than what I have experienced! The difference is not quite as great as I thought it would be when I started to look into this. I was expecting the winds near sea level in a hurricane to be exerting a greater force for the same wind velocity.
We can go a step further and look at the greatest wind velocities that were recorded in the hurricane and look at what the equivalent wind speed would be on the summit if the force was kept the same. The highest wind speed that I could find from Irma was recorded on Barbuda at 155 mph (this anemometer failed and gust likely were higher), the pressure at the time was 922 mb, temperature of 76°F, and relative humidity we can assume again at around 95%. This resulted in a density almost similar to the summit at 1.063 kg/m^3. This would mean that the differential pressure would have been 10.3 inches of water and we can then correct once again for the typical density we have up here in high wind events so the wind equivalent is 155.3 mph. Now since we have a wide range of temperatures and pressures on the summit, the range that the wind speed could have been is 150.1 mph on really cold days to 163.1 mph if it occurred during the summer.
Adam Gill, Weather Observer/IT Specialist