Correcting a wind gust

2008-03-28 22:13:25.000 – Brian Clark,  Observer

My Math

148.5

144.7

At first glance these are just two dimensionless numbers; two numbers with a difference of 3.8 or about 2.6%. Of course, by now you’re probably curious to know what these numbers mean and where I’m going with this.

Well, both are wind gust measurements.

The first value of 148.5 miles per hour was measured a week ago on March 21 as the peak wind speed for the day. This wind gust was measured using our Pitot Tube Static Anemometer in conjunction with the Setra pressure transducer. The pitot anemometer is used to create a difference in pressure(a.k.a. differential pressure) while the Setra is used to measure this value and convert into an electrical voltage that is then sent to our digital database. The database is then able to run through a calculation to determine a wind speed. Note how the pitot anemometer does not, in itself, directly measure a wind speed, rather it measures the force of the wind in the form of a differential pressure. During the past week, I have posted a two part discussion in my AccuWeather.com blog that goes more in depth on how the pitot system works. You can view part 1 here, and part 2 here.

The value of 144.6 miles per hour represents the exact same wind gust AFTER it has been corrected. Why does the gust of 148.5 miles per hour need to be corrected? Well, remember that I mentioned that our pitot anemometer measures the force of the wind, which through calculations is turned into a wind speed. Those calculations have to assume a “standard” air temperature and pressure. In our case, that is 27 degrees Fahrenheit (roughly our average annual temperature) and 800 millibars (roughly our average annual air pressure). The problem is that when that peak wind gust occurred on March 21, the temperature was -5.7 degrees Fahrenheit with an air pressure of 787.098 millibars. These conditions decrease the real world force of the wind.

So, what I had to do was take the differential pressure measured by the Setra pressure transducer during that peak wind gust, and plug it into the same equation our computers use to determine the wind speed, except instead of our “standard” temperature and pressure, I used the temperature and pressure that was occurring at the time of the wind gust. After going through all the math (which made me feel like I was back at Penn State), the result was this 2.6 % correction, from 148.5 miles per hour to 144.7 miles per hour. This corrected value gives us our actual “official” peak wind gust for last Friday and also for the season.

One thing that is important to keep in mind is that at this point in time, we only apply this correction to big and/or record breaking wind gusts. This is the case for a couple reasons. Number one, it would take way to much time for us to compute the corrected value for every wind speed we record. Number two, this is not a linear correction, or in other words, it is not always a 2.6% correction. The lower the wind speed, the smaller the correction. Also, the closer the conditions are outside to our “standard” conditions, the smaller the correction. For lower wind speeds, the correction essentially ends up being trivial. In the near future, we hope to have our digital database calculate this corrected “real wind” value in real time.

In the mean time, this has certainly been a learning experience for me. Before Wednesday afternoon, I knew very little about how to go about correcting a big wind gust and why it is done. This goes to show just one of the reasons that I love my job and love working here; there is always something new to learn.

 

Brian Clark,  Observer