May 24, 2010
All About ASOS
Read this and you will be an expert in ASOS wind reporting.
Now we know why they went to 3 second averaging on ASOS wind reporting --- because they dumped the old "cup" and "propeller" wind anemometers and went with "sonic" instead. I think the entire ASOS network got swapped out in 2007 and 2008. This would include KTTD, KPDX, KVUO and KHIO in the local area. Anyway, check this out from the ASOS manual ---
* The "wind speed" reported in each observation is an average speed for the most recent two-minute period prior to the observation time. This is also considered the "sustained wind" for routine surface observations (just to confuse matters a bit, in hurricane forecasts, the sustained wind is a one-minute average). This two minute average is calculated from a series of 24 five-second average values.
* A wind "gust" is also reported when the peak "instantaneous" wind during the most recent ten-minutes prior to the observation is more than 10 knots greater than the lowest "lull" in the wind during that time. If that is the case, the highest instantaneous wind during that ten minute window is reported as the gust value.
* If the maximum instantaneous wind speed during the entire period since the last reported observation exceeds 25 knots, that value is reported as the "peak wind." It will be at least equal to the reported gust, and may be higher than the reported gust if the "peak" occurred more than 10 minutes prior to the observation time.
* A two minute average is usually a good deal lower than the peak or gust wind in highly variable conditions. Also, the older rotating-cup anemometers measured the wind speed based on the force applied by the wind to those cups. Since there is some mass and inertia involved, even a sensitive anemometer required about a 5-second average for the "instantaneous" values used for gusts and peak wind calculations. The new sonic anemometers still use 24 five-second average for the "two-minute" wind, but they use a running 3-second average to calculate the gust and peak values, and because it is more sensitive to variations in the wind speed and uses a shorter averaging time for these, will likely result in more reports of gust and peak winds, and also in slightly higher values of those variables.
Steve Pierce