Published: 2007-04-21 12:24:36
Updated: 2007-04-21 12:24:36
Posted April 21, 2007
By Sam Matthews
MIKE MOSS SAYS: Sam, For wind observations reported by official stations, here are some pertinent facts about the wind speed measurments.
1. They are taken with anemometers located 10 meters above ground level (about 33 feet), with the anemometer type at official NWS and FAA sites changing from a rotating cup variety in the past to a new sonic version, with no moving parts, in a replacement program that began about a year and a half ago.
2. 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.
3. 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.
4. 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 few additional comments are probably in order here. First, as you might imagine 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.
An example that ties together some of the info above would be an hourly observation for RDU, say, taken at 2:55 pm. It might report a wind speed of 16 knots, a gust to 24 knots and a peak wind of 31 knots. This would be interpreted as the wind averaging 16 knots for the time period 2:53-2:55, the highest instantaneous wind between 2:45 and 2:55 being 24 knots (and, by definition, there also had to be a lull down to 14 knots or below during that time so that the 24 would be reported as a gust) and finally, at some point between 1:55 and 2:45 pm the instantaneous wind reached 31 knots. Since this exceeded 25 knots and occurred prior to the last ten minutes before the new observation, it is reported as the "peak wind" for the period since the previous observation.