"The ASOS also relies on a 10-minute observation period to determine gusts." This is a quote directly from the ASOS User's Guide, Chapter 3, Section 3.2.2.2a. Why? Because it has to. Federal regulations require it. The ASOS could not be commissioned unless the algorithms conformed to the federal standards that are prescribed in Federal Meteorological Handbook #1. Therefore, the algorithms were written to comply with these standards. Here is how it works (the following is obtained from the ASOS User's Guide, and
www.nws.noaa.gov/ops2/Surface/documents/IFWS_BelfordWS_comparison.pdf . Please note: when the old Belfort Rotating Cup Anemometer and Wind Vane Wind Sensor was installed on the ASOS, the ASOS used the highest 5-second discrete average wind speed and direction for the minute for gust and peak wind processing, while the new Ice Free Wind Sensor uses a World Meteorological Organization (WMO) standard 3-second running average wind direction and speed for the minute for gust and peak wind processing):
Every second, the wind speed and direction are sampled by the Ice Free Wind Sensor (IFWS).
Every second, a running average of the most recent 3 seconds of data is computed by the IFWS, producing the "3-second peak".
Every 5 seconds, the average of the most recent 5 seconds of data is computed by the IFWS, producing the "5-second average". The highest 3-second peak is determined by the IFWS and is stored as the 3-second peak.
Every 5 seconds, the 5-second average and the highest 3-second peak collected during the past 5 seconds are collected from the IFWS by the Data Collection Package (DCP) and sent to the Acquisition Control Unit (ACU).
Every 5 seconds, the 5-second average and the 3-second peak are received from the DCP by the ACU to the nearest 0.1 knot and truncated to the nearest knot.
Every 5 seconds, the ASOS IFWS Quality Control (QC) Algorithm evaluates each 5-second sample and the data stream against 10 criteria (9 for the raw samples, and one for the data stream). ONLY when a sufficient number of raw wind samples AND the data stream pass all QC checks will a QC'd wind report be available.
If the ASOS IFWS QC Algorithm has been satisfied, then *****BEGIN ASOS WIND ALGORITHMS***** (else, set wind report to missing).
Every 5 seconds, the average wind speed and direction is computed using the most recent 24 (i.e. 2 minutes) 5-second wind samples received from the IFWS (if 6 or less of the 24 5-second samples have not been removed by the IFWS QC Algorithm, then a wind report will be available). The peak wind is the 3-second peak wind value from the 5-second sample with the highest peak wind speed.
Every 5 seconds, the ASOS computes the 2-minute average wind speed and compares it with the greatest 3-second peak wind value during the past minute. If the current 2-minute average wind speed is equal to or greater than 9 knots, and the greatest 3-second peak wind value during the past minute exceeds the current 2-minute average wind speed by 5 knots or more, then the greatest 3-second peak wind value observed during the past minute is stored in memory as a gust for 10 minutes.
Every 5 seconds, the ASOS compares the highest gust stored in memory for the past 10 minutes with the current 2-minute average wind speed. If the difference between the two is 3 knots or more, the current reported 2-minute average wind speed is greater than 2 knots, and the highest gust exceeds the minimum 5-second wind speed by 10 knots or more, then the highest gust stored in memory is designated as the reportable wind gust. This value is appended to the current 2-minute average wind direction and speed reported in the One-Minute Observation (OMO), the METAR/SPECI observations, and the computer-generated voice messages. The minimum gust speed reported by the ASOS is 14 knots.
The Peak Wind (reported in remarks as PK WND) is determined from the highest observed 3-second running average wind speed which exceeds 25 knots since the last generated METAR.
