ASOS wind reporting

As we move into the cold season, surface wind speeds are about to get strong and gusty. Here's a description of how an Automated Surface Observing System (ASOS) reports wind.

Let's start with a bit of history. Before the implementation of automated sensors nationwide, human observers, typically stationed at airports, monitored an analog or digital wind dial over a brief period to ascertain the average wind direction and speed for their observations. Most locations were also equipped with a wind recorder device that provided a continuous documented record of the measured wind direction and speed, similar to the one depicted on the right. Observers frequently used this device to identify the maximum instantaneous wind speed within the 10-minute period preceding the observation. They relied on visual and mental averaging, along with human judgment, to produce a wind observation. This method often lacked consistency across different sites and observers. That's when automation came in to save the day.

It's important to appreciate that every ASOS processes data in the same way, ensuring site-to-site consistency. ASOS takes a measurement of wind direction and speed every second, providing more frequent, consistent, and accurate data than a human observer ever could. From this, the ASOS calculates five-second averages for wind direction and speed from these individual one-second measurements. These five-second averages are then rounded to the nearest degree and knot and are stored for two minutes. Consequently, these averages serve as the basic units for determining reportable wind values and effectively represent the ASOS version of a manual "instantaneous" wind observation.

Every five seconds, a 2-minute average of wind direction and speed is calculated to determine wind characteristics. If this 2-minute average wind speed is 2 knots or less, it is reported as "calm" and appears in a METAR/SPECI as 00000KT. The current 2-minute average wind is stored every minute in memory for 12 hours and is available for reporting in the one-minute-observation (OMO), computer-generated voice messages (such as ground-to-air radio and telephone dial-in messages), METAR/SPECI reports, and Operator Interface Device (OID) displays.

What about wind gusts? In the manual procedure, a gust is recorded when an observer notices rapid changes in sensor wind speed readings, with a variation of 10 knots or more between peaks and lulls during the 10 minutes before the observation. The reported gust is based on the highest "instantaneous" wind speed observed in this period. The average 2-minute wind is used to report wind direction and wind speed. It's possible for an average 2-minute wind speed as low as 3 knots (observed in the last minute) to be reported with a gust of 10 knots (observed in the last 10 minutes). However, observations of 5 knots with gusts of 10 to 15 knots are more commonly the minimum values reported.

The ASOS algorithm also uses a 10-minute observation period to identify gusts, but it applies this period differently.

(1) Every 5 seconds, the ASOS calculates the highest 5-second average wind speed (and its direction) from the past minute and records this data in memory for 12 hours once per minute.

(2) Additionally, every 5 seconds, the ASOS calculates the current 2-minute average wind speed and compares it to the highest 5-second average wind speed from (1) above.

(3) If the current 2-minute average wind speed is 9 knots or more and the highest 5-second average wind speed from the past minute from (step 1) above surpasses the current 2-minute average speed from (2) above by 5 knots or more, then the highest 5-second average speed from the past minute is saved in memory as a gust for 10 minutes.

(4) Every 5 seconds, the ASOS checks the highest gust recorded in the last 10 minutes against the current 2-minute average wind speed.

(5) If the difference is 3 knots or more, the current wind speed exceeds 2 knots, and the highest gust surpasses the minimum 5-second wind speed in the last 10 minutes from (4) above by at least 10 knots, then the highest gust is marked as the reportable gust.

This value is added to the current wind direction and speed in the OMO, computer-generated voice messages, and METAR/SPECI reports. The minimum gust speed reported by ASOS is 14 knots. Wind speeds between 3 knots and 11 knots may be reported with gusts up to 14 knots. For instance, a 2-minute average wind from 240 degrees at 10 knots with gusts up to 20 knots is reported as: “24010G20KT.”

How is a variable wind defined? Both the manual procedure and the ASOS algorithm use the same definition to identify a variable wind, but they report it differently. In both scenarios, a variable wind is noted when the wind direction changes by 60 degrees or more during the 2-minute evaluation period before the observation. If the 2-minute wind speed is 6 knots or less, a variable wind direction indicator (VRB) is included in the basic wind group. If the 2-minute wind speed exceeds 6 knots, a variable wind direction group is added to the basic wind group in the METAR/SPECI report.

The manual determination of a variable wind is based on visually interpreting the wind instrument reading over a 2-minute evaluation period. In contrast, the ASOS algorithm assesses the range of 5-second average wind directions over the past 2 minutes (24 samples). In both methods, if the current 2-minute average wind speed is 6 knots or less, the wind direction and speed are reported as "VRBff," where "ff" represents the current 2-minute average wind speed in knots. For instance, a variable wind at 3 knots is encoded as "VRB03." If the 2-minute average wind speed exceeds 6 knots, the current wind direction and speed are included in the report. For instance, a current 2-minute wind of 270 degrees at 10 knots, varying from 240 degrees to 300 degrees, is coded as: 27010KT 240V300.

Most pilots are weatherwise, but some are otherwise™ 

Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief™

CFI & former NWS meteorologist