Automated Weather Observation System - AWOS

USE OF AUTOMATED OBSERVATIONS FOR AVIATION BRIEFING PURPOSES

This manual discusses several different types of AWOS observing codes. The AWOS itself generates meteorological data in fixed and free format. These data can be obtained remotely by display software or the VGM (Voice Generator Module), or can be transmitted as an RA bulletin. Code conversions software produces BUFR, METAR, SPECI, 'SA' and synoptic codes. Only METAR and SPECI are used for aviation purposes. Refer to the following excerpt from A.I.P:

(NOTE: This information is subject to change. Please consult latest A.I.P. Amendment, NOTAM or other Transport Canada correspondence for updates.)

(Source: Aeronautical Information Publication, A.I.P. -Transport Canada)

"Automated Weather Observation Systems (AWOS), sometimes called autostations, have been generating weather observation data in Canada since 1969. Starting from the early, basic autostations, Environment Canada gave weather reports, issued in the SA format from the various autostation types, a code number from one to eight. This number appears directly after the word 'AUTO'.

This numbering system is not used in the METAR code. Only METAR AUTO (or SPECI AUTO) reports from AWOS and AUTO2 reports (from an old autostation system) are suitable for aviation.

Pilots now have access to weather reports through DUATS, preflight information terminals at FSSs, and through electronic bulletin boards. It is important that they identify and use only those AUTO reports that are certified for aviation use. In addition, it is important to know the characteristics of AWOS observations. AWOS observations are not made in the same manner as human observations; therefore, they have certain advantages and disadvantages that should be understood.

AUTO2 REPORTS

AUTO2 Reports are from an early type of station that is capable of measuring cloud height and sky cover, visibility, temperature, dew point, mean sea level pressure, wind velocity, and precipitation and rate. A station issuing AUTO2 reports provides only the hourly weather observations, in the old SA format; it does not issue specials. This means that the current observation from this type of station may be up to 59 minutes old when it is used by the pilot. FSS Specialists are required to warn pilots of this shortcoming when they provide an AUTO2 report. The AUTO2 report will not be coded in the METAR format and is not coded in exactly the same way as the old SA or SP. A more important limitation is that AUTO2 reports contain only an atmospheric pressure and not an altimeter setting. For these reasons, AUTO2 reports should not be used by pilots who are self-briefing for flight. They should be used only when provided by a qualified aviation weather briefer.

METAR/SPECI AUTO REPORTS

"AUTO" weather reports are generated by an automated weather observation system known in Canada as AWOS, developed by Environment Canada, DND and TC. There are varying configurations for the AWOS. These configurations are explained in MET 1.2.4. The systems being deployed by Transport Canada and Environment Canada are of the AWOS 3 type. AWOS is a modular system that currently incorporates sensors capable of measuring cloud base height (up to 10,000 feet AGL), sky cover, visibility, temperature, dew point, wind velocity, altimeter setting, precipitation occurrence including freezing precipitation (type, amount and intensity), and the occurrence of icing. It incorporates dual atmospheric pressure sensors for determining altimeter setting that will "fail-safe and shut itself down if there are significant discrepancies between the two sensors. Human observers can add remarks to the weather observations made by an AWOS. The AUTO observation is reported in the normal METAR/SPECI format and can easily be interpreted by aviation users. METAR AUTO is reported on the hour, then SPECI AUTO observations are issued for significant changes in cloud ceiling, visibility, wind velocity, the onset and cessation of precipitation or icing.

The sensors sample the atmosphere and prepare a data message once per minute. There should be little delay between detection of weather that requires a Special Report and the issuance of that report. Where a voice generator, VHF radio and antenna are attached to the AWOS, the most recent observation will be broadcast to pilots listening on the frequency published in CFS."

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