Chapter 24
Observations
ASOS reports the following basic weather elements:
- Sky condition: cloud height and amount (e.g., clear, few, scattered, broken, overcast) up to 12,000 ft (future upgrade may raise height limit).
- Visibility (to at least 10 SM).
- Basic present weather information: type and intensity for rain (RA), snow (SN), freezing rain (FZRA), and unknown precipitation (UP).
- Thunderstorms on site (TS) or in the vicinity (VCTS).
- Obstructions to vision: fog (FG), mist (BR), and haze (HZ).
- Pressure: sea level pressure and altimeter setting.
- Temperature: ambient temperature and dewpoint temperature.
- Wind: direction from which the wind is blowing, speed, and character (e.g., gusts, squalls).
- Precipitation: accumulation.
- Selected significant remarks including variable cloud height, variable visibility, precipitation beginning/ending times, rapid pressure changes, pressure change tendency, wind shift, and peak wind, and may include density altitude.
24.3.2 Automated Weather Observing System (AWOS)
AWOS is a system similar to ASOS. Generally, AWOS does not report all the elements that ASOS reports and may not have the same level of backup sensors or maintenance response levels. Regardless, AWOS provides pilots with the necessary weather information to conduct 14 CFR part 91 flight operations as well as others, depending on their operations specifications (OpSpecs).
AWOS automatically provides computer-generated voice observations directly to aircraft in the vicinity of airports, using FAA ground-to-air radio. AWOS reports are also available via a telephone.
AWOS may be located on airports, at or near ground-based or rooftop-based heliports, as well as on offshore platforms and drill ships.
AWOS are either Federal or non-Federal. Federal AWOS units are owned, operated, and maintained by the FAA. Non-Federal AWOS are owned, operated, and maintained by the site owner.
AWOS generates a METAR at 20-minute intervals and does not report SPECIs. AWOS also provides OMOs available by phone or radio.
The OMOs are not “instant weather;” rather, the clouds and visibility are time averaged (30 minutes for clouds, 10 minutes for visibility). The averaging algorithms are designed to report deteriorating conditions much quicker than improving conditions. For example, if dense fog had been reported and then suddenly dissipated, it might take up to 10 minutes for the OMOs to report VFR conditions.
There are six types of AWOS systems:
- AWOS-A: The AWOS-A system measures and reports altimeter only.
- AWOS-AV: The AWOS-AV consists of an AWOS-A with a visibility sensor.
- AWOS-1: The AWOS-1 system measures and reports wind data (e.g., speed, direction, and gusts; temperature; dewpoint; altimeter; and density altitude).
- AWOS-2: The AWOS-2 system measures and reports all of the parameters of an AWOS-1 system plus visibility.
- AWOS-3: The AWOS-3 system measures and reports all of the parameters of an AWOS-2 system plus precipitation accumulation (rain gauge) and cloud height. AWOS-3 can have optional sensors such as precipitation type/intensity (present weather, P) and/or thunderstorm/lightning (T). The addition of an optional sensors will change the designation to AWOS-3P or AWOS-3PT.
- AWOS-4: The AWOS-4 system measures and reports all of the AWOS-3PT parameters plus freezing rain.
Depending on the type of AWOS unit, the following parameters may be measured:
- Altimeter.
- Wind speed.
- Wind direction (from which the wind is blowing).
- Gusts.
- Temperature.
- Dewpoint.
- Density altitude.
- Visibility.
- Precipitation accumulation.
- Cloud height.
- Precipitation type.
- Precipitation intensity.
- Present weather.
- Thunderstorm/lightning.
- Freezing rain.
- Runway surface condition.