FAR 91.103 specifies that we must obtain weather reports and forecasts. In addition to ATIS, METARs, and TAFs, that means ASOS, AWOS, and all the fun letters and numbers that follow.
Don't know 'em by heart? Here's a quick overview of the high points you can skim in the time it takes to drink a cup of coffee.
Automated Weather Observing System (AWOS)
- AWOS-A: Only reports the altimeter setting
- AWOS-AV: Reports altimeter and visibility
- AWOS-1: Reports altimeter setting, wind data, temperature, dew point, and density altitude
- AWOS-2: Reports the information provided by AWOS-1, plus visibility
- AWOS-3: Reports the information provided by AWOS-2, plus cloud/ceiling data
- AWOS-3T: Reports the same as the AWOS-3 system and includes a thunderstorm/lightning reporting capability
- AWOS-3P: Reports the same as the AWOS-3 system, plus a precipitation identification sensor
- AWOS-3PT: Despite a name beyond its years, it has nothing to do with a gen-AI weather chatbot (but our wheels are turning, and we will accept seed funding to make that happen). AWOS-3PT reports the same as the AWOS-3P system, plus thunderstorm/lightning reporting capability.
- AWOS-4: Reports the same as the AWOS-3 system, plus precipitation occurrence, type and accumulation, freezing rain, thunderstorm, and runway surface sensors
Some AWOS stations report more than the information above, but the data is advisory only. For example, an AWOS-1 station may report visibility even though it's not listed as an available product, but bear in mind that its visibility data is not necessarily as reliable as it would be coming from an AWOS-2.
Automated Surface Observing Systems (ASOS)
Generally, here's what you can expect from these systems:
- Cloud height indicator (it'll have either one or three sensors)
- Visibility sensor (again, one or three sensors)
- Precipitation identification sensor
- Freezing rain sensor (at select sites)
- Pressure sensors (two sensors at small airports and three sensors at large ones)
- Ambient temperature/dew point temperature sensor
- Anemometer (wind direction and speed sensor)
- Rainfall accumulation sensor
- Automated lightning detection and reporting system (excluding Alaska and Pacific Islands)
For either system, you typically can receive a VHF signal within 25 nautical miles up to 10,000 MSL, though often you'll find that a phone number is provided rather than a frequency. Sometimes, you can momentarily disable the COM squelch to hear signals at greater distances.
Since updates for both systems occur approximately every 60 seconds, you can tell ATC that you have the "one-minute weather." Check it often along your route to detect trends.
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