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Automated airport weather observations can be produced by several types of equipment: AWOS-A, AWOS-1, AWOS-2, AWOS-3, and ASOS are the ones I've seen mentioned. What are the differences between these reporting systems? Are the older systems still prevalent, or have they been mostly converted to the new systems?

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ASOS is basically a replacement for AWOS with more reporting parameters, and the first units were installed in 1991. According to the Automated airport weather station Wikipedia page, each different type has the following parameters:

AWOS

  • AWOS A: barometric pressure and altimeter setting (in inches of Mercury).
  • AWOS I: wind speed and wind gusts (in knots), wind direction (from which the wind is blowing) and variable wind direction (in degrees of the compass), temperature and dew point (in degrees Celsius), altimeter setting and density altitude.
  • AWOS II: all AWOS I parameters, plus visibility and variable visibility (in miles).
  • AWOS III: all AWOS II parameters, plus sky condition (in oktas), cloud ceiling height (in feet), and liquid precipitation
    accumulation (in inches).
  • AWOS III P: all AWOS III parameters, plus precipitation type (rain, snow and sometimes drizzle) identification.
  • AWOS III T: all AWOS III parameters, plus thunderstorm detection (via a cloud-to-ground lightning detector).
  • AWOS III P/T: all AWOS III parameters, plus precipitation type identification and thunderstorm detection.
  • AWOS IV Z: all AWOS III P/T parameters, plus freezing rain detection via a freezing rain sensor (Note: this configuration used
    to be called AWOS III PTZ).
  • AWOS IV R: all AWOS III P/T parameters, plus runway surface condition.
  • AWOS IV Z/R: all AWOS III P/T parameters, plus freezing rain detection and runway surface condition.

ASOS

They generally report all the parameters of the AWOS-III, while also having the additional capabilities of reporting temperature and dew point in degrees Fahrenheit, present weather, icing, lightning, sea level pressure and precipitation accumulation.

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    $\begingroup$ What are the practical reasons a pilot would need to remember these differences, or know what type a particular airport has? Just tune to the weather frequency prior to approach and take note of whatever details that are spoken... $\endgroup$
    – bovine
    Commented Dec 18, 2013 at 6:06
  • $\begingroup$ @bovine: Don't ask me, I didn't ask the question. :-) $\endgroup$
    – Lnafziger
    Commented Dec 18, 2013 at 13:09
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    $\begingroup$ @bovine while I agree that you'd just tune in prior to approach or take-off, it might be helpful to know what information is not given for each one. Most of it the difference have to do with relaying the type of precipitation observed, so theoretically, you could be surprised by snow or freezing rain if you thought that the report should include it (but didn't mention it, so you thought you'd be fine) but rather the station was unable to report it due to lack of sensors. $\endgroup$
    – Canuk
    Commented Sep 19, 2014 at 21:49

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