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?
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 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.
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.