Awos & Fzfg?

[Checks]

How does the AWOS determine there is FZFG? Usually when the visibility is 5/8th and below with snow you usually will see the following:

METAR KCMX 301353Z AUTO 33017KT 1/2SM -SN FZFG OVC016 M17/M19 A2987

Sometimes you see just the 1/2 SM -SN with NO fzfg, anybody know how the machine actually can tell there is fog or no fog? I can understand if no precip is falling and the visibilty is reduced then that would equal FOG. I dont understand how the AWOS can tell there is fog when there is rain or snow reducing visibility.

Here is the definition of FZFG:
FREEZING FOG
Used to describe the phenomena when fog is present and the air temperature is below 0 degrees Celsius. It is reported as "FZFG" in an observation and on the METAR.
FOG
A visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of the earth, reducing horizontal visibility to less than 5/8 statute miles. It is created when the temperature and the dew point of the air have become the same, or nearly the same, and sufficient condensation nuclei are present. It is reported as "FG" in an observation and on the METAR.

 

[TZRav8r]

FG is only reported when the visibilty is 5/8 miles or less. 5/8 to 6 miles is considered BR. I would assume that the FZ would be based on OAT.

 

[HAL]

Come on up to Fairbanks any you'll get your fill of FZFG in the winter time! It's called ice fog up here and often comes out when the temp drops below -40. Nasty stuff that can hang around for days and days.

HAL

 

[Noam]

AWOS determines FG vs. FZFG based on the ambient air temp.

If the temp is below 0C, it is FZFG...easy enough!

 

[Checks]

I need clarify the question:

How does the AWOS determine that visibility is reduced by fog verse the precip that is falling (whether it is rain, drizzle, or snow)?

 

[TZRav8r]

PFM!