Re: Visible moisture, below freezing temperatures, but no icing. How is this possible
Twice a year, like the proverbial broken clock?
From a technical perspective, FL's are MSL at QNE (SLP of 2992) and below FL180, except on days when the altimeter happens to be 2992 our indicated altitude which is at QNH (adjusted to sea level), does not match.
For standardization, PiReps do as I did and refer to a particular altitude as FLxxx in hundred feet but PiReps are interesting...
Unlike a metar that reports clouds at QFE (AGL), a PiRep uses their indicated altitude for their report. Below 18,000 MSL this is QNH (AMSL) and above 18,000 MSL this is QNE (MSL at SLP of 2992) for their report which is still different from a metar so a PiRep and metar could conceivably report:
(NOTE: 0A9 is 2.5 NM west of the reporting point and has a field elevation of 1593)
METAR 0A9 2010Z 21004 10SM OVC032 12/0 A3092
UA /OV HMV180005/TM 2010/FL060/TP LJ35/SK OVC048
UA /OV HMV180005/TM 2010/FL190/TP LJ35/SK OVC039/TOPS FL185
So from the 3 reports, we can gather the overcast layer is:
3,200 to 17,800 FT AGL (QFE) - METAR
4,800 to 19,400 FT AMSL (QNH) - PiRep below 18,000
3,900 to 18,500 FT MSL (QNE) - PiRep above 18,000
The clouds aren't changing altitude or thickness but not knowing the difference, it would be easy to confuse the 3 reports and think they were without realizing the actual cloud cover. In this example, you could depart 0A9 on a heading of 240 to 360 without going IFR or come into 0A9 on a heading of 060 to 180 and stay under the clouds skud running out of/in to the airport from clearer skies (highly likely given this cloud cover is likely to at least partly be ruled mountain obscuration) but the terrain in any other direction is between 4800 and 6700 feet. To get out to the North to Northeast VFR, you 'd probably need another 1000 ft to clear the mountains safely and to the Northeast through southwest you need at least 2000 if not 3, to clear the mountains.
(Someone should probably check my math; have to get to work and dont have time to do a full cross check)
Below FL180, its can be very confusing to refer to it as a FL because of these differences in definition. Is FL below 18,000 QNE or QNH? It's still used for shorthand but it doesnt exist for the purpose of flying as you cant actually fly an altitude of FLXXX below 18,000 yet in my post I indicated you'd be "flying FL080 or FL100" and if that were the case, its conceivable control could tell a plane to fly FL039 and another plane to fly FL048 and the first plane flys QNE and the second flys QNH, they're at the same altitude hence, "FL's dont exist below FL180"
