Will Hand wrote:


"Martin Rowley" wrote in message
...
"Norman" wrote ...

0630z NNE F1 7000 HZ 8Sc012 09.3/07.7 1017
[and]
Tudor Hughes wrote:

Is that really haze, Norman? I know I've got a bit of thing
about "haze" and "hazy sunshine" but surely it's thin mist and not
solids in suspension.
[and]
"Norman" wrote ...
I tend to agree with you Tudor but, by definition, it's only considered
to be mist if the relative humidity is 94 percent or more (or is it
more than 94 percent?). On this occasion the RH was only 90 percent so
whatever was obscuring the visibility couldn't be called mist. That
left haze as the only option. Or have the rules changed since I last
did "official" observations?
[and]
"Will Hand" wrote...
AFAIK it is still mist if RH equal to or greater than 95%, haze if
visibility reduced by water droplets and RH 95%.

... that's certainly the "Observer's Handbook" guidance, but I seem to
remember that for practical purposes, we had a bit of latitude in
interpretation, but only by a few %. So U=90% and mist was *just* about
OK. The problem is ... does the screen environment accurately match the
'open atmosphere' conditions?

Cause of many a 'happy' argument between shifts & also with the station
SSA :-)

Martin.


My first posting was to RAF Finningley in Sept. 1970 (station 03360), now
closed. It was close to the industrial towns of Doncaster and Sheffield
and smoke haze was common. I remember one shift where the experienced
observer was reporting vibility below aviation fog limits but also smoke
haze, present weather code 04. Great arguments with the collecting centre
at Manby ensued saying that he should be reporting fog not smoke. But he
was correct as the RH was circa 94% and the air was full of dirty smoke,
you could smell it! He stuck to his guns. Doesn't happen much nowadays of
course with cleaner air generally. Good old days, cough, cough :-)


Even earlier than that and in different circumstances, I remember several
times in the 62-3 winter where we knew what we'd got was fog but the air
temperature and ice-bulb difference resulted in an RH below 95%. As far as
I recall, we still coded it as fog.

--
Graham P Davis, Bracknell, Berks., UK. E-mail: newsman not newsboy