Hello David,
I think we agree on the important parts about fog:
* visibility in air < 1000m
* caused by water (i.e. not by dust, which would be haze)
* near-surface (~ human eyes height)
Coming to the exact wording is more difficult. The term 'cloud' might
include dust (Saharan dust clouds) and exclude radiation fog, so I
wouldn't like to use it. 'humidity' is invisible as you note, so I will
drop that. water droplets might exclude ice-fog. What about:
fog means visibility in air < 1000m due to water
droplets or minute ice crystals close to the surface.
"X_area_fraction" means the fraction of horizontal area occupied by X.
Best regards,
Heik
On 2013-08-06 19:05, David Hassell wrote:
Hello Heiko,
fog_area_fraction:
fog means visibility in air < 1000m due to humidity or water
droplets. "X_area_fraction" means the fraction of horizontal area
occupied by X.
I think that it's fine, but could you confirm the "humidity" part of
your definition? It sounds a little odd to me since water vapour is
invisible, although I appreciate that the relative humidity is often
very high when fog forms. How about:
"Fog means near-surface cloud which reduces the visibility in air to <
1000m. "X_area_fraction" means the fraction of horizontal area
occupied by X."
where "near-surface cloud" is meant to indicate, in some way, that the
cloud is, er, near the surface (or in the boundary layer; or ...). But
I'm not sure about that bit of wording, at all.
I know that some models output fog fraction diagnostics at 1.5m (or
2m) above the surface, but I don't know if that is in any way
standardized in all models or observed datasets.
I'm not a fog expert, so I hope that these points aren't way off the
mark.
All the best,
David
--
David Hassell
National Centre for Atmospheric Science (NCAS)
Department of Meteorology, University of Reading,
Earley Gate, PO Box 243,
Reading RG6 6BB, U.K.
Tel : +44 118 3785613
E-mail: [email protected]
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--
Dr. Heiko Klein Tel. + 47 22 96 32 58
Development Section / IT Department Fax. + 47 22 69 63 55
Norwegian Meteorological Institute http://www.met.no
P.O. Box 43 Blindern 0313 Oslo NORWAY
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