From: elmore@rap.ucar.edu (Kim Elmore)
Newsgroups: rec.aviation.piloting
Subject: Ice Fog (was Re: Engine failure due to cold...)
Date: 26 Jan 93 21:50:56 GMT
Organization: NCAR, Research Application Program

In article <1993Jan26.061032.6856@peck.com> geoff@peck.com (Geoff Peck) writes:
>In article <C1EDEx.I8t@world.std.com> paulc@world.std.com (Paul E Cantrell)
>writes:
>> Well, when I was at the Robinson factory CFI course, a guy from Alaska
>> asked Frank Robinson about that - seems they get something called ice
>> fog when it's really cold. Frank's from Southern California and couldn't
>> tell the guy what was the right thing to do in this situation, but the
>> guy clearly seemed to think that carb heat would make things worse.
>
>Ice fog is one of the most menacing things known to light aircraft.  It's
>sort of like normal fog -- formed when the temperature/dewpoint spread is
>low -- and it tends to be only 10 or 20 feet thick.  Ice fog typically
>forms at night (it's a type of ground fog), but if it gets very thick it
>may take much of the day to clear.
>
>When ice fog comes in contact with a moving aircraft, the aircraft tends
>to ice up instantaneously.  The ice typically looks like a very heavy
>layer of frost.  Not only is it impossible to see through the windows once
>this has happened, but a significant loss in lift and/or increase in stall
>speed may be encountered.
>things start to look a bit murky a few hundred feet up on final.  When one

	I think you guys are mixed up between ice fog and super-cooled
fog.  Here's the Glossary of Meteorology definition of ice fog:

ice fog - (also called ice-crystal fog, frozen fog, frost fog, frost
flakes, air hoar, rime fog, poganip) A type of fog, composed of
suspended particles of ice, partly ice crystals 20 to 100 microns in
diameter but chiefly, especilly when dense, droxtals 12 to 20 microms
in diameter. [A droxtal is a frozen fog droplet, spherical in shape
without any outward crystal chracteristics.]  It occurs at very low
temperatures, and usually in clear, calm weather in high latitudes.
the sun is usually visible and may cause halo phenomena.
	Ice fog is rare at temperatures warmer than -30C or -20F, and
increases in frequency with decreasing temperature until it is almost
always present at air temperatures of -45C or -50F in the vicinity of
a source of water vapor.  Sucj sources are the open water of
fast-flowing streams or of the sea, herds of animals (biofog),
volcanoes, and especially producst of combustion for heating or
propulsion.  At temperatures warmer than -20F, these sources can cause
steam fog or liquid water droplets, which may turn into ice fog when
cooled.

	So what are you seeing that ices up airplanes so rapidly?
Most likely, it's heavy fog in sub freezing temps.  The icing that
you're seeing is simply riming.  It's extremly rapid because the
collection efficiency of just about anything is almost unity at slow
speeds, which includes takeoff and landing roll-outs.  One of the best
(worst) temperatures to collect rime is around -5C to -10C.

	Ice fog is strikingly beautiful: the air sparkles with the
zillions of tiny ice crystals floating around in it.  It's as if the
air is all of a sudden composed of daimond dust!


			Kim Elmore, [N5OP, PP ASEL/Glider 2232456]
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