O. Sami Saydjari wrote:
This question is a question on physcial phenomena, NOT on regulation (so
I am starting a new thread).

As I understand it, icing happens between +2C and -10C. Assuming this
is correct, I have three questions.

(a) Does that mean one is relatively safe if the surface temperature is
below -10C (and there is no temperature inversion, meaning that the
temperature is known to decrease as one goes up in altitude)?



1. How the cloud got formed is as important as the current temperature
of the cloud.

As many have pointed out, cumuloform clouds often retain supercooled
water well below -10.

But not only that, the original air that formed the cumuluus cloud may
have come from low, warm, humid levels. If the air in the cloud started
out at, say, 20 degreec C with a 15 C dewpoint, it will have condensed
out about 11 grams of liquid water per cubic meter by the time it cooled
to -10. (This is also why the worst icing in cumuloform clouds is often
near the top.)

If the air started with a dewpoint of -5, it will have condensed about 1
gram per cubic meter... only about a tenth of the above scenario.

If the air in the cloud STARTED with a dewpoint of -10 or less, the
cloud will have condensed less than 2 grams of liquid water per cubic
meter NO MATTER HOW COLD IT GOT.

2. As Roy mentioned, water condenses only onto a nucleus of some type.
In nature these nuclei are MOSTLY the wrong shape for ice crystals to
form, so it is natural for the condensation to stay liquid as long as
possible, and -10 or so appears to be "common". (By the way, Roy, the
latent heat release when it DOES freeze just goes to warm up the air a
tiny bit). Typically, once a few drops DO freeze, or when snowflakes
are fall into the liquid layer from above, then the liquid will migrate
to the ice fairly quickly...

Below -10, it is common that at least SOME ice crystals will form, and
this usually erodes the water content quickly. However, in vertically
developing cloud (Cumulus, etc.) the updrafts may prevent the
introduction of ice crystals for some substantial period of time.