"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.

There was just a posting from Mike Rapoport in which he said he's seen
icing as cold as -20C.

(b) Is icing from 0C to +2C a possibility only when your aircraft skin
is colder than 0C (probably because you are descending from altitude)?

I don't really know the answer to this one, but I will point out that
just because your OAT gauge reads +2C, doesn't mean it is. If you've
got the standard "meat thermometer" type, it's likely that the last time
it was calibrated was 25 years ago when it left the factory (if then).
I wouldn't trust it to be accurate to +/- 2C.

(c) I have been told that icing is possible from -10C to 0C because
water sometimes get "super-cooled" (which I assume means that water gets
below freezing, but does not form ice for some reason). Is that right?
If so, under what atmospheric conditions does water get super-cooled?

For water vapor to freeze, you need three things. First, you
(obviously) need water. Second, the temperature has to be below the
freezing point. And third (this is the one most people don't realize),
you need what's called a "condensation nucleus". This is some piece of
solid matter providing a surface on which the phase change (i.e. liquid
to water) can occur. It could be a tiny dust spec, or in a marine
environment, tiny salt crystals in the air serve the same purpose.
Think of it like a catalytic converter.

When water freezes, it releases a lot of energy (called the heat of
fusion). If memory serves, it takes 1 calorie per gram per degree to
cool liquid water, and 80 calories per gram to go from liquid at 0C to
solid at 0C. That energy has to go somewhere. I believe what the
condensation nucleus does is provide a heat sink for that energy.
There's also an energy barrier tied up in surface tension, as you go
from a spherical droplet of liquid to an ice crystal.

If you want to see a good demonstration of how much heat it takes to
effect a phase change, go to the drugstore and buy a bottle of rubbing
alcohol. Wipe some on your arm and feel how cold your arm gets. What's
going on is the alcohol is changing phase from liquid to vapor and the
energy (heat) to do that is coming from your arm.

It's been a long time since I took physical chemistry, so I'm afraid I
can't give a better explanation than that. If you want to persue the
topic further, I would suggest googling for "condensation nucleus" or
perhaps consulting an advanced meteorology textbook.

Anyway, what's going on with supercooled water is that there's no
condensation nuclii available for the droplets to freeze onto. Along
comes the leading edge of your wing and the droplets go SPLAT! As far
as the droplet is concerned, your leading edge is just the mother of all
condensation nuclii and it instantly freezes.