Peter wrote:
This is what is seen all the time in ski resorts where you get snow on the hills and rain in the town below. If this was freezing rain then it would be a huge problem because it is a really common thing.
Well indeed, most rain starts with water in the ice phase (Bergeron process) which melts to liquid water as it falls.
Interesting flight today.
SFC +16C (EGHE-EGKA). Climbed FL100 and the OAT was 0C at FL100. Heavy rain all the way up and still heavy at FL100/0C. I had the full TKS on max just in case, obviously, but no ice accreted on unprotected surfaces (which is normal at 0C). Didn’t want to climb any more, or even sit there using up the £5/litre fluid, so went back down to FL080 (+3C).
What was interesting was that the rain showed no interest in reducing at 0C. I think that is unusual.
There was a thick layer above from which the rain was apparently falling.
What should I have expected had I climbed further?
The 2nd is obviously unusual in practice. And normally I find that rain stops before 0C; perhaps at +1C to +3C.
Peter wrote:
There was a thick layer above from which the rain was apparently falling.What should I have expected had I climbed further?
OAT remaining at 0C
Freezing rain?
It’s hard to give definite answer from those few details without knowing the exact layering, but two thoughts:
- Within clouds the OAT is typically a bit higher than outside due to condensation heat. Therefore if it is 0C just below the cloud, at least in the lowest layer of the cloud it should be expected to be above freezing
- Freezing rain typically is created when already existing rain drops are flying through air that is significantly below 0. Can be due to a massive inversion (on a warm front) or more likely due to updrafts in Cb that transport the raindrops up into layers with such low temperatures.
If it is significantly below 0 in the cloud where the rain would form, one typically gets snow and not freezing rain.
Peter wrote:
Clearly freezing rain is so rare that nobody has ever seen it
Just once in my life – and that was luckily not in plane but in a car on the highway – one of the even rarer events when freezing rain made it to the ground. We accumulated more than 1 cm of Ice in 60 seconds – when I stopped under a bridge…
It was IMC so I wasn’t able to see where the layers were, but
If it is significantly below 0 in the cloud where the rain would form, one typically gets snow and not freezing rain.
is a very good point; it may well have been snow. It wasn’t sticking anywhere. At 140kt+ one just sees horizontal streaks.
Malibuflyer wrote:
Within clouds the OAT is typically a bit higher than outside due to condensation heat. Therefore if it is 0C just below the cloud, at least in the lowest layer of the cloud it should be expected to be above freezing
Would that be true even with precipitation falling? The precipitation would cool the air both by convection (being colder) and by evaporation.
Doesn’t freezing rain typically occur as a warm front arrives over cold air or a cold front pushes under warm air? In both cases rain is coming from warmer air into the colder air below. If the air below is 0 degrees C the rain can form freezing rain. The rain might also freeze when hitting a cold aircraft causing what is basically black ice on the aircraft. Not nice!
gallois wrote:
Doesn’t freezing rain typically occur as a warm front arrives over cold air or a cold front pushes under warm air?
That is the main possibility where freezing altitude goes quickly down bellow the warm front as it meets the cold front but you still get rain from the top of the warm front, actually you can argue that the concept of “freezing altitude” does not exist anymore as it extends almost vertically precluding any strategy that involve climbing/descending 
gallois wrote:
Not nice!
Indeed, coming low from the cold front side is death trap: first, climbing higher will not help 1/ as freezing altitude will climb steeply as well and 2/ even above freezing altitude the airframe is still cold and will freeze liquid rain, second, staying level will not help and going down will not help as you will collect freezing rain that falls with gravity
The cold airframe is an extra but it’s the shape of the (-5Cdeg; 0Cdeg) envelop that makes this very interesting 
In raw numbers that shape will encapsulate the typical GA climb/decent gradients, bingo !
Are supercooled water droplets, considerably below zero, but lacking a nucleus to initiate solidification, relevant?
I thought that is what freezing rain is. The droplets don’t freeze because there is no nucleus, no?
