Most people, myself included, would say 0C to -10C and almost impossible in -15C.
And sure enough I have never seen airframe ice in -15C.
So…
About 5 mins at FL190 -15C produced this
and after about an hour in sunshine the rivets just started to clear a little, as can be seen above. The thickness is only about 2mm but sublimation is really slow.
This ice reduced the operating ceiling from FL190 to FL180.
The presence of ice at those temperatures in stratiform clouds depends on how long the clouds have been around and on how many ice nuclei are available. Dust is often a good ice nucleus. Particles from pollution may be.
In stratiform clouds that have been around for a while, you’re most likely to find the liquid at the top of the cloud.
you’re most likely to find the liquid at the top of the cloud.
Yes; I could see the sun just about shining through, when the speed started to drop off.
