Hi Dozy,
Originally Posted by DozyWannabe
In that part of the atmosphere what you're dealing with is not ice as it is generally considered, but supercooled water. This is water that is below freezing point, but still liquid because it has almost zero contaminants. Stick a big piece of metal into that water and it freezes into ice in less than a second. This was demonstrated in the BBC/NOVA documentary.
More likey, some very small ice crystals (which are not detectable by radar) and not supercooled water at this level flight (which is detectable).