[Ref Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 12-24]
Updrafts in a thunderstorm support abundant liquid water with relatively large droplet sizes in the upward current.
When water content carried above the freezing level, the water becomes supercooled.
When temperature in the upward current cools to about –15 °C, much of the remaining water vapor sublimates as ice crystals.
Clear icing can occur at any altitude above the freezing level, but at high levels, icing from smaller droplets may be rime or mixed rime and clear ice.
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