Large amounts of condensation in a convective storm producing large water droplets which then freeze into large snowflakes in the upper levels of clouds.
Weak convection concentrates moisture in the lower parts of clouds where condensation creates small supercooled water droplets or ice crystals.
Strong convective activity lifting large concentrations of moisture to high altitudes where sublimation then creates very small ice crystals.
Strong up-currents that allow large ice crystals to develop through collision and coalescence from small ice crystals.
Refer to figure.
Ice crystal icing (ICI) condition refers to aircraft experiencing icing inflight in high altitude due to high concentration of small ice crystals. At very low temperatures, the water vapour turns directly into solid ice crystals by deposition (often referred to as "sublimation" in meteorology). Engine surfaces and pitot tubes are affected. Several engine power-loss and damage events have occurred in convective weather above the altitudes typically associated with icing conditions. Research has shown that strong convective weather (thunderstorm activity) can lift high concentrations of moisture to high altitudes where it can freeze into very small ice crystal.
- Ice crystals do not adhere to cold airframe surfaces because the ice crystals bounce off. However, the crystals can partially melt and stick to relatively warm engine surfaces. It has also been noted that ice crystals impacting heated windscreens can result in pilots observing “rain” as the crystals rapidly melt on contact with the heated windscreen.
- The main risk of encountering high crystal concentrations appears to be downwind from the tops of large areas of convective cloud - the area where the visible anvil shape is seen when viewed from a distance.
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