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Where are the highest concentrations of ice crystals most likely to occur?
  • A
    In the spreading cirrus from the top of any warm front.
  • B
    At the freezing level surrounding the central uplift core of a convective storm.
  • C
    Within downdraughts associated with a large tropical revolving storm.
  • D
    In the anvil overhead the core of an oceanic mesoscale convective system.

Learning Objective 050.09.01.04.03: Identify weather situations and their relevant areas where high concentrations of ice crystals are likely to occur.


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 far 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 crystals, mostly invisible to the human eye, and to weather radars.

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.

These areas are perfect places for ICI to occur, in and above the tops of huge convective clouds with plenty of moisture carried up to the high altitudes. Therefore, we can keep a good lookout for such clouds using our onboard weather radar, which readily picks up cumulonimbus clouds due to their large water content.

This question calls them oceanic mesoscale convective systems, which simply means that the storms are full of moisture from the sea below, and are at least medium in size. Smaller convective systems would not be powerful enough to cause significant ice crystal icing.

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