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In which of these cloud types can icing be virtually ruled out?
  • A
  • B
  • C


  • D


Refer to figure.
Learning Objective Assess the 10 cloud types for icing and turbulence.

In flight icing occurs when an aircraft flies through icing conditions.

Icing conditions are present within clouds below freezing temperature (0ºC), where the suspended water droplets have not yet become ice crystals. These droplets are held in a "supercooled" state, in that their temperature is less than 0ºC, but they are still liquid. We call them Supercooled Water Droplets (SCWDs).

When an aircraft flies through a cloud of SCWDs, some of them will hit the airframe (which is also below freezing temperature), and this will cause the droplets to freeze, adhering to the airframe in the process.

The type of icing we see depends on how quickly they freeze. If the SCWDs are very cold (between -45ºC and -20ºC), or quite small, then they will usually become completely frozen immediately. This forms rime ice, a cloudy, white icing that occurs on wing leading edges, propeller spinners, and aircraft protrusions such as pitot probes.

However, if the SCWDs are large droplets (found in cumulus and cumulonimbus clouds) and are warmer, between 0ºC and -20ºC, then they may only partially freeze, and the latent heat produced will allow the rest of the droplet to flow backwards over the wing, freezing as it goes. Many of these droplets will combine to form a transparent lacquer of ice over the wing, called clear ice. This type of icing ruins the aerodynamics of an aircraft whilst also increasing its weight, a very dangerous combination.

However, not all clouds contain possible icing conditions. Above 0ºC OAT, the air and airframe are too warm for icing to occur, and below -45ºC, all water is just ice crystals, which bounce off the airframe instead of adhering to it.

This is what this question is referring to, as it is asking where icing would NOT be found. Therefore CS (cirrostratus) clouds are the correct answer, as they are high level clouds made of only ice crystals.

NS, Nimbostratus clouds contain plenty of water and occur at low and medium levels, and are capable of moderate icing (severe icing is also possible in NS, but is much more often found in cumulonimbus clouds - CBs).

AS, Altostratus clouds are medium level, so will contain a mixture of ice crystals and SCWDs.

SC, Stratocumulus clouds are low level clouds with some convective activity, so can also cause icing conditions.

Note: other types of icing include ice crystal icing, hoar frost, and rain ice. These are not caused by SCWDs.

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