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Dangers of airframe icing in stratocumulus clouds

Updated: May 1, 2019

Normally pilots look forward to improving weather conditions after the passage of a cold front. The cold, dense air behind the front becomes negatively buoyant and sinks which tends to dry out the air. Moderate northwesterly winds will often prevail on the cold side of the front making for moderate mechanical turbulence sometimes extending up to 8,000 feet AGL. Other than some turbulent air, we don’t typically encounter much in the way of adverse weather behind such a cold front with few clouds, no precipitation and unlimited visibility.

During the spring, how many pilots think about the airframe icing threat that can occur in an overcast stratocumulus deck after the passage of cold front? Even a thin stratocumulus cloud deck like the one shown above can contain a liquid water contents approaching 0.5 g/m3 - especially near the tops. When the temperature is just right, these harmless-looking clouds can surprise a pilot with some moderate or even severe icing while climbing or descending through them.  This is especially concerning to those pilots flying aircraft without certified ice protection systems (IPSs).

Stratocumulus decks have very distinct characteristics from other clouds. Although not completely smooth on top like a stratus deck, they have rather even tops with a quilted-like or lumpy appearance when viewed from above. While a stratocumulus deck can be broken or even scattered it is quite common for these cloud decks to be overcast when they occur after the passage of a cold front. They are rooted in the boundary layer near the surface similar to other cumuliform clouds, but an overcast stratocumulus deck can extend for hundreds of miles making them difficult to avoid.

Let's examine a case in the middle of April near Atlanta, Georgia where many