Hazard Warning: Ice accretion on aircraft.
While altostratus is often flat and featureless, altocumulus usually creates interesting and varied skies. In some cases, many thousands of small altocumulus clouds will be strung together across the sky in spectacular formations.
As with altostratus, altocumulus normally occurs when a large air mass is lifted to middle levels by a landmass or an incoming frontal system, and condensation occurs over a wide area. The principal difference between the two formations is that altocumulus is affected by instability in the surrounding
atmosphere. This gives rise to its distinctive cumuliform texture.
In isolation, altocumulus does not have great significance for
the weather-watcher, although it can produce light precipitation if the deck is sufficiently thick. However, if the extent of an altocumulus formation appears to be increasing during the course of a day, this may be a sign of an approaching frontal system.
Often altocumulus and altostratus appear together in a
mixed sky. Satellite photography reveals that mixed altocumulus-altostratus formations may extend over thousands of square miles, particularly when associated with a frontal system.
If altocumulus combines with a thick deck of altostratus
at a level where the temperature is below freezing, significant airframe icing may affect the aerodynamics of aircraft flying through the cloud. Otherwise, slight to moderate turbulence
will be the only concern for the pilot. Altocumulus formations are often more distinct and dramatic at sunrise and sunset.