Altocumulus

The small, white, puffy clouds that sometimes slowly drift across the sky can look like dozens, of small, loose cottonballs. This is Altocumulus and it forms between 8,000-18,000 ft (2.5-5.5 km). Altocumulus can develop in several ways. Moist air is cooled by turbulence, then lifted up slightly and cooled to form a layer of cloud at that height. Altocumulus usually forms in a layer of moist air, where air currents undulate gently, like waves on the sea. As a wave rises, water vapour condenses, and there is cloud. In the wave troughs, water evaporates. There the cloud is thinner or the sky may be clear, producing the bands of cloud that are sometimes seen.
Stratocumulus occasionally appears in the evening or early morning, disappearing during the day.

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