Altocumulus is extensive cloud at mid-level in the troposphere, whose lumpy appearance shows the convection within it (Table 8.2). It can take many forms, but is usually a layer of flattened globules of cloud, in groups, waves or lines, often less than 100 metres thick.
One type of altocumulus consists of the wave clouds discussed in Section 8.1. There are also billow clouds, which form a ribbed pattern and occur where the air at different altitudes is moving at different speeds. A layer of air becomes rucked up into a wave-like shape and the ridges of the waves are high enough to reach dewpoint, so that they fill with billow clouds. The exact form of the billows is influenced by convection and the overall pattern of wind. The billows move with the wind, unlike wave clouds, which are anchored to the hills that cause them.
Other ways of forming altocumulus are as follows:
1 The release of any potential instability within altostratus by slow uplift breaks the layer into a series of turrets. Afternoon thunderstorms are possible if this kind (called Ac castellanus) occurs in the morning, though rain from Ac usually evaporates before reaching the ground.
2 Radiative cooling of the top of a thick stratus cloud in the tropics may break up the flat top into a series of turrets. This tends to happen during the late afternoon.
3 Some ice crystals may form in altostratus (As) which consists of supercooled water (Section 6.2), triggering widespread glaciation throughout the cloud (Chapter 9). The consequent release of latent heat of freezing (Section 4.1) causes convection, which extends the As vertically into altocumulus.
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