A stratus cloud (St) may extend horizontally for hundreds of kilometres, but be only 50-500 m thick, with little tendency for vertical growth because the cloud forms by slow uplift within stable air (Table 8.2). The top of the cloud is typically surmounted by an inversion due to radiation cooling of the top of the cloud. Such cloud prevents sunshine reaching the ground in winter at high latitudes, when the Sun is low (and therefore the cloud's albedo particularly high—see Section 2.5), and we describe the sky as 'leaden'.
Stratus sometimes results from mixing in the planetary boundary layer (PBL). This cools the top of the layer to about 10 K below screen temperature if it is 1,000 m thick, for instance. If the cooling reaches dewpoint, a thin layer of stratus forms just below the PBL inversion (Section 7.6). Such stratus forms over the cold water of the Humboldt current (Chapter 11) on the west of South America, where the subsidence/ PBL inversion is only 300-500 m high, and then it may be blown about 50 km inland to the edge of the Andes. The stability of the onshore wind means that the coastal strip receives almost no rain (Chapter 16), yet plants at around 400 m altitude thrive on water they intercept from the cloud.
Altostratus (As) appears as a grey-bluish, striated or fibrous veil, blurring the Sun. It is stratus of the middle troposphere (Table 8.2, Figure 8.4, Figure 8.8). Another reason for altostratus is the spreading and decaying of the anvil from a medium-size cumulonimbus (Section 8.6).
Nimbostratus (Ns) is a large layer of low-level cloud, but precipitating rain, unlike stratus. The rain is usually continuous but light, because the atmosphere is stably stratified and the cloud arises from only gradual lifting. Such cloud can result from orographic uplift; Figure 8.8 shows a stable moist airstream forming a cap of nimbostratus with higher layers of the atmosphere raised to make altostratus.
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