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Altostratus translucidus (As tr) – can form from a descending layer of higher cirrostratus or more commonly from ascending warm air on the a approach of a warm front, thickening into altostratus opacus or nimbostratus indicating impending rain. The outline of the sun is seen through this grey-blue cloud, often accompanied by coronas or iridescence (haloes). |
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Altostratus opacus (As op), or Nimbostratus (Ns) – all too familiar in the UK, these clouds are associated with grey, gloomy, wet days. Often forming from descending and thickening altostratus translucidus, altostratus opacus thickens into nimbostratus as rain starts to fall. |