Nimbostratus: The Grey Layer That Erases the Sun

Nimbostratus is easiest to identify in this order: layer, light, precipitation. Look for a broad, dark-grey sheet with no visible cells or breaks. The thick layer hides the Sun completely, and rain or snow blurs the lower edge. Low ragged cloud fragments, called pannus, may hang underneath.1

WMO classifies Nimbostratus as a middle-level cloud genus, but the main body can extend into low and high levels. In temperate regions, the main body commonly spans about 2–7 km; the base can sit lower. Nimbostratus usually forms when Altostratus deepens and lowers as moist air rises over a broad frontal zone. A front is a boundary between unlike air masses. Once the layer becomes thick enough to hide the Sun, continuous precipitation usually begins.23

Rain or snow usually reaches the ground, but virga evaporates before it gets there. A faint Sun favours Altostratus; a defined low base with drizzle favours Stratus; visible lumpy elements favour Stratocumulus. Lightning, thunder, or hail is the practical signal to classify the storm cloud as Cumulonimbus instead.45

Visual credits: WMO International Cloud Atlas photos — Image 5732, © Edward Engelbrecht; Image 5174, © Martin Gudd; Image 5143, © Jüri Kamenik; Image 4389, © A. Viaut; Image 4717, © Frank Le Blancq; Image 5704, © Art Rangno; Image 5993, © Antonio J. Galindo. Altostratus image: NOAA/NESDIS. WMO image-use terms: 6 Personal, non-commercial use is permitted without further permission; other uses require permission from the copyright holders.