The nimbostratus (Ns) is a thick, grey, featureless cloud layer that produces continuous and often prolonged precipitation — steady rain or snow that can last many hours. It belongs to the middle étage (2–6 km) but its base often descends below 2 km and its top can extend well into the upper troposphere, making it one of the vertically thickest clouds. Unlike cumulonimbus, it does not produce lightning or hail.
Nimbostratus forms primarily along warm fronts and occluded fronts, where warm, moist air is forced to rise slowly over a wedge of cooler air. The cloud develops progressively: cirrus and altostratus thicken into nimbostratus as the front approaches. The resulting precipitation is widespread and moderate in intensity — the classic "grey, rainy day" across northern and central Europe in autumn and winter.
Nimbostratus is classified as opacus because it is thick enough to completely obscure the sun — you cannot determine the sun's position through it, unlike thinner altostratus. Ragged cloud fragments called pannus (or scud) often form beneath the main layer in the turbulent, rain-saturated air. For aviation, nimbostratus is significant because it creates extended areas of low ceiling, poor visibility, and icing. For hydrologists, the prolonged, steady precipitation it delivers is the type most efficiently absorbed by soil, replenishing groundwater — in contrast to the brief, intense downpours from convective clouds.