Nimbostratus is the thick grey layer that brings steady rain or snow. Its base is low and blurred, its top rises into the middle level, and it can cover several regions at once. It hides the sun completely: it is the cloud of the grey days at the heart of a depression, when it rains for hours without a break.
- Level: Middle level (2,000 to 7,000 m)
- Base height: 0 to 2,000 m
- Appearance: dark grey, very thick layer with a blurred base
- Associated weather: steady rain or snow lasting hours
How nimbostratus forms
Nimbostratus forms when a vast mass of moist air rises slowly over hundreds of kilometres, along a front or in the core of a depression. The altostratus thickens and lowers until its base reaches the low level and its depth becomes very large. In such a depth, droplets and ice crystals have time to grow.
The upper part of the cloud is rich in ice crystals. These grow at the expense of the liquid droplets around them, because vapour deposits more easily on ice than on water: this is the Bergeron process. The crystals, now heavy, fall; crossing the warmer lower part they melt and reach the ground as rain, or stay as flakes if the air is cold enough.
How to recognise nimbostratus
Nimbostratus is dark grey, sombre and uniform, with no outline or structure. Its base looks blurred because the precipitation smears it. Beneath the layer, small ragged clouds called pannus hang a few hundred metres above the ground. Visibility is reduced and the light very dim, even at midday.
What sets it apart from altostratus is the continuous precipitation: rain or snow reaches the ground and keeps on. From stratus, which can give drizzle, it differs by its thickness and by the intensity of its precipitation; from cumulonimbus, by the steady, even rain, with no showers or lightning.
What weather nimbostratus signals
Nimbostratus does not announce the weather: it is the weather. When it is overhead, rain or snow continues for several hours, often steady and light to moderate. The wind is usually sustained, and the temperature changes little between day and night.
A warm front brings rain that sets in and lasts; at a cold front it ends sooner and gives way to a showery sky behind. To judge when the rain will stop, watch the pressure: when it stops falling and starts to rise, the nimbostratus is moving away.
Risks and precautions
Nimbostratus does no damage in itself, but the duration of the precipitation tells: saturated ground, run-off, rising rivers. In winter, rain falling on frozen ground can turn to ice. Roads become slippery and visibility drops. In such conditions slow down, keep your distance and follow your country's official warnings.
In the sky over Toronto today
Here is what the Open-Meteo forecast of 2026-11-02 08:13 gives for Toronto, averaged over the next 24 hours: 77% of the sky covered at low level, 58% at middle level and 76% at high level.
For this kind of cloud the level that matters is this one: at middle level the sky is partly covered (58% on average).
Forecast rainfall over the next 48 hours: 2.9 mm.
These percentages are model values on a grid: they say how much of the sky is occupied at each level, not which kind of cloud is there. They change at every update.
Frequently asked questions
How long does nimbostratus rain last?
Several hours, sometimes half a day or more, because the cloud spreads over hundreds of kilometres and moves with the pressure system that formed it. The rain is continuous, light to moderate.
What is the difference between nimbostratus and cumulonimbus?
Nimbostratus gives steady, even rain with no lightning, under a uniform layer. Cumulonimbus gives sudden, intense showers, often with hail, lightning and a gusting wind, under a very tall cloud.
Why is nimbostratus so dark?
Because of its thickness: through such a depth the droplets and crystals absorb and scatter the light, so very little reaches the ground.
Sources
- World Meteorological Organization (WMO), International Cloud Atlas, WMO-No. 407: definitions of the genera, levels and base heights for temperate regions. https://cloudatlas.wmo.int (copyright WMO; no text or image from the Atlas is reproduced here).
- Drawing and text on this page: original to this site, schematic.