Stratus is a grey, uniform, very low layer of cloud that looks like fog lifted off the ground. It has no features and no structure: it covers the sky like a ceiling. It is the lowest of the cloud genera, the one that catches on hills, towers and ridges when the air is damp and still.
- Level: Low level (0 to 2,000 m)
- Base height: 0 to 2,000 m, often close to the ground
- Appearance: uniform grey layer with no features, very low base
- Associated weather: low grey sky; drizzle possible
How stratus forms
Stratus forms when a layer of moist air near the ground cools to its dew point without any vertical mixing. Two causes dominate. The first is radiative cooling on a clear, calm night: the ground loses its heat, cools the air touching it, and the vapour condenses. The second is advection: mild, humid air glides over a colder ground or sea and cools by contact.
Stratus is a close relative of fog; the difference is that its base does not touch the ground. Fog can lift into stratus when the sun warms the ground, or stratus can sink and become fog. The droplets are tiny and hardly fall at all, which is where drizzle comes from.
How to recognise stratus
Stratus is light to dark grey, uniform, with no outlines and no lumps, its base most often a few tens or hundreds of metres above the ground. You see the sun through it only when the layer is thin, with a sharp outline. It can appear in tatters, called fractostratus, after rain or at the end of a fog.
It differs from stratocumulus by having no relief at all, and from nimbostratus by being thinner and having no continuous precipitation. If you are inside the cloud, it is fog: on a hill or in the mountains, a stratus can be fog for some and cloud for others.
What weather stratus signals
Stratus means grey, calm, damp weather. It can give a fine drizzle, made of drops smaller than 0.5 mm, which wets without really raining. The air is stable and saturated, so pollution and particles stay trapped.
It generally burns off during the morning as the sun heats the ground, starting from the edges; in winter, when the sun is weak, it can persist for several days. A stronger wind or a change of air mass sweeps it away.
Risks and precautions
The danger of stratus is visibility. When it touches the ground it becomes fog, and driving gets difficult: reduce speed, switch on dipped headlights and fog lights, keep your distance. On ground near freezing, drizzle can freeze and form glaze ice. In the mountains, a stratus ceiling that is dropping means going back down before you are caught in the mist.
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 low level the sky is mostly covered (77% on average).
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
What is the difference between stratus and fog?
The height of the base. Fog touches the ground and cuts visibility below 1,000 m; stratus has its base above the ground. The same cloud can be fog on a hill and stratus on the plain.
Does stratus produce rain?
Only drizzle, made of drops under 0.5 mm across, and snow grains when it freezes. It does not give true rain.
Why does stratus last all winter?
Because the low sun cannot warm the ground enough to burn the layer off, and the calm air under an anticyclone has no reason to sweep it away.
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.