Stratus Clouds: What They Are, How They Form and What Weather They Bring
Weather Station Analyst and CWOP Contributor
Stratus clouds are low, featureless gray clouds that form below 6,500 feet in a flat, continuous layer. They are composed of water droplets and produce only light drizzle or mist, never heavy rain or storms. Stratus and fog are the same phenomenon at different heights. In winter, stratus produces the most dangerous common weather condition: freezing drizzle and black ice.
What Are Stratus Clouds?
Stratus clouds are the lowest of the ten cloud genera recognised by the WMO International Cloud Atlas. They form a continuous, featureless horizontal layer in the lowest tier of the troposphere and are composed entirely of water droplets. Unlike every other common cloud type, stratus has no distinct individual elements, no visible internal structure, and no edges. It covers the sky like a sheet of grey paper from horizon to horizon.
The name comes from the Latin word stratum, meaning layer or sheet, which describes the cloud’s defining characteristic perfectly. Stratus is the cloud that makes an overcast morning feel oppressive: no drama, no texture, no variation. Just grey. That featurelessness is itself the key identification tool: if the sky has no defining shape, no gaps, and no sunlight casting shadows, stratus is almost certainly the cloud overhead.
Stratus identification in this article follows the WMO International Cloud Atlas genus definition, verified against NOAA National Weather Service JetStream cloud classification standards and UCAR MetEd atmospheric science training materials. Altitude ranges and precipitation behaviour are aligned with NWS operational forecasting guidelines. Content researched, reviewed and analysed against these sources. Last verified July 2026.
What Do Stratus Clouds Look Like?
Stratus clouds look like a flat, uniform grey or white sheet covering the entire sky. They have no gaps, no puffy individual elements, and no vertical development. The base is so flat it looks like a ceiling installed across the sky. In many urban environments, stratus sits low enough to obscure the tops of tall buildings and hills, giving the landscape a compressed, closed-in feeling.
The colour of stratus varies with thickness. A thin stratus layer allows enough diffuse light through to appear white or pale grey, and the position of the sun may be visible as a faint, shadowless disc. A thicker stratus deck blocks all direct sunlight and appears uniformly dark grey across its entire base. Either way, the defining characteristic is the absence of shadow on the ground below and the absence of any structure within the cloud itself.
Two specific stratus forms are worth knowing:
- Stratus nebulosus: the most common form. A diffuse, featureless grey veil with no structure at all. This is what most people picture when they think of a typical overcast day.
- Stratus fractus: ragged, shredded fragments of stratus that hang below the main base, often seen in windy or precipitating conditions. Also called scud. These fragments move quickly and give the sky a turbulent, unsettled appearance even though the overall atmosphere remains stable.
How Do Stratus Clouds Form?
According to the NOAA National Weather Service, stratus clouds form when moist air near the surface is cooled to its dew point, causing water vapour to condense into tiny droplets in a stable, non-turbulent layer. Three mechanisms produce this cooling:
Stratus clouds are the most abundant cloud type on Earth’s surface. They cover roughly 23% of the global ocean at any given time and play a critical role in Earth’s energy balance by reflecting solar radiation back into space. The stratocumulus and stratus sheets over the subtropical oceans reflect so much sunlight that they are estimated to offset several degrees of warming from greenhouse gases. Climate scientists consider their accurate simulation in models to be one of the most important unsolved problems in climate science.
What Weather Do Stratus Clouds Bring?
Stratus clouds bring overcast skies, reduced visibility, and light drizzle or fine mist. They are the product of stable, non-turbulent atmospheric conditions, which means they produce none of the severe weather associated with unstable air: no thunderstorms, no large hail, no heavy downpours, no strong gusty winds. The atmosphere under stratus is calm. The hazard is subtle.
The light drizzle stratus produces consists of tiny droplets too small to fall as conventional rain. They drift downward slowly and can reduce visibility to a few hundred feet without accumulating enough water to run off a road surface quickly. This is relevant in two situations: it makes surfaces slippery without appearing wet, and it accumulates on windshields faster than wipers can clear it at low speeds.
When the stratus layer thickens, lowers further, and the drizzle becomes continuous and moderate, the cloud has transitioned to nimbostratus. The practical rule is: if the precipitation is heavy enough to need an umbrella, the cloud is no longer stratus.
During testing of consumer weather stations across multiple stratus events, relative humidity readings consistently reached 95 to 100% at the surface several hours before a stratus deck became visible overhead. Barometric pressure during stratus events is notably different from frontal cloud sequences: pressure typically remains stable or changes only very slowly, confirming the non-frontal, stable-air origin of most stratus. A humidity sensor logging hourly trends is the most useful instrument for anticipating morning stratus formation from the previous evening dew point data.
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Monitoring dew point and relative humidity gives the earliest warning of stratus formation. See our dew point explainer and home weather station reviews for models that log humidity trends overnight.
Is Stratus Cloud the Same as Fog?
Yes. Fog and stratus are the same meteorological phenomenon at different heights. The WMO defines fog as a suspension of water droplets reducing visibility to below 1 kilometre at the Earth’s surface. Stratus is defined by the same droplet composition but with its base above the ground. The only difference is whether the cloud is touching the surface.
This relationship is directly observable. A fog bank that lifts slightly as morning temperatures rise becomes a low stratus layer. A stratus deck that lowers as temperatures drop in the evening becomes fog. Coastal marine fog that rolls inland across San Francisco Bay is stratus cloud that happens to be touching the ground at the shoreline but rises to a stratus base over the bay itself.
The most common identification error is confusing stratus with altostratus. The key test is the shadow. Under stratus, there are no shadows on the ground at all, because the cloud is thick and close enough to produce completely flat, diffuse light. Under altostratus, the sun appears as a dim disc through frosted glass and casts faint shadows. If you can see which direction the sun is, even dimly, the cloud is almost certainly altostratus, not stratus.
Winter Stratus: The Black Ice Problem
Stratus clouds are most dangerous not in summer but in winter, and not because of heavy snow but because of something far harder to see: freezing drizzle. When air temperatures are below 32 F (0 C) but the stratus layer contains supercooled liquid water droplets (droplets that have not yet frozen despite being below the freezing point), the fine drizzle falling from the cloud freezes instantly on contact with road surfaces, bridges, and pavements.
As the American Meteorological Society defines it, freezing drizzle is drizzle that falls in liquid form but freezes upon impact with the surface. It forms an extremely thin, nearly invisible glaze of ice, commonly called black ice, that provides far less friction than visibly icy conditions. Drivers often have no warning until they have already lost traction.
If you are under a stratus deck and the temperature is at or below 32 F (0 C), treat the roads as potentially icy even if they appear wet rather than frozen. The drizzle from the cloud is liquid when it lands but may freeze within seconds on road surfaces that are at or below freezing. Bridges and overpasses cool faster than road surfaces and are the highest-risk locations. This condition is frequently more dangerous than a visible snow event because it develops without visual warning.
Stratus Cloud Formation in Motion
Time-lapse footage shows how stratus develops as a fluid layer rather than individual cloud objects. Notice how the deck rolls in uniformly from one direction and fills the sky completely, with no individual elements or gaps. This behaviour makes stratus visually distinct from stratocumulus, which shows a similar height but retains individual rounded masses separated by breaks of sky.
Stratus vs Similar Low Cloud Types
Three low-level cloud types are commonly confused because they share altitude and grey colouring. The differences are in texture, precipitation, and the sun test.
| Cloud type | Altitude | Texture | Sun visible? | Precipitation | Key difference |
|---|---|---|---|---|---|
| Stratus | Below 6,500 ft | Featureless flat sheet | No shadows at all | Drizzle only | No structure, no gaps, no shadows |
| Nimbostratus | Low to mid | Dark, thick, opaque | No | Steady rain | Darker, thicker, active rain |
| Stratocumulus | Below 6,500 ft | Lumpy rolls with gaps | Between rolls | Rarely | Has individual elements and gaps |
| Altostratus | 6,500 to 20,000 ft | Grey sheet, slight fibre | Frosted glass | Virga or light | Sun visible as dim disc |
| Fog | Ground level | Same as stratus | No | Drizzle | Base touches the surface |
Where Are Stratus Clouds Most Common?
Stratus clouds are not evenly distributed around the world. They concentrate in specific geographic settings where the conditions for their formation, moist low-level air and a cooling mechanism, occur frequently or persistently.
California and the US West Coast. The California marine layer is the most studied stratus system in the world. Cool upwelling water from the California Current chills the moist marine air above it to dew point, producing a persistent stratus deck that covers coastal areas from May through September. In Los Angeles and San Francisco, “June Gloom” is a cultural reference to this phenomenon: mornings under a grey stratus layer that typically burns off by early afternoon as inland heating draws the marine layer inland and then evaporates it from the top down.
The Pacific Northwest. Western Oregon and Washington receive stratus cloud year-round, but especially in autumn and winter when the Pacific delivers a continuous supply of moist maritime air over the region’s complex terrain. Low stratus and drizzle with bases at 500 to 1,500 feet are the default conditions for cities like Seattle and Portland for months at a time.
The United Kingdom and Northern Europe. The Atlantic Ocean supplies warm moist air that meets the British Isles, where it encounters cooler air and gentle terrain uplift. Persistent low stratus, locally called overcast or “dreich” in Scotland, is one of the most characteristic weather types across England, Wales, Scotland, and Ireland from October through March.
Inland valleys and river basins. Cold air drains downhill overnight and pools in low-lying valleys, where it cools to dew point and produces stratus or radiation fog by morning. The Central Valley of California, the Rhine and Moselle valleys in Europe, and mountain-ringed basins across the Rocky Mountain west all experience regular morning stratus that lifts and dissipates as daytime heating begins.
Winter mornings globally. Any location with moderate humidity and a calm, clear night is susceptible to radiation stratus. Long winter nights give the ground maximum time to cool the surface air, and the lack of solar mixing in the morning keeps the shallow stratus layer intact until the sun is high enough to evaporate it from above.
A home weather station with overnight humidity logging makes it straightforward to predict which mornings will produce local stratus: watch for evening humidity above 85% with calm winds and a clear sky at sunset. That combination almost always produces stratus or fog before dawn.
Sources and Classification Standards
Definitions, altitude ranges, and precipitation behaviour on this page are verified against:
- WMO International Cloud Atlas: primary global standard for stratus genus definition
- NOAA National Weather Service JetStream: operational cloud classification and formation
- UCAR MetEd Training: atmospheric stability and low-level cloud formation
- NASA Earth Observatory: stratus climate role, marine stratus, and radiation balance
- American Meteorological Society Glossary: stratus and freezing drizzle definitions
Content researched, reviewed and analysed against the above sources. Last verified July 2026.
Frequently Asked Questions
Stratus clouds are low-level, featureless gray clouds that form below 6,500 feet in a uniform horizontal layer. They are composed of water droplets and cover the sky in a continuous sheet with no distinct shapes or gaps. They are the lowest of the ten WMO cloud genera and are most commonly associated with fog, light drizzle, and overcast winter mornings.
A stratus cloud looks like a uniform gray or white sheet covering the entire sky with no gaps, distinct shapes, or defined edges. It resembles a low ceiling or blanket of cloud. Unlike cumulus clouds, stratus has no vertical development and no individual elements. It may be thin enough to show the sun as a pale disc, or thick enough to block all direct light and produce a flat grey illumination across the ground with no shadows at all.
Stratus forms when moist surface air is cooled to its dew point, causing water vapour to condense into a stable, flat layer of droplets. The three main mechanisms are radiative cooling overnight, advection of warm moist air over a cold surface such as a cold ocean current, and gentle upslope lifting along terrain. All three produce stable, non-turbulent air which is why stratus always forms as a flat layer rather than developing vertically like cumulus clouds.
Yes. Fog is a stratus cloud with its base at or below ground level. When fog lifts off the surface and its base rises above the ground, it becomes stratus. When a stratus deck lowers and its base touches the surface, it becomes fog. The two are the same meteorological phenomenon at different heights, with the same composition, the same formation process, and the same precipitation type.
Stratus produces only light drizzle or fine mist, not significant rain. The stable, non-turbulent air that creates stratus does not support the updrafts needed to grow large raindrops. When precipitation intensifies and becomes continuous moderate rain, the cloud has thickened and lowered into nimbostratus. The practical rule: if you need an umbrella rather than just a hood, the cloud overhead is nimbostratus, not stratus.
Stratus clouds are the lowest cloud type. They occupy the bottom tier of the atmosphere, commonly with a base below 2,000 feet. In coastal regions and valleys, stratus bases can be just a few hundred feet above the surface. This extremely low altitude is what makes stratus responsible for the poor visibility and reduced aviation ceiling heights that distinguish a stratus day from the merely overcast conditions of higher cloud types.
Stratus brings overcast skies, reduced visibility, and light drizzle. In summer this is mainly a nuisance. In winter, stratus is potentially dangerous: when temperatures are at or below freezing, the drizzle freezes on contact with road surfaces and creates black ice. Stratus never produces thunderstorms, hail, or heavy rain, all of which require the atmospheric instability that stratus conditions specifically lack.