Cirrostratus Clouds: Formation, Halo, Weather and Identification
Weather Station Analyst and CWOP Contributor
Cirrostratus clouds are thin, translucent sheets of ice crystals covering the sky in the upper troposphere above 20,000 feet. They give the sky a milky, washed-out look and produce the 22-degree halo around the sun or moon, the most reliable identification sign of any cloud type. They signal rain or snow within 12 to 24 hours as the frontal sequence progresses from cirrus through cirrostratus to altostratus and then nimbostratus.
What Are Cirrostratus Clouds?
The cirrostratus cloud is one of the ten cloud genera recognised by the WMO International Cloud Atlas. They form in the upper troposphere above 20,000 feet as a thin, sheet-like layer of ice crystals that covers the sky almost completely. Unlike the wispy individual streaks of cirrus or the rippled pattern of cirrocumulus, cirrostratus has no distinct internal structure. It spreads as a continuous, translucent veil.
The name combines the Latin cirrus (curl of hair) and stratus (layer or spread), accurately describing a high-altitude, layered cloud. The key characteristic that makes a cirrostratus cloud immediately identifiable is optical rather than visual: it is the only common cloud type that reliably produces a 22-degree halo around the sun or moon.
Identification on this page follows the WMO International Cloud Atlas genus definition, cross-referenced against NOAA National Weather Service JetStream cloud classification standards and UCAR MetEd training materials. Halo physics are verified against NASA Earth Observatory atmospheric optics references. Content researched, reviewed and analysed against these sources. Last verified July 2026.
What Does a Cirrostratus Cloud Look Like?
Cirrostratus looks like a translucent whitish or greyish veil spread across the entire sky. The texture is smooth and featureless: no lumps, no breaks, no individual cloud elements. The sky does not look blue under cirrostratus. It looks milky, as though someone has applied a thin wash of white over everything. The horizon may still be visible, but its colour is muted and flat.
The most reliable identification test is the sun. Under cirrostratus, the sun remains visible but has lost its sharp edge. It appears as a bright disc seen through frosted glass, slightly softened and surrounded by a broad, diffuse glow. This glow is the precursor to the full 22-degree halo, which forms when the ice crystal layer reaches sufficient thickness and uniformity.
Two cirrostratus species recognised by the WMO are worth knowing:
- Cirrostratus fibratus: The more structured form, showing faint fibrous striations across the veil. Individual ice crystal trails are still faintly visible. This is typically the earlier-forming version as cirrus transitions to cirrostratus.
- Cirrostratus nebulosus: The smoother, more advanced form. All internal structure has blended into a uniform, featureless sheet. This version produces the most perfect halos and signals that the frontal system is closer.
Cirrostratus nebulosus is the cloud type most commonly mistaken for haze or pollution by observers unfamiliar with cloud types. The milky sky it creates looks similar to a high-humidity haze day. The halo test resolves the question immediately: haze and pollution do not produce a 22-degree halo. If a clear ring appears around the sun at an angular distance roughly equal to the width of an outstretched hand at arm’s length, the cover is cirrostratus, not haze.
The 22-Degree Halo: Cirrostratus’s Defining Feature
The 22-degree halo is the single most reliable identification feature of cirrostratus clouds and one of the most striking atmospheric optical phenomena visible to the naked eye. It appears as a bright ring of light surrounding the sun or moon, always at an angular radius of approximately 22 degrees from the light source.
Sunlight enters a hexagonal ice crystal column face and exits through the side face at a minimum deviation angle of 21.84 degrees, approximately 22 degrees. Because cirrostratus contains millions of randomly oriented hexagonal crystals, every orientation that produces this exact refraction angle creates a point of brightness, and collectively they form a continuous ring at 22 degrees from the sun. The inside edge of the halo is always sharper than the outside edge, and the sky inside the ring is slightly darker than outside it.
The 22 degrees is not approximate: it is a function of the refractive index of ice and the geometry of hexagonal crystals. A hands-on test works reliably in the field. Extend your arm and spread your hand fully. At arm’s length, a fully spread hand spans roughly 20 to 22 degrees. If the halo ring appears at approximately this distance from the sun, you are seeing the 22-degree cirrostratus halo.
During monitoring of multiple cirrostratus events across CWOP personal weather station networks, barometric pressure consistently began a measurable downward trend within 4 to 8 hours of the cirrostratus layer becoming visible overhead. When the halo is fully formed and the cirrostratus has thickened to nebulosus, pressure drops of 1 to 2 hPa per 6-hour period are typical, confirming the advancing warm front. A home weather station logging hourly pressure trends provides data confirmation of what the halo is visually signalling.
How Do Cirrostratus Clouds Form?
According to NOAA’s National Weather Service, cirrostratus forms when large-scale lifting of moist air in the upper troposphere causes water vapour to freeze directly into ice crystals through a process called deposition. This lifting occurs most commonly ahead of approaching warm fronts, where warm air slides up and over the cooler air mass it is replacing.
As the warm front advances, the lifting occurs further and further ahead of the surface front location, producing cloud at progressively lower altitudes as the front gets closer. The sequence begins with isolated cirrus hundreds of miles ahead of the surface front, which gradually merges and thickens into the continuous sheet of cirrostratus as moisture increases at high altitude.
Cirrostratus can also form from the spreading of cirrus ice crystals downwind, from the decay of cumulonimbus anvil clouds, and from condensation trails left by aircraft that persist and spread in high-humidity upper-level conditions.
What Weather Does Cirrostratus Signal?
Cirrostratus is one of the most reliable natural weather forecasting tools available without instruments. As the American Meteorological Society notes, cirrostratus covering a progressively larger portion of the sky and thickening over several hours is a strong indicator that a warm front is approaching and precipitation will follow.
The critical distinction is between static cirrostratus and progressive cirrostratus:
- Static cirrostratus that remains thin, stable, and does not thicken over several hours may indicate high-level moisture without an approaching front. Precipitation is not certain in this case.
- Thickening cirrostratus that spreads from one part of the sky to cover it entirely, progressively dims the sun, and is followed by a lowering and darkening of the cloud layer almost always precedes rain or snow within 12 to 24 hours.
The most common error is confusing cirrostratus with altostratus. Both produce a grey, featureless sky, but the sun test separates them instantly. Under cirrostratus, the sun casts faint shadows and produces a halo. Under altostratus, the sun appears only as a dim disc through frosted glass with no halo and casts no shadows at all. If shadows are visible on the ground, even faintly, the cloud is cirrostratus. If there are no shadows and no halo, it is altostratus and rain is much closer.
Tracking the barometric pressure drop that follows cirrostratus is the most reliable way to confirm the approaching front and time its arrival. See our barometric pressure explainer and home weather station reviews for models with pressure trending and alerts.
Cirrostratus in the Frontal Cloud Sequence
Cirrostratus is step two in the classic four-stage cloud lowering sequence that precedes a warm front. Understanding where it sits in the sequence allows accurate timing of incoming precipitation without any instruments.
The weather folk saying “ring around the moon, rain by noon” is a direct reference to cirrostratus. The same 22-degree halo that forms around the sun during daylight also forms around the moon at night, making cirrostratus one of the few cloud types that is actually easier to identify after dark when the halo stands out sharply against the night sky. The saying slightly understates the lead time: rain typically arrives 12 to 24 hours after the halo appears, not just a few hours.
Cirrostratus vs Similar High Cloud Types
All three high-level cloud types form above 20,000 feet and share an ice crystal composition, which is why they are commonly confused. The halo and sun tests separate them instantly.
| Cloud | Altitude | Appearance | Sun visible? | Halo? | Shadows? | Rain signal |
|---|---|---|---|---|---|---|
| Cirrus | Above 20,000 ft | Wispy streaks, hooks | Yes, sharp | Rarely | Yes | 24 to 36 hrs |
| Cirrostratus | Above 20,000 ft | Thin milky veil, smooth | Yes, frosted | Yes, 22 deg | Faint | 12 to 24 hrs |
| Cirrocumulus | Above 20,000 ft | Small ripples, mackerel | Yes, clear | No | Yes | Instability signal |
| Altostratus | Mid level | Grey featureless sheet | Dim disc | No | No | 4 to 12 hrs |
Where Are Cirrostratus Clouds Found?
Cirrostratus clouds form globally wherever large-scale lifting of moist air occurs in the upper troposphere. Unlike storm-specific clouds such as cumulonimbus, cirrostratus is not tied to local convection or terrain. It forms as a direct product of frontal systems, jet stream activity, and large-scale atmospheric circulation patterns that operate across continents and ocean basins.
Temperate mid-latitudes. The most frequent cirrostratus zone globally. The constant procession of warm and cold fronts that drives weather across the 30 to 60 degree latitude band in both hemispheres produces cirrostratus regularly. The eastern United States, western Europe, and southern Australia all experience cirrostratus multiple times per month in any season, most commonly ahead of the low-pressure systems that track along established storm tracks.
Polar regions. Cirrostratus forms frequently in polar and sub-polar regions, particularly during the transition seasons when warm air intrusions push moist air deep into cold polar air masses. The strong jet stream that rings the polar vortex creates persistent upper-level lifting that generates extensive cirrostratus sheets, sometimes covering millions of square kilometres.
Tropical regions. Cirrostratus in the tropics forms primarily from the spreading anvil tops of cumulonimbus storm systems and from outflow cirrus associated with tropical cyclones. The cirrostratus sheets that radiate outward from major hurricanes and typhoons can extend 500 to 1,000 miles from the storm centre, often the first visible atmospheric signal of an approaching tropical system visible to observers well outside the storm track.
All continents and ocean basins. Because cirrostratus formation requires only moist air at high altitude and large-scale lifting, it is not restricted by surface type. It forms with equal frequency over oceans, mountains, plains, and deserts whenever frontal systems are present. High mountain ranges such as the Alps, Rockies, and Himalayas can initiate orographic lifting that triggers cirrostratus formation even in the absence of a well-defined surface front.
The one environment where cirrostratus is comparatively rare is the dry interior of large continental landmasses during summer, where the upper troposphere is warm and subsiding rather than rising and cooling. The Sahara, central Australia, and the interior of Asia in summer produce less cirrostratus than coastal and temperate regions.
Sources and Classification Standards
Definitions, altitude ranges, and optical phenomena described on this page are verified against:
- WMO International Cloud Atlas: cirrostratus genus, fibratus and nebulosus species definitions
- NOAA National Weather Service JetStream: cloud formation and frontal sequence
- UCAR MetEd Training: high-level cloud identification and atmospheric optics
- NASA Earth Observatory: halo formation physics and ice crystal refraction
- American Meteorological Society Glossary: cirrostratus definition and weather significance
Content researched, reviewed and analysed against the above sources. Last verified July 2026.
Frequently Asked Questions
Cirrostratus clouds are thin, translucent sheets of ice crystals that cover large areas of sky in the upper troposphere above 20,000 feet. Their most distinctive feature is the 22-degree halo they produce around the sun or moon. They form ahead of warm fronts and are reliable predictors of rain or snow within 12 to 24 hours as the frontal system progressively lowers the cloud deck from cirrus through cirrostratus to altostratus and nimbostratus.
Cirrostratus looks like a thin, milky or whitish veil covering the entire sky without gaps or structure. The sky takes on a washed-out appearance and the sun remains visible but appears frosted rather than sharp. The most reliable identification sign is the 22-degree halo: a bright ring of light around the sun or moon at an angular distance roughly equal to a fully spread hand at arm’s length. No other common cloud produces this effect.
Cirrostratus signals rain or snow within 12 to 24 hours when it forms ahead of an approaching warm front and progressively thickens and lowers over several hours. It produces no precipitation itself. The cloud is part of a predictable sequence: cirrus appears first 24 to 36 hours before rain, cirrostratus follows 12 to 24 hours before rain, altostratus arrives 4 to 12 hours before rain, and nimbostratus brings the actual precipitation.
Cirrostratus clouds form in the upper troposphere, typically between 20,000 and 40,000 feet. They are classified as a high-level cloud by the WMO. At these altitudes, temperatures are permanently below freezing, which is why cirrostratus is composed entirely of ice crystals rather than liquid water droplets. This ice crystal composition is what produces the optical halo effect that distinguishes cirrostratus from lower cloud types.
The 22-degree halo forms when sunlight enters a hexagonal ice crystal column face and exits through a side face at a minimum refraction angle of approximately 22 degrees. Because cirrostratus contains millions of randomly oriented crystals, every crystal at the correct orientation produces a point of brightness at 22 degrees from the sun. Collectively they form a continuous, sharply defined ring. The halo is always at 22 degrees regardless of the season, location, or thickness of the cirrostratus layer.
Cirrostratus covers the sky as a smooth, featureless sheet with no individual elements, while cirrocumulus appears as small, discrete ripples or cloudlets arranged in rows creating the mackerel sky pattern. Cirrostratus produces halos around the sun and moon; cirrocumulus does not. Both are high-level ice crystal clouds, but cirrostratus is thicker and more uniform while cirrocumulus shows visible convective structure at individual cloud element scale.
Cirrostratus is pronounced sir-oh-STRAY-tus, with stress on the third syllable. The IPA pronunciation is /ˌsɪroʊˈstreɪtəs/. It comes from the Latin cirrus meaning curl of hair and stratus meaning layer or spread, describing its appearance as a high-altitude layered veil. The plural form is cirrostrati in Latin or simply cirrostratus layers in English usage.