Cirrostratus Clouds
Thin High Transparent Layers ☁️
Altitude
20,000 - 40,000 feet (6 - 12 km)
High cloud genus, pure ice
Sun/Moon
Clearly visible, sharp-edged disc
No blurring, unlike altostratus
Optical Effect
22° halo around sun/moon
Caused by hexagonal ice crystals
What Are Cirrostratus Clouds?
Cirrostratus clouds are thin, near-transparent veils of ice crystals that can spread across the entire sky, sitting 20,000 to 40,000 feet up. They're easy to miss on their own — the sky often just looks a bit milky or hazy rather than obviously cloudy — but they give themselves away through their most distinctive feature: a ring of light exactly 22° around the sun or moon, known as a halo. That halo forms because the cloud is made of small, hexagonal ice crystals that bend (refract) light passing through them at a consistent angle, the same way a prism splits light, just concentrated into a ring rather than a spectrum. Unlike altostratus below it, cirrostratus is thin enough that the sun or moon still shows as a sharp, well-defined disc rather than a blurred smudge.
Formation & Characteristics
How It Forms
- 1. Frontal Lift: Usually the very first visible sign of an approaching warm front, hundreds of miles ahead of the rain.
- 2. Slow, Broad Ascent: Moist air is lifted gently over a huge area rather than in a single localized column.
- 3. Ice Crystal Growth: At -20°C to -60°C, any moisture deposits straight as ice, forming fine hexagonal crystals.
- 4. Thickening Sequence: Cirrostratus typically thickens into altostratus, then nimbostratus, as the front gets closer.
Key Characteristics
- ✓ Colour: Whitish, often barely distinguishable from clear sky
- ✓ Texture: Smooth veil, or faintly fibrous streaks
- ✓ Shadow: Objects on the ground still cast shadows through it
- ✓ Sun/moon: Sharp disc, unlike altostratus's blurred glow
- ✓ Composition: Entirely ice crystals
- ✓ Coverage: Can extend across the whole visible sky
Two Recognised Forms
Cirrostratus Fibratus
Shows faint, fibrous streaks or filaments, similar in texture to cirrus but spread into a continuous sheet rather than isolated wisps.
- • Visible fine striations
- • Often the earlier, thinner stage
- • Halo can still form through it
Cirrostratus Nebulosus
A smooth, featureless, milky veil with no visible structure — the classic form that most easily produces a bright, well-defined halo.
- • Uniform, hazy appearance
- • Can be mistaken for a slight haze rather than cloud
- • Usually the more developed stage
Halo vs. Corona — Don't Confuse Them
A 22° halo (cirrostratus, ice crystals) is often mixed up with a corona (a smaller, tighter, sometimes coloured ring seen through thin altocumulus or stratocumulus, caused by water droplets diffracting light). They point to different clouds and different processes:
22° Halo (Cirrostratus)
- • Wide ring, roughly 22° from the sun/moon
- • Caused by refraction through ice crystals
- • Often precedes a warm front by 12-24 hours
Corona (Water-Droplet Clouds)
- • Small, tight ring hugging close to the sun/moon
- • Caused by diffraction through water droplets
- • Not a reliable frontal-approach signal