Cirrostratus is a transparent, milky veil that covers all or part of the sky and whitens the blue without hiding the sun. It is made of ice crystals at the high level, and it is what draws the halo, the ring of light seen around the sun or moon. For anyone reading the sky, it is one of the most reliable signs that a disturbance is on its way.
- Level: High level (5,000 to 13,000 m)
- Base height: 5,000 to 13,000 m
- Appearance: whitish, transparent veil with a halo
- Associated weather: often signals rain or snow within the day
How cirrostratus forms
Cirrostratus forms when a mass of moist air rises slowly over a wide area, as ahead of a warm front. Warm air sliding up over cold air reaches the high level, cools beyond saturation and fills with ice crystals. Instead of the separate wisps of cirrus, you get a continuous sheet, often very extensive.
The layer usually spreads as the front approaches. It starts as a few strands of cirrus, which then join up and whiten the whole sky. The thicker and lower the sheet becomes, the greyer it turns, until it becomes altostratus, a sign that the front is near.
How to recognise cirrostratus
The telltale sign is the halo: a ring of light 22° in radius centred on the sun or moon. It appears because ice crystals shaped like hexagonal prisms bend light by a minimum of 22° as it passes through. The inner edge of the ring is often reddish. At arm's length, 22° is roughly the span from thumb to little finger on an open hand.
Another test: cirrostratus does not stop shadows. Objects on the ground still cast a shadow, softened though it may be, which sets it apart from altostratus, which removes them. The sun stays clearly visible, with a sharp outline, through the veil.
What weather cirrostratus signals
A veil of cirrostratus thickening from the west often signals rain or snow within hours, sometimes within half a day. The rhyme "a ring around the moon means rain soon" rests on this mechanism: it marks the vanguard of a warm front. The rule is not absolute, but it is fairly reliable in temperate regions.
If the veil stays thin, thins further or breaks up, the disturbance is passing elsewhere and the weather stays dry. Watch which way it is going: a thickening veil means rain, a fraying veil means the sun returns.
Risks and precautions
Cirrostratus itself is harmless and gives no precipitation. It filters little ultraviolet light, which remains a trap: a whitened sky gives the impression of a weak sun while exposure stays high. What it does is warn of deterioration, so it is the moment to bring in the washing, plan a shorter outing or cover delicate plants before the rain.
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 high level the sky is mostly covered (76% 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
Why does a halo form around the sun?
Because the hexagonal ice crystals of the cirrostratus bend sunlight by a minimum angle of 22°. The eye then sees a ring of light, sharper on its inner edge.
Does a halo always mean rain?
No, but it often does. It reveals a veil of ice crystals at height, often the vanguard of a front. If the veil thickens and lowers, rain usually arrives within hours; if it fades, nothing happens.
How can I tell cirrostratus from altostratus?
By the sun and the shadows. Behind cirrostratus the sun keeps a sharp outline, a halo may show and shadows stay marked. Behind altostratus the sun looks like it is seen through frosted glass, with no halo and no shadows.
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.