Cirrus is the highest cloud you can see with the naked eye, and one of the thinnest: delicate white wisps that streak a blue sky like brushstrokes. It belongs to the high level, where the air is too cold to hold liquid water, and it is made entirely of ice crystals.
- Level: High level (5,000 to 13,000 m)
- Base height: 5,000 to 13,000 m
- Appearance: white, silky wisps with no shading of their own
- Associated weather: fair weather; a warning of change if it thickens into a veil
How cirrus forms
Cirrus appears when moist air rises slowly to the high level, between 5,000 and 13,000 m in temperate latitudes. As it climbs, the air expands and cools. Once it reaches saturation, water vapour turns straight into ice on tiny particles, without ever becoming liquid. It is very cold up there: in the standard atmosphere the temperature is −56.5 °C at 11 km. A cirrus cloud is therefore made of ice crystals, never of droplets.
The wisps take their shape because falling crystals pass through layers of air where the wind differs in strength and direction. The heavier crystals drift away from the parent cloud and are drawn out into trails: the hooks and commas of some cirrus are streaks of falling ice shaped by wind shear. The ice evaporates before it reaches the ground, because the air beneath is too dry. Persistent aircraft contrails belong to the same family: water vapour from the engines freezes in air that is already close to saturation.
How to recognise cirrus
Cirrus is white, with no grey parts and no shading of its own: the sun shines straight through it. It comes as separate wisps, drawn-out banks, hooks or tufts. It is so high that it seems almost motionless, even when the upper-level wind is carrying it along fast. At sunrise and sunset it glows yellow, orange and pink while the ground is already in shade, because it stays in sunlight long after the lower layers have lost it.
Do not confuse it with its two neighbours at the high level: cirrocumulus, made of small grains, and cirrostratus, a continuous veil that produces a halo around the sun. If you see only wisps, with no grains and no veil, it is cirrus.
What weather cirrus signals
On its own in an otherwise blue sky, cirrus signals nothing: it is a fair-weather cloud, common under high pressure. What matters is how it develops. When the wisps multiply, thicken and merge into a veil, they are the vanguard of a frontal system: cirrus turns into cirrostratus, then altostratus, and rain often follows several hours later, with the warm front.
Long, stretched cirrus streaming past quickly points to a strong upper-level current, which often goes with changeable weather within two days. By contrast, scattered cirrus that fade through the day confirm settled conditions.
Risks and precautions
Cirrus brings neither rain nor wind. It filters little radiation: a sky veiled with cirrus lets most ultraviolet light through, so you can burn without feeling it. For stargazers it makes the night slightly milky. Above all, watch its trend: if it thickens late in the day, the night will be less clear and the next day less reliable.
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
How high is a cirrus cloud?
At the high level: in temperate latitudes the base of a cirrus lies between 5,000 and 13,000 m, according to the WMO International Cloud Atlas. That is the layer where airliners fly and where it is very cold.
Does cirrus bring rain?
No. The ice crystals that fall from it evaporate in the dry air below before reaching the ground. Rain only follows if the cirrus thickens into cirrostratus, then altostratus and nimbostratus.
Why are cirrus clouds sometimes pink at sunset?
Because they are so high that they stay lit by the sun while the lower layers are already in shade. The low-angle light has crossed a great thickness of atmosphere, which strips out the blues and leaves yellow, orange and red.
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.