ObserveCirrus
Ci · High · 5 September 2026 · By
Cirrus: high ice, and the twenty-four hour warning
Cirrus is pure ice crystal at 6000 metres and above, and it is the earliest visible sign that something is about to change. Its wispy hooks and streaks look decorative, but they are the leading edge of the cloud system that will, in a day or so, be raining on you.
Cirrus
Cirrus
- Band
- High
- Altitude
- 6 km – 12 km
- Code
- Ci
- Seasonal window
- Cirrus is visible year-round but the classic thickening-to-rain sequence is most common in autumn, winter and spring in mid-latitudes, when the polar front jet is overhead and frontal systems track more directly across the region. (Jan–Dec)
- Published
- 5 September 2026
- By
Weather clue
Cirrus increasing in coverage and thickening into a milky cirrostratus veil is the single most reliable day-ahead rain signal in the mid-latitudes. Cirrus in a narrow band that stays put and disappears is not.
How to observe it
- Estimate cirrus coverage in eighths (oktas) every hour to see whether the veil is advancing or retreating.
- Block the sun with your hand and look for a 22° halo — its presence means the cirrus is already thick enough to be a rain precursor.
- Note whether the streaks are straight (fibratus), hooked (uncinus) or tufted (spissatus) and record the time.
- Check for a contrail layer at the same altitude before calling something cirrus; contrails are perfectly straight and short-lived.
Why cirrus looks like it does
Cirrus crystals are mostly hexagonal plates and columns that grow in a narrow temperature band, and they fall slowly — a typical 1 mm crystal falls at only a few centimetres per second. That slow fall, combined with strong upper-level winds of 30 to 80 m/s, is what draws them into hooked, streaked and tufted shapes.
Because the crystals fall so slowly, a cirrus streak can persist for tens of minutes after the air that made it has moved on. When you watch a cirrus plume, you are often looking at a fossil of a different part of the sky.
The four species and where the weather is
Cirrus species track the life cycle of the frontal system producing them.
| Species | Look | Meaning in a frontal sequence |
|---|---|---|
| Cirrus fibratus | Fine straight or gently curved filaments | The first sign, typically 24-36 h ahead of rain |
| Cirrus uncinus | Hooked, comma-shaped, 'mare's tails' | Mid-stage; moisture is increasing aloft |
| Cirrus spissatus | Dense, greyish tufts, sometimes from an anvil | Late stage, or debris from a distant cumulonimbus |
| Cirrus floccus | Small rounded tufts with trails | Unstable patch aloft, often brief |
The 24-hour rule, and when it fails
In a typical mid-latitude warm front the rule of thumb is that rain arrives roughly 24 to 36 hours after the first cirrus appears, sometimes as little as 12 hours in a fast-moving system. The sequence to watch is cirrus, then cirrostratus covering more and more of the sky, then the altocumulus and altostratus stages, then rain.
The rule fails in three recognisable ways. If cirrus appears but never thickens into cirrostratus, the system is passing north or south of you. If cirrus appears with a very clean, straight edge and then retreats, it was the outflow from a distant thunderstorm rather than a front. And if cirrus is present with the wind backing and the pressure falling, the front is coming more directly at you than the sky alone suggests.
Optical effects worth looking for
Cirrus is thin enough that light passing through it is barely refracted, so it produces fewer halos than cirrostratus — but it produces excellent ones when the crystals are right.
- 22° halo. A ring of radius 22° around the sun or moon, most often from cirrostratus but sometimes from dense cirrus.
- Sun dogs (parhelia). Bright spots on the halo at the same altitude as the sun, caused by horizontally oriented plate crystals.
- Circumzenithal arc. A bright, upside-down rainbow near the zenith, only visible when the sun is below about 32° elevation.
- Parhelic circle. A faint white horizontal band through the sun, from vertical column crystals.
Rules of thumb
- Cirrus is pure ice crystal, typically between 6000 and 12000 m.
- Rain follows the first cirrus by roughly 24 to 36 hours in a classic warm front, sometimes 12.
- Cirrus that never thickens into cirrostratus is a system going around you.
- Cirrus uncinus, the hooked 'mare's tails', marks the middle of the approach.
| The other cloud | Comparison — Cirrus |
|---|---|
| 卷层云 · Cirrostratus | Cirrostratus is a continuous milky veil, not separate streaks; it is the one that gives a clean 22° halo and makes the sky look white rather than streaked. |
| 航迹云（人为卷云） · Contrail (cirrus homogenitus) | Contrails are dead straight, narrow, and often several parallel lines; natural cirrus is never geometrically straight over long distances. |
The two or three genera this one is most often confused with, and the single feature that separates them.
Observation questions
How can I tell cirrus from an old contrail?
Natural cirrus is essentially never geometrically straight over its whole length and has no starting point. A contrail begins at a specific place on a straight line and stays straight until it spreads out. If you can identify the origin, it is a contrail.
Does cirrus always mean rain is coming?
No. Cirrus that stays thin, or that sits in a narrow band and then disappears, often means the system is passing to your north or south. The reliable signal is cirrus whose coverage keeps increasing and which thickens into a milky cirrostratus veil.