ObserveAltostratus
As · Middle · 27 September 2026 · By
Altostratus: the grey sheet that has already decided to rain
Altostratus is the most consequential cloud in the frontal sequence, because by the time it is overhead, rain is no longer a question of whether but of when. It is a grey or bluish-grey sheet at 2000 to 5500 metres, thin enough to show the sun as a bright patch but thick enough to hide its disc entirely.
Altostratus
Altostratus
- Band
- Middle
- Altitude
- 2 km – 5.5 km
- Code
- As
- Seasonal window
- Altostratus is a year-round cloud, most frequent in autumn and winter with frontal activity. In summer it appears most often as the remnant shield of overnight mesoscale convective systems. (Jan–Dec)
- Published
- 27 September 2026
- By
Weather clue
Altostratus thickening and lowering with the wind backing and pressure falling means steady rain within a few hours. Altostratus that stays thin, warm-toned and static all morning is often the leftovers of last night's storms and will break up.
How to observe it
- Record the exact clock time when the sun's disc stops being visible; log it as the rain countdown start.
- Look for pannus fragments below the main base every 10 minutes once thickening begins.
- Measure pressure if you have a barometer and note the fall rate in hPa per 3 hours.
- Note the wind direction every 30 minutes; backing wind with falling pressure confirms the frontal origin.
The four tests that separate it from cirrostratus
Altostratus and cirrostratus are adjacent stages in the same frontal sequence, and confusing them moves your rain forecast by several hours.
| Test | Altostratus | Cirrostratus |
|---|---|---|
| Halo | None | 22° halo usually present |
| Sun | A diffuse bright patch, disc invisible | Disc visible but fuzzy |
| Colour | Grey to bluish-grey | Whitish, milky |
| Shadow | No shadow at all | Soft-edged shadow |
| Texture | Sometimes shows faint striations or a rippled look | Smooth and uniform |
| Rain | Within 1-3 hours usually | 6-24 hours away |
Why the sun disappears all at once
The transition from 'sun visible as a patch' to 'sun gone' is abrupt in practice, and the reason is that liquid water absorbs much more strongly than ice. Cirrostratus is ice and lets a lot of light through even when it looks thick. Altostratus, at its lower levels, is water droplets, and once the liquid water path exceeds roughly 100 grams per square metre the sun is effectively gone.
That is why experienced observers use the moment the sun disappears as the marker. It is a real threshold, not a gradual fading, and it usually puts rain one to three hours away.
What the base looks like when rain is close
As altostratus thickens toward nimbostratus, several things change in a specific order.
- The colour deepens from light grey to a flat, dull slate grey with no variation.
- The base lowers visibly, often from above 3000 m down to 1500 m or lower.
- Ragged fragments appear below the main base — pannus, or scud — which usually means rain is 30-60 minutes away.
- The wind backs (turns anticlockwise) in the northern hemisphere as the warm front approaches, and pressure falls steadily.
- The light goes flat — shadows disappear entirely and colours outdoors look desaturated.
Altostratus that is not from a front
Not every altostratus sheet is a warm front. Two other common origins are worth knowing, because they forecast differently.
Altostratus from a decaying thunderstorm complex covers a wide area the morning after a night of storms. It is usually thinner, has a slightly warmer, more yellowish tone, and breaks up during the morning as the surface heats. Altostratus from a warm sector ahead of a cold front is thicker and more uniform, and it will be followed by actual rain as the cold front arrives.
Rules of thumb
- Altostratus hides the sun's disc and gives no halo, unlike cirrostratus.
- The jump from thick cirrostratus to altostratus is abrupt because liquid water absorbs far more than ice.
- Ragged pannus fragments below the base mean rain in 30-60 minutes.
- Not all altostratus is frontal; morning-after-thunderstorm sheets behave differently.
| The other cloud | Comparison — Altostratus |
|---|---|
| 卷层云 · Cirrostratus | Cirrostratus keeps the sun's disc visible and usually shows a 22° halo; altostratus hides the disc completely and has no halo. |
| 雨层云 · Nimbostratus | Nimbostratus is darker, thicker, lower and already raining steadily; the transition is marked by the first continuous rain reaching the ground. |
The two or three genera this one is most often confused with, and the single feature that separates them.
Observation questions
Why is there no halo under altostratus?
Because altostratus at its lower levels contains liquid droplets rather than ice crystals. Halos require refraction through hexagonal ice prisms, and liquid droplets scatter light without producing a clean angular ring. No ice, no halo.
How long after the sun disappears does rain start?
Typically one to three hours in a normal mid-latitude warm front, but it depends on how fast the front is moving. A useful refinement is to watch the cloud base: once ragged fragments appear underneath, the rain is usually 30 to 60 minutes away.