Altostratus Clouds:Mid-altitude Gray Layer

Altostratus clouds form a widespread gray or blue-gray mid-level sheet that covers much or all of the sky. The Sun may appear as a dim, watery disk through thinner areas, and a layer that thickens and lowers can lead into steady precipitation.

How to identify altostratus clouds

Sheet of featureless, gray clouds in the middle cloud level capable of masking the sun

Altostratus clouds are found in the middle cloud level. And unlike their altocumulus counterpart, they’re often boring to look at. Along with nimbostratus clouds, these clouds don’t have any species associated with them.

But they do come with a handful of cloud varieties, which can help you make the determination if you’re looking at an altostratus cloud or not. If you see a cloud covering the sky’s entirety that’s not very close to the ground, and it’s positioned such that it’s visible through the clouds but giving off a ‘frosted glass’ appearance, chances are good that you’re observing an altostratus translucidus cloud. On the flip side, if the cloud is opaque and you can’t see the sun’s position, then consider it an altostratus opacus.

These clouds can also be responsible for precipitation, though it’s short-lived and not very common. You might also find scud clouds (pannus cloud accessory) underneath the main cloud layer, which can be one more indication that you’re looking at an altostratus cloud.

Altostratus cloud examples

Is it altostratus? Check these four clues

Use the Sun's appearance, the sheet's color, and the absence of distinct cloud elements to separate altostratus from nearby layer clouds.

Wide gray sheet
The layer covers a large part of the sky with a smooth or softly textured blue-gray appearance.
Watery Sun
In thinner areas, the Sun is visible only as a diffuse disk without sharp shadows on the ground.
No halo
Unlike cirrostratus, altostratus normally does not produce a clear halo around the Sun or Moon.
Middle-level base
The main layer is higher than stratus, though precipitation and ragged lower clouds can obscure its base.

Read how the altostratus layer thickens

The Sun's visibility and the cloud base are useful signs of whether the layer is still thin or developing toward widespread precipitation.

Classifying altostratus clouds beyond the cloud type

Begin with altostratus as the cloud type, then add species, varieties, supplementary features, accessory clouds, or origin terms only when each detail is visible. Every illustration below links to its complete field note.

Altostratus cloud varieties

5 cloud varieties are associated with altostratus clouds. Open any field note to compare the illustration with photographs and identification guidance.

Illustration of altostratus duplicatus clouds

Altostratus duplicatus (As du)MultilayeredExplore this subtype →

Illustration of altostratus opacus clouds

Altostratus opacus (As op)Opaque, hides the Sun or MoonExplore this subtype →

Illustration of altostratus radiatus clouds

Altostratus radiatus (As ra)Parallel bands and stripsExplore this subtype →

Illustration of altostratus translucidus clouds

Altostratus translucidus (As tr)Translucent, Sun or Moon position visibleExplore this subtype →

Illustration of altostratus undulatus clouds

Altostratus undulatus (As un)Wavelike, undulatingExplore this subtype →

Altostratus supplementary features

3 supplementary features are associated with altostratus clouds. Open any field note to compare the illustration with photographs and identification guidance.

Illustration of altostratus mamma clouds

Altostratus mamma (As mam)Sac-like, resembling cow uddersExplore this subtype →

Illustration of altostratus praecipitatio clouds

Altostratus praecipitatio (As pra)Precipitation reaching the surfaceExplore this subtype →

Illustration of altostratus virga clouds

Altostratus virga (As vir)Precipitation trails that do not reach the groundExplore this subtype →

Altostratus accessory clouds

1 accessory cloud is associated with altostratus clouds. Open any field note to compare the illustration with photographs and identification guidance.

Illustration of altostratus pannus clouds

Altostratus pannus (As pan)Ragged frazzles, scudExplore this subtype →

Compare altostratus with similar cloud types

Use cloud-base level, texture, structure, and sky coverage together before choosing the closest match.

Altostratus vs. cirrostratus

Altostratus is a gray mid-level sheet that strongly dims the Sun. Cirrostratus is a thinner high ice veil that may create a halo.

Best clueA halo points to cirrostratus; a watery disk behind a gray sheet points to altostratus.

Illustration of altostratus clouds — Altostratus

Illustration of cirrostratus clouds — Cirrostratus

Altostratus vs. nimbostratus

Altostratus may be thinner and can reveal the Sun through parts of the sheet. Nimbostratus is darker, deeper, and associated with widespread steady precipitation.

Best clueIf steady rain or snow is reaching the ground beneath a fully opaque layer, favor nimbostratus.

Illustration of nimbostratus clouds — Nimbostratus

Altostratus vs. stratus

Altostratus is a broad middle-level sheet. Stratus has a much lower base, can touch hills, and often resembles fog lifted just above the ground.

Best clueA low deck obscuring nearby terrain favors stratus; a higher blue-gray sheet favors altostratus.

Illustration of stratus clouds — Stratus

See a altostratus cloud classification in action

Follow a real photograph through the visible clues that lead to a complete cloud classification.

Altostratus opacus (As op)See how height, shape, coverage, and other clues build this full classification.Read the step-by-step lesson →

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