Altostratus Clouds

Altostratus Clouds

Thin Mid-Level Gray Clouds ☁️

Altitude

6,500 - 20,000 feet (2 - 6 km)

Mid-level cloud genus

Sun Visibility

Faint, like ground glass — no halo

Too dense for a clean disc

Precipitation

None at the surface, usually

Signals rain within 12-24h

What Are Altostratus Clouds?

Altostratus clouds are featureless gray or blue-gray sheets that spread across large stretches of the mid-level sky, roughly 6,500 to 20,000 feet up. Look up through one and the sun or moon shows only as a dim, blurred patch — as if you were looking at it through a pane of ground (frosted) glass. That faint, hazy disc is the single clearest way to identify the cloud: it is dense enough to blot out sharp shadows and any halo, but still thin enough to let some light through. Altostratus almost never reaches all the way down to affect ground-level visibility the way stratus or fog does, and it very rarely drops rain that survives the fall to the surface — its real significance is as a forecasting signal, not as weather in its own right.

Formation & Characteristics

How It Forms

  1. 1. Warm Front Approaches: A mass of warm, moist air rides up and over the retreating cold air ahead of a warm front.
  2. 2. Gradual Ascent: Unlike the sharp, localized lift that builds cumulus clouds, this is broad, gentle lifting spread over hundreds of miles.
  3. 3. Mixed-Phase Layer: At this altitude the air straddles the freezing level, so the cloud is made of a mix of supercooled water droplets and ice crystals.
  4. 4. Thickening: As the front gets closer, the layer thickens and lowers — the next stage in the sequence is nimbostratus and steady rain.

Key Characteristics

  • ✓ Colour: Grey or blue-grey, darker than cirrostratus
  • ✓ Sun/moon: Visible only as a faint, diffuse smudge
  • ✓ Halo: None — it's the giveaway that separates it from cirrostratus
  • ✓ Coverage: Broad, often covers the whole sky
  • ✓ Structure: Largely featureless, sometimes faintly fibrous or striated
  • ✓ Composition: Water droplets, supercooled droplets and ice crystals together

Altostratus Varieties

Altostratus Translucidus

The thinner variety. Sun or moon still shows through as a soft, watery glow, though never sharp enough to cast a defined shadow.

  • • Sun/moon position is easy to judge
  • • Earlier stage of an approaching front
  • • No precipitation reaching the ground

Altostratus Opacus

Thick enough to hide the sun or moon completely. Marks the layer thickening as a front draws nearer, often just hours from rain.

  • • No sun/moon visible at all
  • • Sky looks uniformly dull and grey
  • • Often thickens into nimbostratus next

Telling It Apart From Similar Clouds

vs. Cirrostratus

  • • Cirrostratus is higher (20,000ft+) and pure ice
  • • Cirrostratus produces a sharp 22° halo; altostratus never does
  • • Cirrostratus lets the sun show as a clean disc, not a smudge

vs. Stratus / Nimbostratus

  • • Stratus sits much lower, often below hilltop level
  • • Nimbostratus is thicker still and actively raining
  • • Altostratus is the “not yet raining” stage in between

Key Facts About Altostratus Clouds

Physical Properties

  • • Thickness: often several thousand feet
  • • Mixed-phase: water droplets and ice crystals
  • • Can produce virga (rain that evaporates before landing)
  • • Colour deepens as the layer thickens

Weather Significance

  • • Classic warm-front indicator
  • • Usually precedes rain by 12-24 hours
  • • Thickening/lowering means the front is getting closer
  • • Follows cirrus and cirrostratus in the sequence