Altostratus (As) is a mid-level cloud layer found at 2–6 km altitude, appearing as a grey or bluish sheet that covers the sky partially or completely. It is composed of a mixture of water droplets and ice crystals. A key diagnostic feature: the sun can be dimly visible through altostratus as if through ground glass — a "watery sun" — but it does not produce halos (which require the ice crystals of cirrus).
Altostratus is most commonly associated with approaching warm fronts. In the classic frontal cloud sequence, cirrus gives way to cirrostratus, which thickens into altostratus. As the front draws nearer, the altostratus thickens further and its base lowers, eventually becoming nimbostratus — the principal rain-bearing cloud. Light rain or snow (known as virga if it evaporates before reaching the ground) can fall from thick altostratus.
For pilots, altostratus is significant because it can contain supercooled water droplets that cause moderate icing on aircraft surfaces. The cloud often extends over hundreds of thousands of square kilometres ahead of warm and occluded fronts. In the Alps and other mountain ranges, altostratus frequently produces persistent light precipitation that can last for days during frontal passages. Observing the transition from thin, translucent altostratus to thicker, opaque forms is a practical skill for predicting the onset of rain within 6–12 hours.