Altostratus
Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Class 11, Ch 10 "Water in the Atmosphere"
Meaning
Altostratus is a middle-level cloud. "Alto" means middle and "strato" means layered. It is a grey or bluish sheet that covers the sky. The Sun looks dim through it, as if seen through frosted glass. It often thickens into nimbostratus, the cloud that gives steady rain. So in the cloud sequence of a warm front (where warm air slides up over cold air) it is a sign that rain is near.
Example
Over north-western Europe, for example over Britain, the sky ahead of an Atlantic warm front first shows cirrus and then cirrostratus. These turn into a grey altostratus sheet that dims the Sun. Soon after, the cloud thickens into nimbostratus and steady rain begins.
Don't confuse with
- Cirrostratus: a higher, thinner ice veil through which the Sun stays clear and often shows a halo. Through altostratus the Sun looks dim and blurred, usually with no halo.
Related concepts
- Cloud classification by height
- Cirrostratus
- Cirrocumulus
- Altocumulus
- Stratocumulus
- Nimbostratus
- Cumulonimbus