GlossaryAltostratus
Genus · Middle · As
Altostratus
Altostratus
Definition
A genus of middle-level cloud forming a greyish or bluish-grey sheet, thin enough to show the sun as a bright patch but thick enough to hide its disc completely. No halo is produced.
Field note
The moment the sun's disc disappears behind altostratus is a real threshold: rain is usually 1 to 3 hours away. From then on, watch the base for ragged pannus fragments below it.
- Added
- 5 July 2026
- By
- Class
- Genus
- Band
- Middle
Read the full field note
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.