Altostratus
6,500 - 20,000 ft (2,000 - 6,000 m)
“Grey skies are just clouds passing over”— Duke Ellington
Altostratus is the grey blanket of the sky, the cloud that makes the sun look like it's shining through frosted glass. It's not the most exciting cloud to look at, admittedly, but it plays an important role in the atmospheric drama.
This featureless layer of grey or blue-grey cloud can stretch for hundreds of miles, dimming the light without completely blocking it. You can usually tell it's altostratus rather than its higher cousin cirrostratus because altostratus is thick enough to eliminate any halo effects around the sun.
When altostratus moves in and begins to thicken, it's often a sign that steady rain or snow is on its way. It's the transition cloud, the in-between stage as a weather front approaches and the sky prepares for a more thorough soaking.
things worth knowing
- Altostratus is thick enough to hide the sun but thin enough that you can still sense its position
- It can extend vertically for thousands of feet
- This cloud type often produces virga—precipitation that evaporates before reaching the ground
- Altostratus frequently thickens into nimbostratus as a front approaches
weather wisdom
Usually indicates continuous rain or snow within hours