Definition
Clouds whose bases typically lie between 6,500 feet and 20,000 feet above ground level in the middle latitudes. They are composed primarily of water droplets, but can contain ice crystals when temperatures are low enough. Common types include altostratus and altocumulus. They are identified by the prefix 'alto-' in their names.
Plain English
Clouds that sit at medium height in the sky — higher than the everyday low clouds you see most days, but not as high as the wispy ones near the top of the sky. In normal flying terms, their bases are usually somewhere between about 6,500 and 20,000 feet above the ground.
Context Anchor
Seen in aviation weather reports, forecasts, and cloud-identification sections when cloud layers are grouped by height.
Derivation
The 'alto-' prefix used for middle clouds (altostratus, altocumulus) comes from the Latin 'altus' meaning 'high.' That is slightly misleading in English, where 'alto' suggests 'highest' — but in cloud naming it specifically marks the middle layer, not the top.
Why Pilots Care
Affects decisions on icing risk, turbulence, and visibility at cruise altitudes.
Grounding Statement
If the cloud layer is above the low cloud range but below the high cloud range, it is being described as middle clouds.
Intuition Check
Do not read “middle” as meaning halfway up the whole atmosphere. In this context, “middle” means a specific cloud-height band, generally about 6,500 to 20,000 feet above the ground.
Example Sentence 1
The forecast called for a layer of middle clouds around 10,000 feet, so we planned to cruise at 8,500 to stay clear of them.
Example Sentence 2
Middle clouds can obscure the sun while still allowing some light through, unlike thicker low clouds.