Definition
A prefix used in cloud classification to indicate clouds of the middle level of the troposphere, typically with bases between about 6,500 and 20,000 feet above the surface. Combined with a root form, it produces names such as altostratus (middle-level layered cloud) and altocumulus (middle-level heaped cloud).
Plain English
Alto, when used in a cloud name, means the cloud sits at the middle height of the sky — not low and not high. So altostratus is a middle-height sheet of cloud, and altocumulus is middle-height puffy cloud.
Context Anchor
Seen in cloud names such as altostratus and altocumulus when studying or describing cloud types.
Derivation
From the Latin altus, meaning 'high.' In everyday English 'alto' suggests something high, but in meteorology it was assigned specifically to the middle layer of clouds — above the low clouds (stratus, cumulus) and below the high clouds (cirrus). Knowing this prevents the natural assumption that 'alto' means the highest clouds.
Why Pilots Care
Middle-level clouds can signal changing weather, possible icing, or reduced visibility at typical cruising altitudes.
Intuition Check
Alto does not mean the highest clouds here. In cloud names, alto means the cloud is in the middle-level group.
Example Sentence 1
The forecast called for a layer of altostratus moving in ahead of the warm front, so the pilot expected lowering ceilings during the cross-country.
Example Sentence 2
Altostratus, an alto-type cloud, began to cover the sky and reduced the ceiling for the VFR flight.