Definition
A cloud-coverage and type code used in pilot weather reports (PIREPs) and meteorological coding to indicate altocumulus clouds at a single layer with little vertical extent. The 'AC' identifies the cloud genus (altocumulus, a mid-level cloud) and the '1.1' indicates one cloud cell or layer with minimal vertical development.
Plain English
A short code meaning 'one thin layer of altocumulus cloud, not building upward.' It tells weather readers there is a single, flat sheet of mid-level puffy cloud rather than tall, growing clouds.
Context Anchor
Seen in FAA handbooks, glossaries, training material, and footnotes when the text is pointing to an outside FAA guidance source.
Derivation
AC stands for altocumulus. 'Alto' comes from Latin altus meaning 'high,' and 'cumulus' from Latin meaning 'heap' or 'pile.' So altocumulus literally means 'high heaps' — mid-level heaped clouds. The numeric suffix is a coding convention, not a Latin term: the first digit counts cells/layers, the second describes vertical development.
Why Pilots Care
Advisory Circulars supply practical details that help pilots comply with regulations and improve safety without creating new legal requirements.
Intuition Check
Do not read AC 1.1 as an aircraft system code or radio phrase. Here, AC means Advisory Circular: a written FAA guidance reference.
Example Sentence 1
The pilot's PIREP coded the mid-level cloud as AC 1.1, indicating a single thin altocumulus layer with no vertical buildup.
Example Sentence 2
Training materials often point to AC 1.1 when discussing standard operating practices.