Definition
In sky condition reporting, a descriptor applied to a cloud layer when the summation of sky cover at and below that layer is predominantly transparent — meaning the layer is reported but the sun, moon, blue sky, or higher clouds remain visible through it.
Plain English
A cloud layer is called thin when you can mostly see through it. The clouds are there, but they don't fully block the view of the sky or sun behind them.
Context Anchor
Seen in weather reports and handbook discussions about what does, and does not, count as a ceiling.
Derivation
From Old English thynne, meaning sparse or not dense. In aviation weather reporting, the everyday sense carries through directly — a thin layer is one where the cloud material is sparse enough to see through.
Why Pilots Care
A thin layer does not constitute a ceiling. Ceilings are based on broken or overcast layers that are opaque, not thin. Misreading 'thin broken' as a ceiling can lead to incorrect VFR/IFR decisions.
Grounding Statement
Picture a pale veil of cloud spread across the sky while higher clouds or blue sky are still visible through it.
Intuition Check
Thin does not mean simply “not many clouds” or “a low layer.” In this context, it means the layer is see-through enough that something above it can still be seen.
Example Sentence 1
The report showed a thin scattered layer at 3,000 feet, so the sun was still clearly visible from the ramp.
Example Sentence 2
Although ceilings were reported at 1800 feet, the thin overcast permitted the pilot to see stars and continue under VFR.