Definition
A NOTAM contraction used to describe a cloud layer that is mostly transparent, allowing the sky, sun, moon, or stars to be seen through it. In aviation weather reporting, a thin layer indicates that the cloud coverage at that level is not dense enough to fully obscure objects above or below it.
Plain English
Clouds you can partly see through. They're there, but they don't completely block the view of the sky or the ground.
Context Anchor
Seen in NOTAMs and other short aviation messages where space is limited.
Derivation
From Old English thynne, meaning 'not dense' or 'lacking thickness.' In aviation use, it keeps that everyday meaning — a layer that lets light and sight pass through rather than blocking it.
Why Pilots Care
Helps pilots assess whether a thin layer will permit visual references or affect flight planning for altitude and visibility.
Intuition Check
Do not read THN as meaning harmless or unimportant. It only means thin; the condition may still affect the flight.
Example Sentence 1
The NOTAM described a thin overcast at 4,000 feet, so the pilot expected to still see the sun faintly during the climb.
Example Sentence 2
The METAR included THN BKN to indicate the broken cloud layer was thin enough for possible VFR operations.