Definition
The gases that make up a very small fraction of Earth's atmosphere — less than 1% combined — including argon, carbon dioxide, neon, helium, methane, and water vapor. They are present in tiny amounts compared to nitrogen (about 78%) and oxygen (about 21%), but several of them play significant roles in weather, climate, and atmospheric behavior.
Plain English
All the other gases in the air besides nitrogen and oxygen. There isn't much of any of them, but they still matter — water vapor in particular drives most of what we call weather.
Context Anchor
Seen in basic atmosphere discussions when the Pilot’s Handbook explains what the air around an aircraft is made of.
Derivation
"Trace" comes from the Latin tractus, meaning a track or mark left behind — something barely there. In chemistry and atmospheric science, a "trace" amount means a quantity so small it's only just detectable. That's the sense used here: gases present in only tiny amounts.
Why Pilots Care
One trace gas — water vapor — is responsible for clouds, precipitation, icing, thunderstorms, and visibility changes. Despite being a small percentage of the atmosphere, it shapes nearly every weather decision a pilot makes.
Intuition Check
Trace does not mean a drawn line or flight path here. It means a very small amount of a gas in the air.
Example Sentence 1
After nitrogen and oxygen, everything else in the atmosphere is grouped as trace gases.
Example Sentence 2
Changes in trace gases can affect long-term visibility and weather patterns encountered on cross-country flights.