Plain English Meaning
Normally, the higher you go, the colder the air gets. In a surface-based inversion, that flips: the air right at the ground is the coldest, and the air a few hundred or thousand feet up is warmer. It usually happens overnight when the ground cools off quickly under clear skies.
Full Definition
A surface-based temperature inversion is a layer of the atmosphere immediately above the ground in which temperature increases with altitude rather than decreasing as it normally does. It typically forms on clear, calm nights when the ground rapidly radiates heat away, cooling the air just above it while the air higher up remains warmer.
Context Anchor
Seen in weather study and briefings when pilots are considering early morning , , , , and near the .