A thermal layer is a phenomenon which can be found in the atmosphere of a gas giant.

In 2154, Arik Soong had a Denobulan medical ship Barzai and its pilot jettisoned into the atmosphere of a gas giant, where it became trapped between two thermal layers, forcing the starship Enterprise to break off their pursuit of Soong and mount a rescue. (ENT: "The Augments")

In 2372, the Defiant slipped between two thermal layers were speeds were easier to mange. The Defiant later passed into another thermal layer, where it encountered the Karemma and the Jem'Hadar. (DS9: "Starship Down")