 Day-night heat redistribution

Day-night heat redistribution transports absorbed stellar energy from an illuminated hemisphere to a dark one through atmospheric motion or an ocean. A simple comparison uses the atmospheric advection time, $L/U$, and the radiative relaxation time in a planetary atmosphere. Fast transport relative to cooling allows gas to retain heat while crossing the planet, warming the nightside and reducing the day-night contrast.

An exoplanet thermal phase curve measures the resulting variation in brightness as different hemispheres rotate into view. HD 189733 b provides an observed example. The inference remains wavelength-dependent because different opacities sample different pressures. A small brightness contrast can therefore constrain redistribution at the sampled levels without proving that every level has the same temperature.