Abstract Nitric oxide (NO) radiative cooling governs much of the energy budget in the Earth’s lower thermosphere and damps out temperature perturbations. The radiative relaxation time (RRT), the timescale that defines how efficiently infrared radiation damps out the perturbations to the thermal structure to 1/e of the perturbation’s initial value, is derived from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) NO observations, and is used to investigate the evolution of the thermospheric thermal perturbation. The RRT is sensitive to solar activity, being a factor of 3–4 smaller for solar maximum conditions than for solar minimum. The major factor driving this difference is the NO concentration, which is anti‐correlated with the RRT and contributes to nearly 80% of its change. The RRT is quasi‐insensitive to the strength of the thermal perturbation. The RRT during the main phase of the geomagnetic storm is ∼2–3 days regardless of the severity of the storm.