At absolute zero, all molecular movement stops. All actual temperatures are above absolute zero. Absolute zero would occur at -273.16°C, -459.69°F, or 0 Kelvin.

Ratio of radiant energy absorbed by a body to the corresponding absorption of a blackbody at the same temperature. Absorbance equals emissivity on bodies whose temperature is not changing. (A = 1 - R - T, where R is the reflectance and T is the transmittance.)

Spectral radiation regions not absorbed by atmospheric gases. These windows are transparent to radiation at those wavelengths. The most obvious window is the visible light window.