The net longwave radiation is made up of two components, upwelling longwave radiation and downwelling longwave radiation. Upwelling radiation is the thermal energy emitted from the earth's surface to the atmosphere. Downwelling is the radiation emitted from the by the atmosphere towards earth's surface. Downwelling is influenced by the water vapor, clouds, and other radiative properties of the atmospheric column. Longwave radiation methods in HEC-HMS estimate the downwelling radiation. Temperature and emissivity are two principal values in the estimation of longwave radiation.

Kirchoff's Law states that the absorptivity of an object is equal to its emissivity. Emissivity is a measure of the effectiveness of a material in emitting thermal radiation with values varying between 0 and 1.0. A blackbody is an idealized object that absorbs all incident energy has an emissivity of 1.0. Object emissivity is a ratio of the emissivity of a perfect blackbody. For example, a blackbody emits radiation at a rate of 448 W/m2 and aluminum foil has an emissivity of 0.07, so aluminum foil emits radiation at a rate of 0.07 x 448 W/m2 = 31.36 W/m2.