All living and non-living bodies emit Longwave Radiation. The magnitude of the radiation is proportional to the temperature (measured in Kelvin degrees) of the body raised to the fourth power. Significant sources of longwave radiation in hydrologic applications include the atmosphere itself, and any clouds that may be present locally in the atmosphere. Clouds usually have a higher heat content and higher temperature than clear atmosphere, and therefore there is increased downwelling longwave radiation on cloudy days. Whether the atmosphere and clouds are a net source of longwave radiation to the land surface depends on their temperature relative to the land surface temperature. In most cases, the net longwave radiation is incoming during the daylight hours, and outgoing during the night hours.
The Longwave Radiation Method included in the Meteorologic Model is only necessary when Energy Balance Methods are used for Evapotranspiration or Snowmelt. Each option produces the Downwelling Longwave Radiation arriving at the land surface.