The energy absorbed by the surface is radiated from the Earth as terrestrial longwave radiation (L↑). The amount of energy emitted is primarily dependent on the temperature of the surface. The hotter the surface the more radiant energy it will emit. The gases of the atmosphere are relatively good absorbers of longwave radiation and thus absorb the energy emitted by the surface. The absorbed radiation is emitted in all directions with the downward directed portion being longwave atmospheric counter-radiation (L↓).
The difference between incoming and outgoing longwave radiation is net longwave radiation expressed as:
Recall that under normal conditions air temperature decreases with an increase in altitude through the troposphere. This occurs because the earth is the immediate source of energy for heating the air above it. Knowing that heat is transferred from warmer to cooler bodies, this means the surface is normally hotter than the air above. So for most situations, net longwave radiation is a negative value as more longwave radiation is emitted by the Earth than it gains from the air. Some of this radiation is emitted out to space is lost from the earth system. That which is not lost to space is absorbed by the atmosphere and drives the "greenhouse" effect.
Under other circumstances net longwave can be zero or a positive number. If zero the amount of radiation emitted by the surface would be equal to that of the air. This is true when the air and surface have the same temperature. If the air is warmer than the ground a positive value exists. This could occur with heavy cloud cover (clouds are good absorbers and radiators of longwave radiation) or if a warm air mass travels over a colder surface.
This work is licensed under a