Downward longwave radiation, often referred to as back radiation, is the thermal radiation emitted by the atmosphere and directed towards the Earth's surface. While instruments can measure this radiation, it is not a true measure of the radiation absorbed by the surface for several reasons:
Atmospheric Interactions: The downward longwave radiation is influenced by interactions with the atmosphere. The atmosphere contains gases and particles that can absorb, scatter, and re-emit radiation. As a result, some of the downward longwave radiation may be absorbed or scattered by the atmosphere before reaching the surface. This means that not all of the back radiation actually reaches and contributes to the energy absorbed by the surface.
Surface Emissivity: The surface of the Earth has its own thermal radiation, known as surface emissivity. The surface emits longwave radiation based on its temperature and emissivity properties. The downward longwave radiation includes contributions from both the atmosphere and the surface. However, the surface emissivity affects the net energy transfer between the surface and the atmosphere. If the surface emissivity is different from 1 (perfect emitter), it means that not all of the back radiation is absorbed by the surface.
Energy Balance: To determine the true measure of radiation absorbed by the surface, one needs to consider the overall energy balance. It involves accounting for all forms of energy transfer, including solar radiation, longwave radiation, conduction, convection, and latent heat flux. The net energy absorbed by the surface depends on this energy balance, which is influenced by various factors such as surface properties, atmospheric conditions, and surrounding environment.
Therefore, while instruments can measure downward longwave radiation (back radiation), it is important to consider these factors and the overall energy balance to obtain a true measure of the radiation absorbed by the surface.