Solar Loading in temperature sensors occurs when thermocouples are used to measure parameters like the atmosphere temperature or water temperature in an open area where they may be exposed to sunlight.
The sunlight which falls on the sensor can itself induce heat and thereby cause an error in the temperature measurement.
This is known as solar loading. The Solar loading increases with the increase in thickness of the sensor. Fine wire thermocouples, which have a very thin cross section have very negligible solar loading.
Solar loading can be minimized by the fitment of a radiation shield which is a reflective cover made of material such as metal foil which can reflect the radiation. The sensor can be placed inside this shield.