Source of Information on why grass temperature is lower than air temp

We have been debating the physics behind grass and air temperature - noting in the winter grass temp can be around 2 degrees lower than air under certain conditions

does anyone know any good sources to explain physics behind this phenomena

Dave

Anything that is exposed to the sky can be cooler than air temperature, a good example is frost on your car when the air temp is above freezing. The cause is Radiative Cooling.

Niko
Thanks your reply and link appreciated
David