Effective radiant temperatures (ERTs) of five wheat canopies in different stages of development were measured during morning and noon periods. The observed variability in nadir sensor response was quantitatively described as a function of canopy structure and the vertical temperature profile of canopy components. In many cases, the nadir sensor ERT was a poor measure of vegetation temperature due to effects of soil emissions. Strong vertical temperature profiles of vegetation components were also observed. The theory and measurements presented document that remote measurements of vegetation canopy temperatures cannot be made indiscriminately over large spatial regions without consideration of the underlying physical principles.
D. S. Kimes, S. B. Idso, P. J. Pinter, Jr., R. D. Jackson, and R. J. Reginato, "Complexities of nadir-looking radiometric temperature measurements of plant canopies," Appl. Opt. 19, 2162-2168 (1980)