The aerodynamic temperature is required for prediction of the surface heat flux using Monin-Obukhov similarity. This fictitious temperature is not systematically equal to the actual air temperature near the surface and is not directly available from observations or in numerical models. The aerodynamic temperature is normally replaced with either the canopy air temperature or the surface radiation temperature, sometimes accompanied by adjustment of the thermal roughness length or specification of an excess resistance. In this study, the relationship between the aerodynamic temperature and the surface radiation temperature is examined in terms of the surface energy budget and simple representation of canopy structure. Previous inconsistencies are discussed. The observed behavior of the aerodynamic temperature is studied using Canadian Twin Otter aircraft data from BOREAS over nine different forested and nonforested sites. Variation of the behavior of the aerodynamic temperature between different parts of the boreal forest is found to be strongly correlated with the red reflectance. ¿ 1997 American Geophysical Union