Most parameterizations used in Large Eddy Simulations of the atmospheric boundary layer are based on the assumption that subgrid-scale fluxes are aligned against spatial gradients of transported quantities (down-gradient closures). Based on field experiments, we determine the distribution and most probable relative orientations of the subgrid-scale (SGS) heat flux relative to parameterizations based on the temperature gradient. We show that, under neutral and unstable atmospheric stability, the SGS heat flux most likely lies within the geometric span of the so called mixed tensor eddy diffusivity model.