Samenvatting
In order to ensure that the turbulent scalar flux is along gradient the components of the tensor of turbulent diffusion must be restricted. The author shows that the symmetric part of Kij must be a positive definite matrix. The tensor Kij is obtained by an algebraically approximated second-order closure. It is suggested that the dimensionless negative lapse rate Gh must be smaller than 0.029 in order to satisfy this restriction.