An exact expression for solair temperature, a parameter widely used in solar-energy system modelling, has been presented which appropriately accounts for ground temperature variation (with depth). Numerical results illustrate the dependence of solair temperature on the ground temperature gradient. In addition it is erroneous to assume the thermal emittance of the ground to be unity, i.e. that of a black body, in the computation of solair temperature. The simplicity of the exact expression poses no problems for its practical acceptance.