The usual procedure in calculating the cooling process in a wake behind a blunt object is to assume a region of pure expansion up to a distance where the pressure has reached its ambient value, followed by a region where the mechanism of pure heat conduction is operative. In the present paper both mechanisms are assumed to be valid simultaneously, and the result is compared with previous calculations. The following criterion is established: the minimum radius of a hemisphere-cylinder configuration, above which a simultaneous conductionexpansion calculation is not needed, is given by the approximation