In most previous studies of three‐dimensional turbulent diffusion in the lower atmosphere, it was assumed that the principal axes of the diffusivity tensor coincide with the x direction of the mean wind, with the vertical z, and with the lateral y [Sutton, 1934; Monin, 1959; Laikhtman, 1964; Yeh and Brutsaert, 1971]. Thus the diffusivity was taken as a diagonal tensor with components K_xx, K_yy, and K_zz. However recent experimental evidence [Zubkhovskii and Tsvang, 1966; Wesely et al., 1970] and theoretical analysis [Lettau, 1952; Matsuoka, 1961; Gee and Davies, 1964; Brutsaert, 1970] have suggested that the principal axes of the diffusivity tensor do not coincide with the vertical and the mean wind direction but are tilted and that there is an additional diffusivity, namely an off‐diagonal component K_xz = K_zx.