It can be shown that the freedom available in Ertel's potential vorticity definition allows one to arrive at a pair of (q, x(8))-variables [q is the modified potential vorticity (MPV), X(8) is the appropriately taken function of potential temperature e which enters MPV-definition], such that the air mass enclosed within solenoids formed by intersection between the families of equiscalar surfaces q=const and x=const is nearly preserved despite the inftuence of diabatic heating and turbulent friction. It is hypothesized and attempted to prove that in (q,X)-co-ordinates the long-term atmospheric climate processes admit a simple statistical description.Pages: 131-13