Abstract: The mechanism of the ageostrophic motion in the macroscopically quasistatic subsynopticscale systems is studied. Under the prerequisite that the time scale of the whole motion is far larger than that of its inertia-gravitational component, the relations of both the rotation and divergence components of the ageostrophic motion to various kinds of forcing factors are revealed more concretely and detailly than that done before. And it is proved that the above said prerequisite of slow motion is ensured in all the common cases even in most of the familiar cases of strong systems. In addition to the known fact that the Rossby number in traditional form R0≡(U/fL)≈○(1) can be accepted in the strong quasi-two dimensional systems, here it is also proved that the horizontally small but vertically deep and thick systems containing strong convective cloud ensemble can just tolerate the especially strong diabatic heating and maintain their slow evolution. And correspondingly, the divergence motion can be only half an order smaller than the rotation motion in such systems. The overrestriction of the divergence motion in all the filtered models seen so far is breaked then.