The atmosphere is a very dynamic place with air masses moving in response to various forces, mainly, ultimately due to the re-distribution of heat from the equator to the poles as modulated by the rotation of the earth. . . some very interesting fluid mechanics going on.

Now air masses form and move around. When two different air masses are juxtaposed often one wins and pushes the other away by flowing underneath (cold front) or on top of warm front; but at times both air masses have about the same O MPH! and we have a stationary front. Such beasts are not long lived as eventually new driving forces come in to play, and the general circulation will destroy the approximate steady state situation.

One way of thinking about the general movement of the atmosphere (a metaphor) is to go down to the beach and watch the surf line. One moment the water rushes in and it is moving up the beach and it may be a few feet deep… but then it washes out changing direction and the water depth becomes zero … then new water flows in. and so the earth atmosphere is a three dimensional version of this at the scale of the PLANET.

The most important physics is the equator to pole transport of heat (since the equator gets more incident solar energy), the rotation of the earth and the coupling between the oceans and atmosphere.

This is a very complex system that is in an incessant state of change . . . that we call WEATHER.