Abstract: In this paper, the two-level quasi-geostraphic model is used, the concept of assimilation of space and time is adopted, the idea of space instability is introduced and combined with time instability to study the effects of any disturbance introduced at an unidentified latitude to the general circulation. A general theory about the extent of existence of the planetary waves, the criteria of space and time instabilities and their physical implications, and the coupling of the high and low level planetary waves is presented. The extent of existence of the wave length of the baroclinic planetary waves is closely related to the velocity field and the vertical stratification. The more unstable the vertical stratification is, the shorter the length of the stable, moving planetary waves may appear will be. The vertical stability in the moist baroclinic atmosphere is much less than that in the dry baroclinic atmosphere, therefore, it is possible for stable, moving planetary waves of much shorter wave length to appear in the moist baroclinic atmosphere. Under the ordinary vertical stratification in the troposphere, it will be no stable, stationary ultra-long planetary waves. If the vertical strafication is as stable as that in the stratosphere, there will be stable, stationary planetary waves with wave length longer than 8500 km.