How to understand the unstable atmosphere under heating quality control? Isn't it sunny under the control of the warm air mass?

Well, generally speaking, a warm air mass refers to a collection of gases with a higher temperature than the surrounding area. Generally speaking, under the control of the heating mass, its temperature is higher than the surrounding temperature, so the air flow here will rise (which can be understood as heat expanding and cold contracting, becoming lighter), thus forming a low-pressure area, and the surrounding air will move and replenish here, thus forming the wind near the ground. Because this sentence is a dry heating mass, it often happens inland. Because the humidity is too small to rise to form clouds, it can't rain (similar to my autumn and winter seasons). At this point, if cold air strikes, if it carries enough water vapor, it will form clouds, and finally it will rain. This is the next sentence. In fact, for example, in winter, it is sunny and it doesn't rain for a long time, but it feels very cold. It is often the cold air mass that stably controls the climate here. Of course, the warm air mass is sometimes stable, but according to the winter scene described in the poem, it is easy to know that the warm air mass is not active relative to the cold air mass, so it is naturally not as stable as the cold air mass. Ask again if you don't understand!