Geographical question: Why does the weather change the most when the troposphere is hot and cold?

The density of hot and cold air is right, but it is not the low density that spreads to the high density, but the cold air is dense and heavier than the hot air, so the cold air will sink. After the cold air sinks, the amount of gas in the upper air will decrease, and there will be horizontal air supplement, so that circulation will form a circular flow.

As for why it is cold in the upper atmosphere and hot in the lower atmosphere, this is a relative concept, because the upper atmosphere reflects and refracts sunlight, but hardly absorbs it, while the lower atmosphere has many greenhouse gases that will absorb the long-wave radiation on the ground, thus playing the role of heat preservation, so there will be a phenomenon that the troposphere air is cold in the upper atmosphere and hot in the lower atmosphere.