Geographical problems: 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, and after the cold air sinks, the gas volume in the upper air will be reduced, and the horizontal air will come to supplement it, thus forming a circulation.

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