Hot air rises and cold air sinks due to varying densities. Given the same volume of air, hot air is generally less dense compared to cold air.
The difference in densities between hot and cold air is determined by the absorption of heat energy that primarily comes from the sun. When air is heated, the particles present in the atmosphere begin to move faster and expand rapidly. The increased distance between air molecules results to the reduction in the density of air. Cold air undergoes the reverse process. Due to less absorption of solar energy, air particles are compressed and there is minimal movement in the air. This causes an increase in the density of air.