What causes the cold air at the top of the mountain to sink at night?

At night, the air at the top of a mountain cools down due to loss of heat through radiation. This cooler air becomes denser than the warmer air below it, causing it to sink. Additionally, the process of gravity pulls the denser cold air down the slopes, leading to a phenomenon known as katabatic winds. As a result, the cooler air accumulates in the valleys and lower elevations.