Heavier gases sink, lighter gases rise. That is how a hot air balloon works: the warm air expands making it lighter than the surrounding air and it rises, lifting the basket.
After sunset, when the sky is clear, the heat radiates out of the ground creating a layer of very cool air near the ground.
When this happens on a slope, this cold dense air starts flowing downhill. This creates a wind known as a ‘katabatic wind’.
Katabatic winds are often gentle, but I did once experience a powerful one off the coast of Iceland when freezing air rolled down the snowy mountains. I was sailing at the time and the wind went from Force 4 to Force 7 almost instantly and stronger gusts followed.
Here is a video of me enjoying a much gentler version:
When the cold air causes water vapour to condense this causes fog or mist to form. This gives us a rare oppportunity to watch the katabatic wind in action, as the fog maps the flow of the air for us:
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