If you have ever spent an evening on the slope of a mountain, you know the temperature will precipitously drop as soon as the sun sets. Part of this is the dry mountain air that does not hold on the heat well. However, most people also notice that the wind picks up dramatically, and you had better throw on another layer or two before it gets too cold.
These reason for this change in wind is the presence of katabatic winds. These winds, which roughly translate to, descending winds, in Greek, rush down the side of the mountain every evening. The katabatic winds are created by the sun heating up air during the day, causing it to rise up the side of a mountain or other slope. As the sun heats the air, it rises up like a slingshot being pulled back. As the sun sets behind the mountain, the dry mountain air rapidly cools and becomes denser. The dense air flows back down the mountainside, gaining momentum and it rolls down the slope — often creating a significant drop in wind chill.
Most katabatic winds are mild in nature, usually just 10 to 15 miles per hour, but some winds, such as those in Antarctica can reach hurricane speeds of up to 200 miles per hour. I have personally experienced 60 to 70 mile per hour katabatic winds in the Kali Gandaki river valley in Nepal, which sits in between two 26,000 foot mountains in a very dry high mountain plain.