If you live in mountainous terrain you know why one side of the mountain tends to have cloudier, perhaps ever rainier or snowy weather, while the other side has drier, sunnier, and warmer conditions. This is a common occurrence, on a larger scale, for the Rocky Mountains.

As air moves over a mountain, it cools. This causes water vapor to condense into clouds and give rain on the windward side of the mountain. Once the air crosses the mountain it begins to descend again as drier air. Dry air can warm quicker than moist air, so on the lee side of the mountain things are often sunnier and warmer.

The “Foehn Effect” can happen anywhere in the world, as long as there are winds and some mountains. It was a foehn wind which was responsible for the world’s biggest temperature change on record. In 1972, the temperature in Montana rose from a bone-chattering -54F to 48F. This was thanks to the winds crossing the enormous Rocky Mountains. The larger the mountains, the larger the warming effect can be.