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Anyone who’s ridden a lift on a cold February morning has experienced it: First the chair climbs through frigid, gloomy clouds on the lower mountain, and then you suddenly cross a threshold and break into bright sunshine and warm skies above. This stunning natural phenomenon is called a cloud inversion. But what causes it?
Normally, air temperature decreases as altitude increases. This is largely because of radiative heating from the Earth’s surface— and it’s why mountains tend to be cold and snowy. But cold air is also denser than hot air, which means it naturally wants to migrate downhill, allowing less-dense warm air to remain on top.
In mountainous terrain, these inversions can trap the cold air in confined valleys (think Telluride or Jackson Hole), where winter temperatures and low sun angles compound the effect. With sufficient moisture in the cold layer, fog will form and stay trapped beneath the warm layer—as can air pollution such as car emissions or smoke from fires. This all creates a dreamy ski environment: an atmospheric ocean of clouds, broken only by powdery mountain summits.