Inversions occur in basins. For example, air temperature is normally warmer at the surface of Lake Tahoe than higher in the sky. An inversion happens when this situation is reversed. During an inversion, air is colder at lake level and warmer higher. In short, cold, damp air is trapped under a layer of warmer air.

Tahoe and Truckee occasionally get inversions in the winter between storms because of a lack of wind to mix the air. At night, cold air from the surrounding mountains sinks (it's heavier than warm air) to lake or ground level. The heavier, cold mass pushes the warm air up and out of the basin. Next day, because the sun is low in the sky, the sun does not warm the surface air much. The result is dull gray. See the attached photos.

Summer, Too
Inversions also occur in the summer. For example, it is fairly common for Truckee to have air temperatures lower than at Tahoe during summer months. This happens when Truckee has a mass of cooler air flow down from the mountains and settle at the surface; there is not enough wind to mix the levels of air in the Truckee Basin. But in the summer, the (somewhat) heated water of Lake Tahoe itself generally prevents the lake basin from having inversions.

Think about all this as you ride the lift up into the sunlight!

Mark McLaughlin is a weather historian and writer who lives on the North Shore.