In mountainous areas most live in the valleys and often hike in the mountains – normally a colder environment as you gain elevation. However in winter, often a surface temperature inversion will form in these areas as cold air, in contrast to the norm of temperature decreasing several degrees for every 1000 feet gained, remains trapped in the valleys, and often along with the any air pollution that may exist.

Inversions are likely to happen during periods with long winter nights, calm winds, and clear skies. The long clear nights create a scenario where the ground and air closest to the surface is allowed to cool the its greatest extent, is not mixed into warmer air higher in the atmosphere by wind, and the resulting cold air naturally settles in lowest spots and is overrun with warm air.

Typical Winter Hiking / Inversion Considerations

This is in contrast to typical weather patterns where the sun warms the air closest to the ground, with that air gradually losing heat as it rises. While those below sub-zero winter hikes and backpacking trips can be quite rewarding, taking advantage of an inversion may find you leaving your house in near arctic conditions to find chilly, but pleasant hiking weather higher in the hills. As such, inversions can at times, be a winter hiker’s delight.

To learn more about weather patterns and phenomenon Peterson’s Field Guide to Weather is a good resource. See our Winter Backpacking Guide for more on winter backpacking considerations.

Editor’s Note: This Jargon installment originally appeared in Issue 32 of TrailGroove Magazine. You can read the original article here.