On long stretches of lingering fog or days when the mountains are warmer and sunnier than down at sea level, a temperature inversion is often to blame. It is a common occurrence here in Vancouver ...

This produces a temperature inversion. The normal temperature profile of the troposphere (left) and the temperature profile with an inversion present (right). Credit: Scott Sutherland ...

Temperature inversions usually begin to form at the end of the day around dusk (3-5 hours before sunset) and last through the evening (intensifying throughout the night) up to sunrise.

A temperature inversion occurs when a layer of warmer air sits over a layer of cold air. Keep in mind, warmer air rises. So, if the cold air near the ground warms up and tries to rise, it ...

A temperature inversion is now in place. Under an inversion, moisture can build up, causing clouds and fog. If the inversion is strong enough, dense fog can become a hazard.

When the temperature increases with height, this is known as an inversion. In the Southern Appalachians, this rather common occurrence happens in the evening and can last into the night.

What is a temperature inversion? Chris Fawkes explains why this phenomenon makes it warmer on the top of a mountain than in the valleys.