What is an Inversion?

Simply put an inversion is when a layer of warm air traps a layer of cold air underneath it.

In Mendocino County we have frequent (80-90% of the time) inversions. These inversions are strongest on winter nights when it is not raining and there is little or no wind. Inversions trap pollutants at or near ground level and do not allow them to disperse. Inversions usually break one to three hours after sunrise. However, in extreme cases inversions do not break during the day and can last for several weeks.  Inversions lift when either the wind blows strong enough to mix the air layers or the sun heats the cold air below enough to cause the layers to mix on their own.

(1/31/05 7:45 AM, Airport Temp 34F)

Below is a picture from NASA showing an inversion in northern India -  

Open burning is prohibited on days when strong inversions are likely and, the District encourages people to limit the use of woodstoves and fireplaces during those times.