Meteorologists call this phenomenon a temperature inversion. It’s what happens when early morning temperatures fall, trapping pollutants in the chilly air. Warm air above the cool air traps the cooler air below, and keeps it from rising.

That’s what causes the haze, as airborne pollutants hang closer to ground level. But as the sun rises and the cool air warms, the hazy air begins to rise, carrying airborne pollutants with it. The hazy air moves higher and higher, dispersing as it goes, and improving visibility.

This often yellowish-brownish haze occurs mostly in winter, when nights are longer and the air stays cool until the sun rises in the sky.

Tucson is surrounded by mountains, and they contribute to the winter inversions. The mountains cause an overnight downward flow of cold air onto the desert floor and that cold air can increase the strength and duration of these morning temperature inversions.

What this means is that air quality can be poorer in winter as pollutants from motor vehicles, industry and fireplaces remain trapped longer in the air we breathe.

People can help reduce winter air pollution levels by driving less often, limiting idling our vehicles to a minute or less, and reducing our use of fireplaces.

Fireplaces are inefficient sources of heat. When you have the option, choose natural gas, electric or solar-powered heat or a wood stove certified by the Environmental Protection Agency.

To reduce the use of polluting fossil fuels, carpool or take the bus if you can, and do more than one errand while you’re out, to reduce the number of trips you make. And, if you can, drive a vehicle powered by an alternate energy source, such as electricity.

For more on local air pollution levels in Tucson, including particulates, ozone and carbon monoxide readings, go to www.AirInfoNow.org.  Also, call Pima County’s Department of Environmental Quality air pollution hotline: (520) 882-4AIR or (520) 882-4347.

Beth Gorman