Thursday and Friday, January 12 and 13, the air quality in Hamilton was harmful for "sensitive groups." Most of the time this week, the air quality has been "Moderate."

The air layers get trapped in the mountain valleys during the winter. You can tell by the incidence of freezing fog and the general haziness when you're looking at the Bitterroot Mountains.

In Missoula, air quality standards regulate when you can use wood as a heat source. They have instituted that almost every year since the rules were adopted by Missoula County.

For more information and to check for the latest information, go to the Montana Air Quality website. You can find out the data from Hamilton and other places in the state. has a direct link on our "trending bar" near the top of the home page. Just click on Air Quality.