The Colorado Department of Public Health and Environment has issued an air quality alert for Adams, Araphoe, Boulder, Denver, Douglas, and Jefferson Counties until 4 p.m. Wednesday.

The alert is described as an “Action Day for Particulates”. Just like on Tuesday, particulate matter mainly from pollution will be trapped near the ground because of an inversion over the Front Range. Normally air gets colder as you ascend higher into the atmosphere. But when temperatures aloft are warmer than temperatures at the ground, it’s called an inversion. That warm air aloft is what traps the particulates near the ground and what causes the brown cloud to be visible for many miles.

Source: CBS4