Denver's location at the foot of the Rocky Mountains make it prone to temperature inversions in which warm air traps cooler air near the ground, preventing pollutants from rising into the atmosphere.

What causes Denver's brown cloud?

The brown cloud over Denver comes mostly from vehicles, which emit nitrogen dioxide, formaldehyde, and benzene. Some days, these noxious chemicals dissipate, whisked away by prevailing westerly winds. Other days, pollution hangs in the air.

Why are inversions a concern for Denver?

The longer an inversion goes, usually, the worse air quality gets. Pollutants at the colder surface accrue over time, and they have no way to “escape,” because of the lid of warmer air above. This week has featured a fairly consistent temperature inversion, allowing pollutants to stay trapped and accumulate.