Brown Cloud
Updated: September 3, 2024The Brown Cloud, a pollutant-related visibility problem, is caused by an accumulation of tiny carbon-containing particles smaller than 10 microns in size, and the chemical reactions of certain gases, including NOx, NH3, and SO2. The collective pollutants block light, causing the distinctive Brown Cloud and impeding visibility. This was primarily a wintertime issue in Denver for many years, due to the temperature inversion.