Causes of Poor Visibility

Extremely small particles are the principal cause of poor visibility. These tiny particles, too small to be seen without a microscope, are measured in microns, with one micron equal to about one-seventieth (1/70) of the diameter of a human hair.

Particulate matter less than 2.5 microns, often referred to as PM2.5, is a significant cause of haze. Each particle, about the size of a single grain of flour, can float in the atmosphere for days, behaving much like a gas. Over half of the PM2.5 in Phoenix is caused by the burning of gasoline and diesel fuel in vehicles (sometimes referred to as on-road mobile sources) and in off-road mobile sources, such as construction equipment like loaders and bulldozers, locomotives, lawn mowers, leaf blowers, and other devices that emit air pollution as they move¹. PM2.5 particles containing carbon, like soot from tail pipes, are particularly effective in reducing visibility because they both scatter and absorb light.

Nitrogen dioxide and sulfur dioxide gases from burning of fossil fuels also contribute to the brown cloud. Nitrogen dioxide gas is brown, giving that color to the haze. Chemical reactions in the atmosphere convert these gases to fine particles.

Dust, principally from driving on paved roads, is also a contributor. Natural sources, like carbon particles from wildfires and dust from the Salt River bed, are small contributors to the haze.

Weather conditions such as temperature, wind speed, and humidity make the brown cloud look different on different days. Phoenix area nightly temperature inversions, which are stronger in the winter, combined with its location in a valley, play the largest role. Every evening after sunset the surface of the land cools off more rapidly than the air above. As a result, fine particles and gases from combustion produced that day are trapped under the inversion. At the same time, a mass of cooler air slides down from the mountains, pushing the pollution across the valley from east to west. On a relatively calm, hazy day, if you look to the west from the top of Piestewa Peak (formerly Squaw Peak) a of brown haze will be apparent. If observed for several hours, the haze layer will rise, as the temperature rises and the inversion lifts. Around mid-morning the direction of the air flow in the valley reverses, as the relatively warmer air makes its way from west to east, moving up toward the mountains. In the afternoon the brown cloud will become less visible to the west, but more visible to the east.

¹Brown Cloud Summit Final Report, Appendix 3, Sources of Fall and Winter Visibility Impairment in Phoenix, page 5-2.