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.
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
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
other devices that emit air pollution as they move1.
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
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.
Summit Final Report, Appendix 3, Sources of Fall and Winter Visibility
Impairment in Phoenix, page