Photo by The Maricopa County Air Quality Department

In theory, Welch has the most privileged views of anyone in the Valley. From 3,000 or 4,000 feet, it should be possible to see the entire sprawling metropolitan area laid out like a street atlas – from the southeastern reaches of Queen Creek to the northwestern limits of Sun City West. But the air is frequently so dirty that he can barely make out the Chandler air traffic control tower as he’s headed back to the airport from the south.

“Imagine a picture of the morning fog over the Golden Gate Bridge, and then paint that brown over the city,” he says. “On the hot, still days of fall, after the monsoons are past, that’s what you get. When it’s at its worst, you can’t even make out the skyscrapers Downtown, even if Picacho Peak is crystal clear.”

Phoenix’s infamous “brown cloud” is bad enough from the ground. Seen from the air, it assumes a science fiction-like menace. No longer a soupy haze, but a cohesive, creeping fog that seems to be smothering the city, the brown cloud could be the aftermath of an apocalypse or the harbinger of an alien invasion.

Which begs the question: What, exactly, is it?

The answer: a suspension of fine particles spewed from our tailpipes and kicked up by our tires that is literally choking us to death. And because only certain types of pollutants make a visible contribution to the brown cloud, the problem is even worse than it looks. The air in Phoenix is inducing heart attacks and keeping kids home from school. It’s not an exaggeration to say that you’re shaving years off your life simply by living in the Valley.

Why Is the Sky Brown?

By its simplest definition, a brown cloud is a haze with a brown appearance – a haze being a suspension of particles that are too small to see individually but impair visibility in the aggregate when combined. Brown clouds aren’t unique to Phoenix; they occur over most large cities in the western United States. Ironically, it’s our lack of regional haze that makes “brown cloud” more of a buzz phrase west of the Mississippi than east of it. On the East Coast, large, multi-state regions are so generally hazy that it’s hard to pick out the urban clouds that hang over individual cities.

In environmental literature, the kinds of particles that contribute to air pollution are referred to as particulate matter, or PM. They are broken down by size. Particles with a diameter of 10 micrometers or less are called PM10. Particles that are still smaller, 2.5 micrometers or less, are called PM2.5. That’s incredibly small; by comparison, the diameter of a typical human hair is around 75 micrometers.

Hazes impair visibility by scattering and absorbing light, thereby decreasing the amount of light that travels from distant objects to our eyes. The amount of light scattered by a particle is mostly a function of its size, and there’s a “Goldilocks” phenomenon at work here. The smallest particles in the atmosphere are too small to scatter much light; the largest particles (generally coarse dust particles) are too large. Maximum scattering occurs from particles that are “just right” – those with a diameter roughly equal to the wavelength of light, or 0.5 micrometers. So most of the light scattering that occurs in the atmosphere is due to particles that fall into the PM2.5 category.

According to a 1999 report commissioned by the Maricopa Association of Governments – and still frequently cited in brown cloud literature – the particles that contribute most to haze in western urban areas are organic compounds such as ammonium nitrate, elemental carbon, fine soil dust particles and ammonium sulfate. The exact composition of brown clouds varies by region: Not surprisingly, dust contributes more to the brown cloud problem in Southwestern cities such as Phoenix than it does in the Pacific Northwest.