Atmospheric Uprisings

By now we've turned west onto the main highway toward Kitt Peak and the Tohono O'odham Nation. Cruising in our VW at a breathtaking forty-five miles per hour, we can't easily see the "heat waves" that are rising all around us above the desert floor. Heat has expanded the air layer at ground level, making it less dense and lighter than the cooler air above it. Everywhere bubbles of air are rising like hot air balloons-but without the balloons.

We can't see these ascending bubbles directly, but light passing though them is refracted in randomly-changing directions, causing distant objects to ripple and dance-an effect known as shimmer or atmospheric boil. Telephoto shots through shimmering desert air are a staple of western movies, evoking an impression of heat for comfortable viewers in climate-controlled theaters.

High above the ground the rising bubbles can be quite large. These are the thermals described in the chapter "Desert Storms." Thermals, too, are usually invisible, but there are sometimes clues to their whereabouts. A hawk wheeling in the sky may be saving energy by riding the rising air of a thermal. Soaring turkey vultures gain another benefit besides lift. Unlike most birds, they have an excellent sense of smell, and occasionally the ascending air carries with it the tempting fragrance of decaying flesh!

Toward the western side of the valley two dust devils are parading slowly across the desert floor. A dust devil, which looks something like a miniature tornado, is a special kind of thermal. Like people, some thermals are better organized than others, and dust devils are thermals of the best organized kind.

The exact conditions that create these grit-charged whirlwinds are somewhat mysterious. Dust devils form most frequently around mid-day or in the early afternoon, when solar heating is most intense. As heated air rises above a surface "hot spot," nearby air spirals inward and upward to take its place-like water whirling into a sink drain, only upside down. The rotation of the air (or water) accelerates as it approaches the center of the action, as a spinning skater speeds up when he pulls in his arms.

The analogy between a dust devil and a draining sink is apt for another reason. An endlessly repeated "factoid," easily refuted by any skeptic who puts it to the test, holds that draining sink water rotates in opposite directions north and south of the equator, thanks to the Coriolis effect. The Coriolis effect, a consequence of the earth's rotation, does indeed cause hurricanes to spin in opposite directions in the northern and southern hemispheres, but it has negligible influence on systems as small as sinks.

Or as small as dust devils. Not surprisingly, the sink drain myth has extended itself to these whirling dervishes, which are commonly believed to spin one way in Australia and another way in Arizona. In fact, no matter where on earth dust devils appear, about half turn clockwise and the other half counter-clockwise, in stubborn disregard of what they're "supposed" to do.

Dust devils are much less powerful than tornadoes, but they are capable of doing damage. A very large one could conceivably knock over a tall, flat-sided vehicle like ours. No dust devil could lift Dorothy (or Toto) off the ground, but it's not hard to imagine one levitating a lizard.

Do dust devils have any other effects on living things? No doubt they sometimes disperse the seeds of desert plants over longer than usual distances, and they help spread the fungal spores that cause the disease coccidioidomycosis, better known as valley fever. Perhaps more important, dust devils raise tiny soil particles high into the air, where they may drift hundreds of miles. Airborne dust is a major factor in the formation of the limy desert soil layers called caliche, as well as clay-rich argillic layers, both of which have profound effects on desert vegetation, as explained in the chapter "Desert Soils."

Floating dust has interesting effects on the desert sky, too. More on that farther down the road.