Desert Air and Light

David Wentworth Lazaroff

Rocks, rattlesnakes, roadrunners, coyotes, cacti-all familiar and tangible parts of that great and complicated whole we call the Sonoran Desert. But the desert is also made of less substantial things, and these, too, contribute to its special character. Among these elusive ingredients are many subtle and mysterious phenomena involving air and light. These aren't matters to ponder indoors! Let's take a drive in my Volkswagen bus. You can sit up front.

Phantom Water

It's 2:00 p.m. on a very hot April afternoon, and we've just pulled out of the entrance to the Arizona-Sonora Desert Museum, west of Tucson, Arizona. Far ahead of us a puddle seems to be continuously evaporating off the sun-baked pavement, its receding edge matching its speed to ours. Even when I floor the accelerator we can't catch up to the water, but somehow the cars passing our Volkswagen easily do. They drive into it without a splash, and they seem to be reflected in it, upside down.

Of course, real water doesn't behave this way. The retreating puddle is just the most familiar form of that often misunderstood phenomenon of air and light, the mirage.

The puddle mirage (which can also be seen on warm days in more temperate climates) starts with simple physics. A shallow layer of sizzling air lies on the surface of the hot pavement. When light from the sky encounters this superheated layer it's bent, or refracted, upwards toward our eyes. The effect is very much as though a mirror were laid flat on the road. We see an image of the bright sky, and even upside down images of cars and cacti.

The puddle mirage. Light from the sky is refracted upward by a layer of superheated air on the pavement. Light entering the superheated layer close to the bus isn't bent upward sharply enough to reach the driver's eyes.

We can't catch up to the water because light entering the hot air layer close to us isn't refracted upwards steeply enough to reach our eyes. The mirage looks like a puddle because when we see sky and automobiles apparently reflected off the ground, our brains insist on interpreting the scene as something familiar. In everyday experience, the most common reflective object we see on the ground is a body of water.

Is the puddle mirage real or is it an illusion? It's actually a little of both-straightforward physics plus the workings of the human mind. Add a little dehydration, which we almost always experience on hot days like this, and perhaps we're even more inclined to see water where it isn't!

Ground-level layers of hot air aren't restricted to pavement. Under the right conditions they can conjure up larger "ponds" and "lakes" in the open desert. And these layers have important consequences for living things. Many spring annual wildflowers begin their lives in the fall as rosettes of leaves spread flat on the desert floor, where they bask in the thin layer of warmer air on cool, sunny days. When the weather heats up in the spring a plant may lift these leaves off the ground, or the rosette may die back as the stem grows upward, and cooler, higher leaves take over the duties of photosynthesis.

Air temperatures can drop so rapidly in the first few centimeters above the soil that long legs can be a real advantage for a small desert animal. When they find themselves on hot ground, many desert lizards stretch out their legs full length and lift their bodies as high as they can. Some small day-active animals escape the natural oven near the ground by climbing or flying to higher perches, and of course many simply seek shade. Ground-hugging desert creatures live in a world very different from the one you and I experience at the "higher altitudes" of human existence.

When Light Needs a Brake Adjustment

Why does light bend when it meets a hot ground-level layer of air, as in the puddle mirage?

The well-known "speed of light," about 186,000 miles per second (300,000 kilometers per second), is actually light's speed limit. Light zips along that fast only in an absolute vacuum. When light travels under less ideal conditions, such as through ordinary air, it's slower. In effect air puts on the brakes, and the denser the air is, the more forcefully the brakes are applied.

Imagine our Volkswagen bus is a light wave. It's been shrunk to matchbox size and turned on its right side, and it's hurtling obliquely downward toward the ground. Because we're driving through air the brakes are dragging, and the speedometer reads just under light's speed limit-a white-knuckle situation if there ever was one!

At the last instant before impact our right (lower) wheels enter the superheated layer of air just above the pavement. The hot air has expanded, so it's less dense than the cooler air above it. Suddenly the brakes on the right wheels aren't dragging as strongly as those on the left. The right side of the bus starts to move faster and the vehicle pulls to the left, away from the ground. Saved by refraction!

The part of the light wave in the superheated air near the ground travels faster than the part in the cooler, denser air above it. As a result the wave veers upward, the way a bus with bad brakes pulls to one side.

Refraction is simply the changing of a wave's direction when different parts of it move at different speeds. Any kind of wave. Sound waves in the atmosphere travel faster in the warmer air at lower altitudes than in the cooler air higher up, so they tend to bend upward, like light in the puddle mirage. That's why sometimes you can't hear the thunder from a distant monsoon storm, even though you can see the lightning: the sound passes above your head!

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.

Got Those Bouncing Photon Blues

Speaking of the desert sky, why is it so blue? For that matter, why is the sky blue anywhere? Oddly enough, we owe the answer to the latter question largely to a trio of scientists who worked in the foggy climate of the British Isles.

In the late 19th century the physicists John Tyndall and Lord Rayleigh showed that the sky is blue because of the way sunlight interacts with air. Most ordinary visible light passes through the atmosphere relatively undisturbed, but occasionally a light particle-a photon-runs into an air molecule and bounces off it, a process called scattering. The light we see in the sky is sunlight that has been scattered off air molecules. But why is it blue instead of white?

The visible spectrum, as seen in a rainbow, or more often nowadays in the surface of a compact disk. Also shown are the invisible infrared and ultraviolet radiation just off the ends of the visible spectrum.

About two centuries before Tyndall's and Rayleigh's investigations the great Sir Isaac Newton had demonstrated that ordinary white sunlight is a mixture of all the colors of the rainbow, from red through violet. Tyndall and Rayleigh showed that how strongly light is scattered by air molecules depends on its color (more precisely, on its wavelength). Light from the blue-violet end of the spectrum is much more likely to bounce off an air molecule than is light from the red-orange end. As a result, most reddish light travels through the atmosphere more or less unimpeded, but enough bluish light is scattered into our eyes to make the sky appear blue.

At least it appears blue to us. Other creatures may see it differently. Invisible to us, the radiation lying just beyond the violet end of the spectrum-the ultraviolet, or uv-is scattered even more than the violet. The desert sky is so bright with uv around midday that an exposed surface receives about as much of it scattered from the sky as directly from the sun-a good reason for humans to wear broad-brimmed hats! Unlike us, hummingbirds and honey bees can see ultraviolet light. What color might the sky look to them?

Sonoran Desert skies are such a deep blue (to human eyes) because desert air is unusually pure, that is, compared to the air above many other places on the planet; it's relatively free of the tiny floating particles and droplets called aerosols. Aerosols come in a wide range of sizes, and the larger ones reduce the blueness of the sky. Unlike air molecules, they scatter light of all colors about equally. As a result, they seem to fill the sky with white light, diluting the blue.

Desert air has so few of these large aerosols partly because it's so dry. In more humid climates water vapor condenses on microscopic airborne particles, forming tiny droplets that we see as hazes and fogs. This is especially true in coastal areas, where tiny salt crystals from evaporating ocean spray are especially good at capturing water vapor and creating water droplets. In fact, morning fog is a routine occurrence in parts of the Sonoran Desert along the western coast of Baja California, as described in the chapter "Biomes and Communities."

Of course, even inland desert air is far from aerosol free. Beautiful water droplet hazes can form in the Sonoran Desert overnight after rainstorms, but they often evaporate quickly after sunrise. On spring days like this one the desert sky can be noticeably brightened by airborne pollen. And, unfortunately, even Sonoran Desert skies can be sullied by particulate pollution from cities and factories. But the most characteristic desert aerosol is dust. A major dust storm can make the car in front of you disappear from view, but small everyday dust particles have more benign effects, as we'll see.

Deceptive Distances

As we drive westward onto the Tohono O'odham Nation we can see desert mountain ranges receding into the distance in every direction. Nearby slopes show the warm colors of volcanic rocks and blooming palo verde trees, but more distant mountains seem bluish and washed out. That's because air between us and any object in the landscape scatters blue light into our eyes. The more distant the object, the more air intervenes, and the greater is the bluish tint and the loss of contrast.

This effect, called aerial perspective, is well known among landscape painters and photographers, who exploit it to create the impression of depth. It's one of the cues we all use unconsciously to gauge distance in the outdoors. In regions where the atmosphere is thick with aerosols, aerial perspective is strong and obvious, but here in the desert it's often quite subtle. More than one desert traveler has suffered dire consequences because the mountain with the next water hole was much farther away than it looked!

Blue skies, blue mountains-and blue birds. Among the oaks and telescope domes on the summit of Kitt Peak (now disappearing in our rear-view mirror), Mexican jays are preening their plumage. The blue in their feathers isn't a pigment; it's caused by the scattering of sunlight off tiny structures in the feathers-the same process that makes the sky blue. Scattering even creates the blue in the open eye of the astronomer awakened by the scolding jay outside her window!

Speaking of sleep, it looks like you could use a siesta, too. That's another familiar effect caused by warm desert air....   

Left-over Light

I see that last pothole woke you up. We've just left the Tohono O'odham Nation. I've turned onto this dirt road so that we can park among the creosote bushes and enjoy the closing act in the day's drama of air and light-that unparalleled boon to the photographic industry-a desert sunset.

Balanced on the jagged horizon, the sun has turned the orange color of a desert globemallow blossom. Believe it or not, it looks that way because of the same phenomenon that makes skies, jays, and eyes blue: light scattering. At the end of the day, sunlight travels a much longer path through the atmosphere to reach us than it does at noon. Along the way so much violet, blue, and even green light is "scattered out" that mostly red, orange, and yellow light gets through. In other words, the reddish light we're seeing now is merely the sunlight that's left over after helping to create blue skies for travelers farther west.

The same purity of the air that makes desert skies so blue also helps make desert sunsets so beautiful. There usually aren't enough large aerosols here to quench the light of the setting sun or to reduce visibility. However, smaller aerosols, mainly in the form of airborne dust, can actually intensify the colors of a sunset.

The earth's shadow and the colors of a thundercloud just after sunset. The bottom of the cloud (1) is in darkness. The middle of the cloud (2) is lit by sunlight reddened by scattering in the lower atmosphere. The top of the cloud (3) is white; it's lit by sunlight that has passed only through the rarefied upper air.

Unlike larger aerosols, which, as you'll remember, scatter light of all colors about equally, very small particles scatter bluer light preferentially, much as air molecules do. They brighten the blue sky and make sunsets even redder. It's because of these tiny particles that desert sunsets are usually more colorful than sunrises. Breezes and dust devils increase the dustiness of air by the end of the day, but some of the dust settles out during the calm of the night. (When you see a dust devil, think of it as a sunset in the making!)

Now the sun has dropped below the horizon, and we need to turn 180 degrees to watch the next important event. A blue-gray band is rising slowly above the eastern horizon. This is the earth's shadow-literally the shadow of our planet on its own atmosphere. From the point of view of any hawks still soaring in this darkened region of air, the sun is below the horizon. Just above the shadow is a band of reddish light. Hawks soaring up there can still see the setting sun. We see the reddish band because reddened sunset light is being scattered back to us by dust in the air.

The curtain of night rises more quickly, then merges into the darkening sky. If this were a cloudy evening, these few minutes would be the most spectacular-a time of shifting shadows and rapidly-changing hues, as fiery sunlight finds its way through openings between the shifting clouds.

Part of the fun of watching a desert sunset a few months from now will be following the changing appearance of a monsoon thundercloud. As the sun slips farther below the horizon, the up-tilting shadow of the earth swallows the cloud from the bottom up. Just above the rising shadow the cloud is bathed in the warm light of the setting sun, but for many minutes the top of the cloud remains bright white, because at that great altitude the air is too thin to appreciably dim and redden the sunlight. Then, just before it's extinguished, the summit turns pink as it's lit by sunlight that has passed close to the earth's surface far to the west.

Out of the Oven, Into the Refrigerator

As twilight colors fade we can feel the coolness of night settling over the desert. The sun-warmed ground is cooling by radiating its heat into the sky. Most of the radiation lies in the infrared, or ir, just beyond the red end of the visible spectrum. (See the illustration above.) While we can't see infrared, in a sense rattlesnakes can. On nights like this they use sensitive ir detectors on their heads to find the warm bodies of rodents in the dark. You can read about that in the "Rattlesnakes" chapter. (Watch your step out here, by the way.)

As the ground cools it in turn chills the air just above it. Gradually, the layer of hot air that seethed above the sun-baked soil during the day will be replaced by a quieter layer of cold air. Unlike warm air, which is light and floats upward, cold air is dense and flows downhill. (You can prove it. Stand in front of your refrigerator in your bare feet and open the door.)

Later tonight cool air will slide downward off the slopes of nearby mountain ranges and collect here on the valley floor. It will pool especially in low places, which will be noticeably colder by morning than places only a few meters higher. This cold air drainage is most apparent on clear winter nights. You do not want to lay down your sleeping bag on the bottom of an arroyo in January!

Sometimes the lake of cool air that gathers overnight in a desert basin causes morning mirages very different from the puddle mirage we saw this afternoon. Under these conditions light is refracted downward instead of upward, and, thanks to a peculiar atmospheric astigmatism, distant objects can seem to stretch out vertically, creating illusory cliffs and towers. If there happens to be a city in the basin, pollutants may be trapped in the cold air. This situation is often called a temperature inversion because the air's temperature rises with increasing height, instead of falling, as is usually the case.

Night-time radiative cooling and cold air drainage can pose a real threat to some desert plants, especially on the cooler northeastern edge of the Sonoran Desert. Frost sensitive plants like desert ironwoods may be confined to mountain foothills from which cold air drains away on winter nights. Radiative cooling is one reason why young saguaros are more likely to survive under a nurse plant. At night the overarching branches of a tree or shrub are slightly warmed by the infrared radiation from the ground. The branches then reradiate some of the heat back downwards, keeping the young cactus above freezing through a chilly night.

Radiative cooling operates well only when the desert sky is clear. Clouds work like nurse plants on a larger scale-they, too, intercept upward-traveling infrared and reradiate it. Even invisible water vapor absorbs infrared, so there's less cooling when there's more moisture in the air. In fact, water vapor is the most important gas responsible for the famous greenhouse effect, which keeps most of the earth, Sonoran Desert included, from resembling Antarctica.

Dry, cloudless skies let sunlight in during the day and heat out at night-that's why in a desert there's often such a great temperature difference between day and night.

The sky is dark now. Stars are twinkling overhead, and even more toward the horizon. (The scintillation is caused by random refraction by moving air of varying temperatures-an effect much like the shimmer we saw this afternoon.) Later tonight the atmosphere will settle down, the "seeing" will improve, and astronomers back at Kitt Peak will train their telescopes upward through the clear window of the desert sky. Perhaps they'll take a look at that red planet up there, where dust devils whistle across a rock-strewn world so dry that the Sonoran Desert seems like a rainforest by comparison.

Alas, there are some deserts where even a Volkswagen can't go!

Additional Readings

Bohren, Craig F. Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics. New York: John Wiley and Sons, 1987.

Gedzelman, Stanley David. The Science and Wonders of the Atmosphere. New York: John Wiley and Sons, 1980.

Meinel, Aden and Marjorie. Sunsets, Twilights, and Evening Skies. Cambridge: Cambridge University Press, 1983.

Minnaert, Marcel. Light and Color in the Outdoors. New York: Springer-Verlag, 1993.

Schaefer, Vincent J., and John A. Day. A Field Guide to the Atmosphere. Boston: Houghton Mifflin, 1981.