Getting a handle on dust devils

Saturday, July 19, 1997 | 11:04 a.m.

Dust devils on Earth are a minor and somewhat misunderstood weather phenomenon, but apparently on Mars there have been some big whirlwinds that have left their mark on the planet's desert-like surface.

Steve Metzger, a graduate research assistant with the Desert Research Institute, has done extensive studies on dust devils in the Eldorado Valley. He says additional research into the swirling vortices can answer many questions about conditions on both planets.

"Mars is so close geologically in many ways to Earth," the 42-year-old Reno resident said. "This is an excellent opportunity to compare and contrast."

Dust devils on Mars, he said, can stretch four miles high and a half-mile wide. By contrast, on Earth, most big dust devils range from 1,000 feet to 1 1/2 miles high. Most are just a few feet to several yards wide.

Metzger said photos taken during Viking missions to Mars confirm that there are dust devils there. He is studying photos sent back to Earth from the Pathfinder, looking for more evidence of the effects of dust devils on the planet's surface.

Metzger says he is looking forward to the Mars Global Surveyor, a weather probe that is slated to arrive at the Red Planet in September and begin a two-year orbiting mission that will study meteorological conditions like dust devils.

Dust devils resemble tornados and often are called "little twisters" because, to a lesser degree, they gather dirt and debris, pushing it skyward.

However, dust devils form much differently than tornados. A dust devil forms on the ground in hot weather and travels upward. A tornado is accompanied by a storm front and forms in the atmosphere, touching down on the ground.

A dust devil ordinarily does not form during a storm because the clouds blot out the sun, preventing the ground from getting warm enough for it to develop.

The National Weather Service at McCarran International Airport says this is the heart of dust devil season.

"As warm as it has been with the thermals rising, dust devils are a daily occurrence," said Barry Pierce, a weather service meteorologist based in Las Vegas. "They are not very dangerous. Usually you get a mouthful of dust."

As evidence of their ordinarily weak force, Metzger drove his truck with sophisticated meteorological equipment and computers right through the heart of dust devils to gather data for his study.

But, on rare occasions, dust devils can cause damage.

The weather service, in naming its top five Southern Nevada weather events of 1996, gave honorable mention to several destructive June dust devils.

One of those twisters was blamed for the death of a skydiver when his parachute collapsed in a developing swirl. Another dust devil ripped the awning off a mobile home in northeast Las Vegas.

But the real damage caused by dust devils may be more long-term than immediate.

"Studies could lead to answers regarding smog chemistry and global climate change," said Metzger, whose research has found that dust devils have a horizontal wind speed of up to 60 mph and a vertical lift of 25 mph.

They travel along the surface at about 30-35 mph during their relatively short lives -- generally a few minutes.

Still, such speeds are significant enough to push a lot of pollution into the atmosphere, Metzger said, noting that dust is a key ingredient in the formation of smog.

"Dust from Washington (state) could result in smog in Chicago that mixes with industrial particles there and comes down in the form of acid rain in Maine," Metzger said.

Also, he said, sunlight that is reflected off dust and does not reach Earth's surface could affect global climates.

"Southwest U.S. cities have significant dust pollution problems," Metzger said. "We need to determine how much of that is from the natural process (dust devils, etc.) and how much is caused by humans (construction, etc.)

"No one has yet put a number on that. That is what I am going to do."

In studying the recent information that has come from Mars, Metzger has noticed a lot of dark rocks called basalts.

"The basalts on Mars are similar to those found in the El Dorado Valley," Metzger said. "And dark rocks absorb heat a lot better."

Although heat is a key factor in forming a dust devil, it does not necessarily have to be hot for one to develop. Mars, for instance, is very cold even when the sun hits it.

"There are dust devils on Antarctica," Metzger said. "To form dust devils you need clear skies, the sun beating down on the ground and very little wind. Even if it is cold, you can still have a dust devil as long as the ground air is warmer than the air above it."

While tornados are rare in Southern Nevada -- the last one touched down in the southern part of the valley on March 30, 1992, causing considerable damage to one house -- dust devils are plentiful.

"During my research last summer, there were 25-96 dust devils per day that could be seen from a distance of 3 1/2 miles," Metzger said.

"Dust devils occur a lot more often than people may think. How much this affects the air we breathe, we just don't know for sure."

Metzger became interested in dust devils while doing research on the Nevada Test Site. He studied how much dust the wind might have been blowing off the old nuclear testing range.

Dust, he said, also is comprised of salts distilled from the water of now-dry lakes. These salts dissolve when they hit a moist surface, such as your lungs. Some dust consists of microscopic bits of rock which also is harmful to the respiratory system, Metzger said.