Paul Ostapuk thought a band of dark air extending across the western horizon looked a bit odd as his plane descended into Page, Ariz., but he had no way of knowing then just how odd it would turn out to be.
That's one of the few areas left in the country where the air is normally so clear you can see for 100 miles, and there had been no suggestion of an approaching storm in the weather report, so Ostapuk grew increasingly curious about the enormous cloud.
"It was pretty dramatic," says Ostapuk, a meteorologist and air quality specialist with Arizona's Salt River Project, the second largest producer of electricity in the state. The cloud, which he saw for the first time at around 4:30 p.m. on April 12, appeared to undulate, and within a couple of hours had spread across the entire sky.
The air was so thick with tiny particulates that it seemed as though the sun was setting prematurely, and astronomers at the Lowell Observatory in nearby Flagstaff would later report seeing rings around the sun, a clear indication that this was no ordinary cloud.
"My first impression was that there had been a volcanic eruption," says Ostapuk, but a quick review of news reports and other resources ruled that out.
And there were no wildfires or other possible sources in the area, so this was clearly a very strange beast.
Ostapuk turned to a NASA Web site with rich data available from a satellite launched less than five years ago. The Total Ozone Mapping Spectrometer (TOMS) provides continuous mapping of ozone and other aerosols, such as sulfur dioxide from volcanic eruptions, around the world.
He then backtracked the cloud, checking where it had been on each previous day.
And finally it hit him. It was clear that the dust he saw descending on Page had come from a massive storm so far away that it seems unthinkable it would have had such a dramatic impact in the small Arizona town.
The storm had occurred in the Gobi Desert of Mongolia and China. That's right folks — the Gobi Desert.
The dust had been carried to high altitude by winds so fierce that airports had to be closed, and then transported 7,000 miles across the Gulf of Alaska, down through British Columbia and over the vast expanse of open land in this country known as the Colorado Plateau.
A high pressure system trapped it in the area, causing some of the particulates to drop down to the ground and turning the sky yellow.
It had taken six days for the gunk to move from Mongolia to Arizona, and it came from what Chinese officials were already calling the worst sand storm of the spring season. Winds had swept across lands that had been laid bare by overgrazing, kicking up enough dust to reduce visibility to near zero and making even ground transportation nearly impossible.
It is a problem that China has been struggling with for several years now. A similar storm in 1998 was blamed for 12 deaths, and the storms appear to be increasing in frequency.
Clouds of dust from storms in China frequently blanket Korea and Japan, adding new fuel to centuries-old disputes between those three countries. A Korean research report described the fallout from the most recent storm as "yellow dust," and said it contained 16 times more "metallic substances" than your ordinary dust storm, contributing to respiratory illnesses.
Ostapuk didn't find that conclusion too far-fetched.
"It has the potential to cause respiratory problems, if you're sensitive to fine particulates," he says. "It effected me a little bit."
Choking on Sand
Long range transport of dust is not a new problem. Occasionally, dust storms in Africa's Sahara Desert reach Florida, and it's not altogether a bad thing. Dust from those storms transport many nutrients, including iron, enriching soils in one area of the Earth at the expense of another.
And of course we all know these days about what dust did to the dinosaurs. Most scientists now believe that dust from the impact of the Earth with a giant meteor or asteroid blackened the sky so intensely and for so long that food resources around the globe died out, killing off the dinos.
No one is suggesting that the dust storms in Mongolia pose any such threat. At this point it is more a matter of intense scientific interest than anything else. NASA, the National Science Foundation, and the National Oceanographic and Atmospheric Administration are all intensifying their research in the area.
It turns out the timing is ideal. A team of American researchers is already in Asia studying aerosol particles and their effect on Earth's climate, and a second dust storm has kicked up enough stuff to give them much to ponder.
On Tuesday the National Oceanic and Atmospheric Administration reported the second storm was already looming over Colorado. If it holds together, the dusty system is expected to reach the East Coast by Saturday.
As for the cloud that darkened the normally pristine skies of northern Arizona, it eventually moved on, according to NOAA.
By April 19, it had reached the East Coast, stretching from Hudson Bay to Northern Florida, and then it moved on out to sea.
What did it leave behind? Some dust, and some marveling over the complexity of a planet that just seems to be getting smaller and smaller.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.