SUNSETS AROUND the world have been more spectacular than usual for a year and a half, and University of Utah researcher Steve Krueger knows why: the explosion of Mount Pinatubo in the Philippines in June 1991.
"The volcano put up a layer of volcanic aerosol in the atmosphere," Krueger said during an interview at his office in the university's William B. Browning building. An assistant professor of meteorology, he has been creating models of cloud behavior for the past two years.And since shortly after Pinatubo blew, he has been photographing the brilliant sunsets. He now has about 400 slides in his collection.
"By aerosol, we don't mean volcanic ash, but actually small droplets of sulfuric acid. The sulfuric acid formed from sulfur dioxide gas that came from the volcanoes."
Some volcanoes, like Mount St. Helens in Washington State, dump mostly ash; some loose rivers of lava, as those in Hawaii; and some are really gassy, like Mexico's El Chichon, which erupted in 1982, and last year's Pinatubo.
In fact, an estimated 25 million tons of sulfuric gases blasted into the atmosphere when that volcano blew its top.
"The first thing that happens is the initial spreading of the clouds," Krueger said. Within three weeks, the Earth was girdled by the cloud, which spread along an equatorial belt.
This fine mist hangs high above the Earth, reaching the stratosphere at an altitude of about 80,000 feet.
Soon after a volcano's sulfur cloud spreads along the equator, winds fan it to the north and south. "It has a worldwide distribution after several months," he said.
On July 11, 1991, Krueger was on the Baja peninsula in Mexico to view the total eclipse of the sun.
At sunrise that day, he noticed a cloud formation that looked strange to his trained eye. "That's the first evidence I saw of the volcanic cloud," he said. It hadn't shown up farther north yet.
Later in July 1991, he went to Florida on a weather study. Of course, Florida is a lot closer to the equator than most of the country, and at that time the Pinatubo cloud was still spreading along the equator.
While driving one evening near the old Mercury launching pads, he noticed "very brilliant clouds - much higher than the normal cirrus."
The ordinary cirrus clouds there were already dark, yet a strange cloud layer was still lit up, showing that the sunlight was hitting something high above the Earth. "They also had a peculiar wavy structure to them," he said.
For several days, he photographed the strange cloud layer, which would appear before sunrise and after sunset. It was invisible during the direct sunlight of the daytime.
"I wasn't sure if they were volcanic clouds or something else," he said.
Later, he learned that satellites were tracking the spread of Pinatubo's gasses. The clouds were rippled because at first they didn't diffuse evenly through the atmosphere.
Krueger returned to Utah and the volcanic clouds arrived here, too. He continued to photograph them, often from his perch high on the Browning Building.
"I never saw much structure here," he said. As the clouds covered the globe, they had become more uniform.
The aerosol layer is so thin that it's invisible except under extreme lighting conditions, such as at sunset, when the raking light illuminates droplets against the dark night.
"The clouds act like a projection screen that we see the distant sunset colors on," Krueger said.
Because of variations in cloud thickness and conditions at the distant sunset, the colors vary. His studies show that they typically follow a pattern of silver turning to salmon, then purple, orange, and sometimes a red glow.
A fascinating aspect of the Pinatubo sunsets is that dark rays, or bands, often stretch across the high layer. These are shadows of mountain peaks in the Sierra Nevadas, 500 miles away.
Ordinarily they don't show up because the clouds aren't high enough to catch the light of the sun when it's going down so far off. But with the Pinatubo aerosols, the sunset colors are from sunlight reflecting off droplets 80,000 feet above the Earth, two to four times as high as the ordinary cirrus clouds.
Utah's sunsets were most brilliant in October 1991 and have been fading since then. But although the droplets are gradually precipitating, many continue to hang in the high reaches of the atmosphere, giving a brighter tinge to sunrise and sunset.
When the weather permits, Krueger continues to keep track of the acidic layer.
He is interested in seeing how Pinatubo's gas will affect a total eclipse of the moon, which will take place on Dec. 9. The amount of material in the air may absorb so much of the moon's light that very little penetrates to the surface.
"It should be very dark," he said. "The moon may almost disappear."
In Utah, the eclipse will be in full swing when the moon rises. The eclipse may be so dark because of the volcano's sulfur that it will be hard to spot until totality begins to end.
For next year, Krueger predicts the morning and evening sky shows will be a little less spectacular in the spring, but will brighten for the summer.
"That's been noticed in the past, that there seems to be a decrease in the spring," he said. Eventually, the droplets will drop lower, form ice crystals, and precipitate from the atmosphere.
Meanwhile, the show starts at sundown. "There's going to be a constant change in the color pattern from that time," he said.
"I find the best time is about 20 minutes after sunset. That's the time when you're likely to get rays, which don't always occur, and the widest variety of colors.
"The most brilliant colors actually come a bit later, actually 20 or 30 minutes after sunset."