Rare "night shining" clouds called noctilucents, which are usually confined to northern latitudes, have been spotted as far south as Utah and Colorado, according to a NASA news brief.
"If you ever saw them, you wouldn't forget them - iridescent blue and silver," said James Russell, professor and co-director of the Center of Atmospheric Sciences at Hampton University in Virginia, who spotted the clouds in Helsinki, Finland.
Noctilucents usually occur in the warm months of spring and summer, with most sightings occurring in June and July.
They form 50 to 60 miles above the Earth's surface in atmospheric zones called the mesosphere and thermosphere. Their formation at this altitude places them roughly 40 miles higher than normal clouds.
Because noctilucent clouds form at such great heights, the sun's rays continue to illuminate them after sunset.
Thus, prime viewing conditions usually occur a half-hour to an hour after sunset when the sun is 6 to 16 degrees below the horizon. At this time, observers can scan the western horizon for tendrils of luminous, silver and electric blue formations.
"They take up a big part of the sky," said Scott Bailey, professor of physics at the University of Alaska. "If you look up into the sky and move out of the orange (of sunset) you can see them. There is some blue among them, but the streaks are silvery. It's incredible."
Scientists do not usually think of cloud forms spreading as would a species of animal. Nevertheless, noctilucent clouds are indeed spreading.
"They are being sighted at lower latitudes than ever before," Russell said. "They are becoming brighter and increasing in number."
Noctilucents were first reported in 1885 after the Krakatoa volcanic eruption in Indonesia. The volcano spewed volumes of ash and chemicals into the atmosphere. The newly-saturated sky caused spectacular red and orange sunsets and turned people's eyes skyward.
Shortly after the eruption, British sky watcher T. W. Backhouse reported seeing a glowing cloud in the night sky. He submitted his findings to the journal Nature.
Since Backhouse's sighting, amateur observers and scientists have reported noctilucents throughout the polar regions - Canada, Russia, Britain and Australia. More recently, the clouds have migrated south.
Mike Taylor, a professor of physics at Utah State University, photographed a noctilucent formation in Logan on June 22, 1999.
"That set off a firestorm of realization that they came down to lower latitudes," Bailey said.
Noctilucents sighted in the summers of 1999, 2000 and 2001 in Utah, Colorado and even New Mexico make the possibility of seeing them this summer more than a wish, Russell said.
Although scientists understand the mechanics of regular cloud formation, the formation of noctilucents is still a mystery.
In order for clouds to form, the air must contain water molecules and dust particles. Through a process called nucleation the water molecules collect around the dust particles and each other, forming a cloud.
Scientists know noctilucents readily form during spring and summer because warm air heated near the Earth's surface rises and brings water to the nether reaches of the atmosphere.
On the other hand, scientists do not know how dust particles are getting 50 to 60 miles high into the atmosphere. Some researchers say the dust comes from burned-up meteoroids in the upper atmosphere, Russell said.
Noctilucent clouds may also be indicators of increasing global temperatures.
Greenhouse gases, such as carbon dioxide, trap heat in the lower atmosphere, but actually make the upper atmosphere colder.
Therefore, as the effects of global warming become prevalent around the globe and the upper atmosphere becomes colder and colder, noctilucents can migrate southward.
Scientists cite the Industrial Revolution as evidence that greenhouse gases play a role in the cloud's formation.
During the Industrial Revolution, factories began pouring greenhouse gases into the atmosphere. The first sightings of noctilucents occurred during this time and have increased ever since as the world became more industrialized.
"There is now increased interest in these clouds because of their possible role as markers of global change in the upper atmosphere," said Gary Thomas, professor of physics at the University of Colorado.
A new satellite, the Aeronomy of Ice in the Mesosphere, developed by Russell and his colleagues, will make the necessary measurements to answer many questions that noctilucent clouds present.
The satellite is set to launch Sept. 30, 2006.
Besides noctilucents' scientific value, both Bailey and Russell said the clouds are important because they are relatively new phenomena and are capable of sparking human imagination.
"Every record of what we've seen in the sky has been the same," Bailey said. "As far back in human existence as we know, among the Egyptians, the Native Americans and the Chinese, there is no record that clouds glow in the dark."
