It's noctilucent cloud-viewing season in the far north.

By now, six weeks after the summer solstice, those who live in the far north know the Midnight Sun will soon set for the first time in months. The Earth has moved to a point in its orbit where the sun finally dips below the horizon enough to allow nightfall and the opportunity to see celestial shows like the aurora borealis — and another sublimely spectacular phenomenon known as noctilucent clouds.

"Noctilucent clouds are night-shining clouds" says Richard Collins, a professor of atmospheric science with the University of Alaska Fairbanks. "They're clouds that form 50 miles high in the sky. They're composed of small ice crystals. They're clouds that are visible from Interior and Southcentral Alaska in the first two weeks of August every year."

Collins has studied noctilucent clouds for years as part of his work for the university's Geophysical Institute.

"These are clouds that exist right on the edge of space," he said.

Collins says the two-week window of opportunity to observe the wispy, obliquely illuminated clouds opens when the temperature in the upper reaches of the atmosphere over that part of the Earth is at its coldest, and when the sun is in the right position to sort of back-light the clouds' ice crystals.

"They're only visible when the sun goes below the horizon and then the light comes from behind the Earth and illuminates them from below," he said in a recent interview.

Bob Fischer, a National Weather Service meteorologist in Fairbanks, says noctilucent cloud-viewing season begins around the same time as civil twilight returns to the far north, when the sun is below the horizon but there's still some light in the sky.

"Civil twilight ends when the sun is 6 reaches below the horizon," he said.

Fischer says the season ends when the sun dips lower farther below the horizon.

"When the sun gets below 16 degrees, basically it gets too dark to see them," he said.

Collins says people who live between 55 and 65 degrees north latitude live in the ideal noctilucent cloud-viewing area.

"Say, Juneau (to) just north of Fairbanks," he said, referring to the area in Alaska that lies between those parallels. "Southern Sweden, Stockholm and Britain and Ireland. That's the sweet zone for seeing them."

But Collins says recent sightings farther south, and changes in the upper atmosphere where the clouds occur, have led researchers to suspect the influence of carbon dioxide buildup.