XXXXX DRUDGE REPORT XXXXX THU, JUNE 24, 1999 20:59:22 ET XXXXX FREAK: RARE CLOUD -- WITH POSSIBLE WARNING -- SEEN OVER COLORADO High-flying clouds of ice crystals were spotted in the sky over Colorado Tuesday night, the first time they've been seen this close to the equator. The cloud clusters are common closer to both poles of the Earth during the summer. They had never been spotted in the U.S. south of North Dakota. "This could be a signal that something is happening to our upper atmosphere," Gary Thomas, a professor at the University of Colorado's Laboratory for Atmospheric and Space Physics, tells Firday's DENVER ROCKY MOUNTAIN NEWS. The NEWS' Charlie Brennan reports that Tuesday's sighting was recorded by three men in three locations: CU meteorology instructor Richard Keen near Boulder, Utah State University physics professor Mike Taylor in Logan, Utah, and Mark Zeilcik, a noctilucent cloud researcher in Edmonton, Canada. There are diverse theories as to what caused the clouds to form at such a southerly latitude. "It could be an indication that we're polluting our atmosphere with greenhouse gases, or it could be part of a natural cycle of long- term trends," said Thomas.