I was sitting outside facing south, in northglenn co. about 2:25pm. i was watching a plane headed east when i spotted a white circular object moving sw even faster than the airplane. it was roughly about 18000 ft up. traveling in a straight line. it traveled about halfway to the horizon and appeared to be above some low hanging clouds, with blue skies all around the cloud. that's when the craft seemed to turn a bit to the west and descended into the cloud. as it changed direction, i could see a silver/grey circle with a ring of white light around the outside, just before it went in the cloud. then it was gone. didn't exit from the cloud. i'm positive it was no aircraft. what it was? well a ufo is all i can figure.