I was was flying across the Rockies at 35000 feet. I was sitting in a window on the plane looking down at the mountains covered with snow. I was not sure where we were in relation to my destination but we were about an hour from san francisco flying east to west. we came into some cloud cover over the mountains and all of a sudden there were bright too bright lights below the plane that I could see through the clouds. my first thought was we were too close to the mountains and we were going to hit it because I thought the lights could have been a building. all of a sudden the clouds broke open and I was staring at a large triangular shaped object with lights on it. this object was below the plane over the top of the mountain and looked stationary as we flew over the top of it.the object was silver in color with two lights on the narrow part of the craft. I believe we were around 5000 feet over the peak of the mountain, and the craft covered a good portion of the top of the snow capped mountains. the mountains again were covered with clouds and I never saw the object again. I clearly saw the object for about 15 seconds. there was nothing to distort my view of what I was looking at.