In the Rocky Mountains, autumn descends a little earlier than it does at lower elevations, so if we wait until the locust tree in the front yard is yellow, we will probably have missed all the mountain fun. In general, late September is the best time to see changing colors at elevation, but paying close attention to changing temperatures can result in a surer bet. In general, warm, sunny days followed by cold nights will result in the most dramatic fall colors. That's why mountain climates are so conducive to fall vibrancy. In the mountains of Colorado, for example, September days usually stay sunny and warm, but overnight lows start to drop dramatically. The temperature changes so quickly that leaves change all at once, and trees that were green just a few days earlier become otherworldly tufts of yellow and red. At lower elevations and in more humid climates, temperatures change more gradually, and so do the trees. Watching temperature trends is a good way to determine the best time to see changing trees, but time of day is also important. At high noon, direct sunlight tends to give forests a flat, washed out look. No autumn forest is anything short of gorgeous, even at midday, but taking a mountain hike in the morning or midafternoon will result in a more breathtaking experience. Morning and afternoon light is indirect and comes in at an oblique angle, especially in fall. This creates shadows and colors that add depth and drama to any scene, and it's in the morning and afternoon that fall aspens and other changing trees are at their most stunning. The spectacle of autumn in the mountains is a tourist destination for thousands each year. When fall finally arrives in the suburbs and the cities, those who went to the mountains in late September are treated to a second view of this annual beauty. By late October, when piles of fallen leaves line suburban streets and fill trash bags printed with jack-o'-lanterns, the mountains will already be blanketed with snow. Some will have begun the winter tradition of ritual weekend trips to the ski slopes. Those of us who prefer the depth and mystery of autumn; however, will be out enjoying the crisp, cool autumn air as long as we possibly can.