The plot at the right shows hourly temperatures Sunday evening into Tuesday at Keystone Ridge and Goldstream Creek. Focus on Monday evening. With clear skies and no wind, temperatures on the valley floor dropped like a rock after sunset, with the low temperature for the (calendar) day occurring near midnight. Meanwhile, 325 meters up the hill, after briefly falling a few degrees toward sunset, temperatures not only slowly rose, but the high temperature for the (calendar) day occurred about the same time the valley was reaching the low temperature, not long before midnight. This was possible because, in spite of plenty of sunshine during the day Monday, the high temperature on Keystone Ridge during the afternoon Monday was just seven degrees higher than the morning low temperature. Maximum solar elevation Monday was 24º above the horizon, which is about the same as Chicago on winter solstice.