Tuesday, September 25, 2012

Summer is Gone

Summer is gone, and we know this how? Well, in addition to looking outside and seeing the bare trees, solar heating is now no longer the dominant driver of temperatures in Interior Alaska. Rather, surface cooling (longwave radiation) and horizontal movement of air masses with different thermal characteristics (advection) are starting to overwhelm daytime heating from the sun.

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 24ยบ above the horizon, which is about the same as Chicago on winter solstice.

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