Dramatic evidence of the frigid depths of Berkeley's brutal winter: ice in the improvised backyard watering dish for the chickens. This has happened twice even though the recorded air temperature from a sensor about 6 feet off the ground in back of the house hasn't been lower than 36 degrees this week and I don't see reports from any of the many Wunderground and other stations in the area of readings of 32 or lower. Berkeley's record low for Dec. 29, for what it's worth, is 30, set in 1905, and the average low for December, going back to 1893, is 43.7. (The town's all-time low was set Dec. 22, 1990, and coincided with a Christmas visit by family members who thought they were visiting someplace warmer than Chicago.)
Yes, there are some physical explanations for how water can freeze even if the ambient air temperature isn't yet freezing -- evaporative cooling, for one, and the fact the ambient temperature can be colder at ground level then it is a few feet above.