We have a wild Thursday on tap. A strong but fast moving cold front will pass through the region kicking up some strong winds (a valley High Wind Warning is in effect from 9 am to 7 pm), and mountain snow (a Winter Weather Advisory from 9 am to 10 pm at Tahoe.) Valley winds could gust in excess of 60 mph in the wind prone areas, and the mountains could get anywhere from a few inches at the lake to a foot on the ridgetops. The valley could see some rain, but probably not until the late afternoon to evening, and those could turn to snow showers late, but little valley accumulation is expected. Temperatures will warm to the upper 50s Thursdays before falling back into the 40s for the weekend.
So why didn’t the water in Bob’s cup not freeze after being left overnight when the temperature got into the mid-20s? There could be several possibilities. If there were any impurities in the water, that could affect the freezing point. Also water does not radiate energy as fast as solid objects, so at night, the ground will cool much faster than the water will. And if the cup of water was left under a covering of some sort (a porch or an eave), then that would block the radiational release of the heat in the water, keeping it warmer, while the thermometer may have been out away from the house.
But I think the real answer deals with thermodynamics. More tomorrow.