To answer the above observations;
The quoted temps were both Fahrenheit, and it was just regular tap water (filtered from the fridge dispenser). It could be ride length...BUT...The ride when the temp was in the high 20s was a 25 mile ride. The ride when the temp was 10 degrees was only a 15 mile ride (that was all I could handle). BUT...when it froze in the 20s ride I noticed that it was frozen about halfway through that ride. In both cases there was little wind. I had thought about the sun factor. But the sun was also about the same in both cases.

Dan