It looks like the Tsnow at 0.5m on the UW tower has gone awry.
I don't expect this one to come back. These are tiny thermistors that can easily break, and I am impressed that they have all lasted this long.
I also think the 0.4 UW has been reading incorrectly since about March 11, where is started slowly creeping up into the positive temperatures. Then starting March 13th the 0.5 UW also crept into positive territory, until March 16 where it starting reporting unreasonable results.
