It rained, so I thought that the outdoor sensor had been covered with water, and was thinking about the thermodynamic properties of things when they get soaked in water, which is that the molecules with a higher kinetic energy evaporate from the liquid, which reduces the average kinetic energy of the molecules in the liquid, so as a result, the temperature decreases. It's like why you get cold once you step out of the shower, even though the room was a comfortable temperature before you got in. I was remarking how surprising it was that it was able to consistently keep my temperature sensor about 30 degrees below ambient simply due to evaporation.