"thermometers out in the open can register the cold night sky more than the ambient temperature"
Wow, how does that happen? That IS weird. It was clear, I'll grant you that. But the device be affected by the temperature of something that is far away, is that what you are saying? That blows my mind.
"it can be 10 degree F colder"
More like at least 15, I'd think, maybe 20. It just wasn't very cold here last night, and weather reports back me up on that. But then again, see below...
"maybe when it fell on the ground it broke"
Not too likely on that - it "fell" from a height of one inch or so :-) But the ice water idea is a good one. If accurate, that will reassure me that the thermometer is reasonably OK, and I presumably will have to somehow keep it from registering the sky (put it IN my shoe?)
"Measuring temperatures is full of weirdness."
That's for sure. I wish I understood more about the causes of at least some of the weirdness. For some reason, the winter nighttime low in my backyard often appears to be at least 9 degrees lower than the forecast for my city - I say this because if the forecast is 41, I have a likelihood of frost hitting my garden, and also because of how much lower than the forecast my car registers after a long drive arriving home from work late on a winter night. I've always attributed that partially to the "official" thermometer possibly being located nearer to the SF Bay than my house, which is nearer to the hills (either that or perhaps cold air flowing down from the hills past my house). I also have had the experience of my car thermometer registering 38 at 11 pm (already lower than the forecast low of 41, which presumably would arrive hours later than this) at my house when I arrive home, and finding ice on my boyfriend's car in the driveway when I exit MY car.
Thanks to both of you. I just googled the first sentence of Rex's that I quoted at the beginning of my reply to you guys, and got a little more elaboration (not that I doubted, mind you!). The phenomenon in the preceding paragraph is making a little more sense to me now.
edit @ Dale: it resets the minimum once a day. And before I went outside last night, it was reading 68 as the minimum. It reset again this afternoon and is now reading 70 as the minimum.