Acu-Rite thermometer: question

Speculation: Sometimes on clear nights (you said it was windy), thermometers out in the open can register the cold night sky more than the ambient temperature. That's one reason weather station thermometers are in wooden boxes or other enclosures.

If you were cowboy camping, that same cold night sky would be sucking warmth from your sleeping bag (along with the wind), so the low reading might closer to your experience.

Measuring temperatures is full of weirdness.

— Rex

what Rex said – it can be 10 degree F colder, or

maybe when it fell on the ground it broke. Put thermometer in plastic bag to protect it. Put bag in ice water – as much ice as possible with enough water to fill in spaces. Ice should be all the way around it. Let it sit for 15 minutes. Should read 32 F, maybe 33 F because it's hard to get it down to freezing point.

Does it show the minimum since the last time it was cleared? In other words, it wasn't the lowest that night, but another time?

"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.

"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.

That happens because of radiant heat transfer.

Objects get colder when they lose heat energy.

There are three mechanisms for removing heat energy:

1. conduction – come in contact with a colder object and heat flows to the colder object … pick up an ice cube and your hand skin gets colder
2. convection – colder fluids (like water and air) come in contact with a a warmer object and absorb heat (that part is conduction) and then move away … that is why 40 degrees in wind cools you faster than 40 degrees with no wind
3. radiation – that is why a 40 degree sunny day feels warmer than a 40 degree cloudy day. Or, in the case of night time, the temperature drops faster on a clear night than a cloudy night. Also, as yo describe, an object has frost on it on mornings when the air temp did not get down to 32F … the object gave up heat energy to the upper atmosphere via radiant heat transfer. Your thermometer was one such object.

I measured it in http://www.backpackinglight.com/cgi-bin/backpackinglight/adams-torso-simulator.html

Like Jim said – radiant heat loss. Night sky is cold. The ground loses heat to sky by radiant heat loss so it gets colder. The air right next to ground gets cold from the ground. Your thermometer on the ground gets cold.

I've camped in trees where it was 40 F, and then went to meadow where it was somewhat below freezing.

If you're cowboy camping, the ground next to you will get colder so the air temperature will be colder, plus your bag will lose heat due to radiant heat loss, so it's doubly cold.

If the sky is clear, and your sleeping bag is marginally warm enough, sleep under shelter – tent or better yet treed area.

I understand about radiant heat loss, but wasn't aware that it extended to the upper atmosphere; I guess I thought of it more in terms of losing heat to the air or objects directly around you. I'm still surprised at the magnitude of the difference in this situation – that the air could be close to 50F yet the thermometer would register below freezing at some point. And this on a night when there is no frost, not even in my area – hasn't been any for a few weeks now, in fact.

dk,

I own several of these thermometers. Every once in a while I'll get a "way off" min reading. I have no explanation but I just ignore it.

On a clear night you're radiating to space, average temperature 2.76 Kelvin. Really friggin cold.

For small temperature differences, radiation effect is small, but it is proportional to T^4 so it grows quickly.

If I needed to measure the actual air temperature reliably, I'd put the sensor in a double-walled tube to prevent it from "seeing" the sky, and provide a slow flow of air through the tube. The walls could be very thin and don't have to insulate. The inner one comes pretty much into equilibrium with the air and eliminates almost all the radiation effect.