How cold is the night's sky?

[J.C.MacSwell]

From the earth's surface, on a clear night, what is the equivalent temperature of the night's sky?

 

I notice that (as I expected) water will freeze (shell) over in a bucket at +2 C when I am watering my rink at night. When it is overcast at night I wait until it is below freezing. I'm roughly at sea level.

 

Obviously the water is evaporation cooling and radiating more than it is receiving radiation and conduction in order to do this.

 

I know it is not as cold as 2.7 K because the clear atmosphere is "clear" in the visible range but not all of the rest.

 

So what is the equivalent black body temperature of the open sky?

[Klaynos]

I don't actually know, but I do know that it's ALOT hotter than 2.7k because there is alot of reflected heat from the earth, that's why on cloudy nights it feels warmer, the clouds reflect more of the earths heat back to the surface...

[J.C.MacSwell]

I don't actually know, but I do know that it's ALOT hotter than 2.7k because there is alot of reflected heat from the earth, that's why on cloudy nights it feels warmer, the clouds reflect more of the earths heat back to the surface...

 

Exactly. Reflect, absorb and re-radiate. Even when it appears clear this effect is still there but to a much less extent.

 

I'm basically looking for an equivalent temperature for a clear night at sea level. I know it would depend on the temperature and humidity which would vary all the way up through the atmosphere.

 

My experience (guess) is that a clear night vs overcast is worth at least 4 degrees Celsius in terms of freezing water. Not saying that is right, but that would indicate that the clear sky must be equivalent of well below freezing temperature in terms of a radiation heat sink.

[Klaynos]

I'd guess it depends ALOT on the daytime average temp too... the best option I can give you is get a black body thermometer and pop outside Can anyone tell I'm an experimentalist?

[J.C.MacSwell]

I'd guess it depends ALOT on the daytime average temp too... the best option I can give you is get a black body thermometer and pop outside Can anyone tell I'm an experimentalist?

 

Is their such a thing?

[Klaynos]

http://en.wikipedia.org/wiki/Infrared_thermometer

 

I'm pretty sure I've seen one of them or something like it in my physics department measureing stuff at quite a distance... some third years found it in a lab... damn pesky kids!

[Mr Skeptic]

You could try putting a thermometer out in the open on top of a slab of styrofoam. Compare to a thermometer below the styrofoam. Not the best of measures, but it may give you an idea.

[swansont]

You could try putting a thermometer out in the open on top of a slab of styrofoam. Compare to a thermometer below the styrofoam. Not the best of measures, but it may give you an idea.

 

That won't give the answer. The issue is what is the temperature of the reservoir involved in the radiation exchange, and that depends on the opacity of the air at a wavelength around 10 microns.

 

If you are "sampling" the atmosphere, there's a good chunk of it that is down in the 220 K - 230 K range http://spacescience.nrl.navy.mil/introupatmsci.html

 

And, of course, to any extent the sky is transparent at those wavelengths, it will look even colder.

[DrP]

Sorry if I'm being dim here, but, Isn't this like asking how long is a bit of string? The answer will depend on altitude, global position, weather conditions, summer or winter etc....??

[swansont]

Sorry if I'm being dim here, but, Isn't this like asking how long is a bit of string? The answer will depend on altitude, global position, weather conditions, summer or winter etc....??

 

Temp near the ground in the vicinity of 0 ºC, clear sky, are specified in the OP.

[DrP]

Temp near the ground in the vicinity of 0 ºC, clear sky, are specified in the OP.

 

OK - Clear sky then. Ground level summer in Botswanna compared to Winter in Northern Norway?

[J.C.MacSwell]

OK - Clear sky then. Ground level summer in Botswanna compared to Winter in Northern Norway?

 

Nova Scotia, Canada, sea level, temperature at ground level 0 degrees Celsius, typical temperature gradient/distribution above that, say 75% (average for a clear night?) relative humidity, typical relative humidity distribution above that. Midnight on a typical night in early March

 

Anything else anyone could think of being typical values.

 

Botswana or anywhere else in other conditions would make interesting comparisons.

[swansont]

OK - Clear sky then. Ground level summer in Botswanna compared to Winter in Northern Norway?

 

Are either of those going to be anywhere near 0 ºC ?

[DrP]

Are either of those going to be anywhere near 0 ºC ?

 

Norway in the north will be way below zero! Being honest - I do not know about Botswana - it's very hot in the day but I do believe it gets very cold in the African deserts at night, so I don't know about this one, I would guess that it's abouve freezing though. (?).

[John Cuthber]

The temperature of the sun's surface is about 6000 C.

We don't need to go there to measure it; we just need to study the spectrum of light it emits.

We could do the same experiment from pluto (though it would be a bit tricky) and we would still get the same answer.

The temperature of the sun is the same wherever you measure it from (I don't want to know abouth the super hot corona etc. here). The same is true for "space"; it has a temperature- we can measure it (it will change with the deffinition you use for temperature)

In much the same way we could measure the temperature of the sky by averaging the spectrum- it would give a pretty odd set of answers depending on what wavelengths you chose to look at but you could average over all those to get some sort of an answer.

An easier way would be to find something that never sees the sun (tricky) or seldom does- the dark bits of the moon might be close enough, and measure its temperature.

[swansont]

Norway in the north will be way below zero! Being honest - I do not know about Botswana - it's very hot in the day but I do believe it gets very cold in the African deserts at night, so I don't know about this one, I would guess that it's abouve freezing though. (?).

 

But a condition of the OP is being near 0 ºC.

[Mr Skeptic]

An easier way would be to find something that never sees the sun (tricky) or seldom does- the dark bits of the moon might be close enough, and measure its temperature.

 

I think he still wants the earth's atmosphere to be included. It seems to be more of a practical question. I think the temperature of space was like 2.7 K or something.

[John Cuthber]

Then the question's not got a lot to do with the night sky.

[thedarkshade]

I think he still wants the earth's atmosphere to be included. It seems to be more of a practical question. I think the temperature of space was like 2.7 K or something.

Last time I read something about this it was written -270C, and it was also written that this is expected to decrease due to expansion!

[Cap'n Refsmmat]

-270.3C = 2.7K, so that's about the same thing.

[J.C.MacSwell]

I think he still wants the earth's atmosphere to be included. It seems to be more of a practical question. I think the temperature of space was like 2.7 K or something.

 

That's the idea. No atmosphere would amount to 2.7 K more or less depending on the direction.

 

I want to include the radiation component of the atmosphere (not any conduction / convection contribution) which should be greater than the contribution of the CMBR.

[swansont]

That's the idea. No atmosphere would amount to 2.7 K more or less depending on the direction.

 

I want to include the radiation component of the atmosphere (not any conduction / convection contribution) which should be greater than the contribution of the CMBR.

 

I think what you'd have to do is account for the optical depth of the atmosphere for all of the wavelengths in the blackbody spectrum. IOW, is the atmosphere opaque at 10 microns, or partially transparent? If the latter, you'll couple to both the atmosphere and the CMB.