Chris Peterson wrote:Remo wrote:Have you ever slept out at night on a clear night in the desert or someplace else with a dry atmosphere. It can feel brutally cold even though the air temperature is not that low, and you can get frost in the morning on your sleeping bag. Why? Because half your body is staring out at 2.73 K, with little IR radiating back down from the warmer air. The atmosphere is "optically thin".
Not that thin!
The temperature of a clear, dry sky is usually around -40°C (the lowest I've measured is -70°C). That is determined by the radiative properties of several constituents, particularly water.
The sky is cold, which is why we can easily get frost when the air temperature is above freezing. But thankfully, the sky isn't at 2.7 K!
I monitor the weather here, and my thermometer is on a rise, so it pretty accurately reflects the true air temperature. The biggest extreme I've seen from radiative cooling was a measurement of 14°F from my station, and a simultaneous measurement a couple of miles away, in a valley, of -34°F. I've had dog water bowls ice over when the air temperature was around 50°F.
Eh, Chris,
Point taken. You are absolutely correct that the sky temperature we "see" on a cold clear night, is not the temperature of the cosmic background radiation but some mix of the CMBR plus IR radiation coming from the atmosphere.
To clarify my point (I know I got a little technical; now I, guess I get even more technical), the atmosphere is "optically thin" because you can see through it in both the optical and the IR spectrums. May get some haziness, but the absorption is less than ( 1 - 1/e ) or ~63.2%. See
http://en.wikipedia.org/wiki/Optical_depth and
http://en.wikipedia.org/wiki/Optical_de ... rophysics). This is less than the mean distance a photon would travel before it gets absorbed.
As we both agree, on a clear night, we do see the 2.7K polluted by the IR absorption of by H2O ( and to a lesser extent CO2 and other greenhouse gases )from ground radiating IR and then reradiating the IR back to the earth. And it is noteworthy that the absorption/radiation spectrum of CO2 and H20 are not a blanket in the IR, but have selective (and kind'a mushy) frequencies. So if you pick your frequencies right, you can get some really cold background radiation readings.
If, as you said, you got a -70C sky reading (impressive!), an awful lot of what you were seeing is background temperature mixed in with some IR pollution from the atmosphere. Don't forget that blackbody radiation is proportional to the T^4 so ~ 75% of what you were seeing was the cold, cold universe.
Given that those Antarctic temperatures were done at 4000m (where air pressure is halved), at virtually zero humidity, and with a cold land temperature and no sunlight for months, the sky temperature must be really, really cold.
http://asterisk.apod.com/images/smilies ... iggrin.gif