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I have noticed this winter that nights without clouds (especially when the 12h before the night where not cloudy) were way colder than the cloudy ones.

It makes sense to me because the clouds can reflect the heat (infrared?) earth have accumulated during the day rather than loosing it into the atmosphere (like a kind of blanket). But I could be "swayed" by a confirmation bias here, so here is my question:

Is there a correlation between temperature and cloud, during the night?

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    $\begingroup$ @AdamDavis - Insignificant? No. It can be extremely significant. $\endgroup$ – David Hammen Jan 9 '15 at 16:17
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Is there a correlation between temperature and cloud, during the night?

Very much so. It's called radiative cooling. Three factors come into play: cloudiness, relative humidity, and windiness. Nighttime radiative cooling is greatest under clear skies, low relative humidity, and light or no winds. The temperature drop (in degrees per hour) can be a factor of more than four greater under conditions of clear skies, low humidity, and light winds compared to that under conditions of thick low clouds and high relative humidity. That increased cooling can make for a significant temperature drop on a long winter night.

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    $\begingroup$ Four degrees per hour is pretty significant! Here's a resource you might want to point to that backs up your answer: sjsu.edu/faculty/watkins/cloudiness.htm $\endgroup$ – Adam Davis Jan 9 '15 at 17:09
  • $\begingroup$ "four degrees" isn't unambiguous. Do you mean four kelvin? $\endgroup$ – EnergyNumbers Jan 9 '15 at 17:19
  • $\begingroup$ Whether radiative cooling is largest under low or now wind depends on the perspective. When measuring temperature in a valley it's certainly true. When measuring outgoing longwave radiation from space, I can't think why it would be. $\endgroup$ – gerrit Jan 9 '15 at 18:26
  • $\begingroup$ @gerrit Cooling usually refers to change in temperature. Wind speed does not affect the radiative flux itself. $\endgroup$ – milancurcic Jan 9 '15 at 18:55
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    $\begingroup$ @gerrit - A marked nighttime temperature inversion sets up under conditions of clear skies, low humidity, and no wind at all, and this grows in height throughout the night. Surface temperature can be 6C cooler than air 1000 meters above the surface by the end of the night. With light winds, there is some turbulent mixing of the cooler air below and warmer air above. The cooling isn't as strong. The inversion never sets up with even stronger winds. In this case, turbulent cooling dominates over radiative cooling. $\endgroup$ – David Hammen Jan 9 '15 at 19:16

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