User:EWS23/clouds

Why are the cloudy nights warmer than the clear nights?

Because the clouds prevent heat from escaping, so it is warmer.--βjweþþ (talk) 13:26, 12 February 2006 (UTC)

Clouds transmit short wavelength radiation, but reflect long wavelengths. Radiation from the sun is short wavelength, and radiation escaping earth is long wavelength, because it has less energy. So it gets trapped (greenhouse effect). deeptrivia (talk) 13:39, 12 February 2006 (UTC)

Well, during the day the clouds mostly transmit the sun's shortwave radiation (that is, allow it to pass through to the Earth), but some of it is reflected back into space due to the clouds' albedo. During the night, the clouds absorb and then re-emit the longwave radiation of the Earth as a function of the clouds' temperature (see Stefan-Boltzmann law). So, some of that emission is into space, and some of the emission is back to the Earth. That added input of radiation from the clouds make the surface warmer on a cloudy night than it would be on a clear night. EWS23 | (Leave me a message!) 18:08, 12 February 2006 (UTC)

P.S.- The result of the Stefan-Boltzmann link above is that lower clouds warm up the surface more than high clouds (because the low clouds are warmer). So, if you prefer a warmer night, you'd rather have a deck of low stratus clouds than a high, wispy cirrus cloud. EWS23 | (Leave me a message!) 18:20, 12 February 2006 (UTC)

Also, remeber that days with clouds are going to be warmer than days with no clouds because warm air sucks up more moisture than cold air. It's kind of a chicken and the egg thing: most often the heat is causing the clouds, not the clouds causing the heat. -Quasipalm 18:28, 12 February 2006 (UTC)

Warm air does not "suck up" more moisture than cold air. In fact, air cannot suck up water at all. When the air is warm, liquid water evaporates more quickly; when it's cold, the water vapour that's already in the air condenses more easily. --Bowlhover 19:56, 12 February 2006 (UTC)

I think what Quasipalm meant is that warm air can hold more water vapor than cold air. Of course, with the same amount of water vapor, this means you will have more clouds (condensed, liquid water) in cold air than you would in warm air. EWS23 | (Leave me a message!) 20:42, 12 February 2006 (UTC)

That's exactly what I meant, in simple terms. See this link for more info. I like this summing up, "at temperatures near freezing, you can expect big honking snow flakes and lots of them. One those comparatively rare occasions when it snows near 0 F, you can expect individual snow crystals, but not very many of them because such cold air can't "hold" as much water vapor. Below about -40º, you can expect only very small crystals to fall, and very few of them at that. "

The idea of "holding" a substance only applies if that substance is being dissolved, and water vapour does not dissolve in air. With the same concentration of water vapour, more clouds are formed if the air is cold, not because cold air cannot hold as much vapour as warmer air (which is false), but because water vapour condenses more readily if the temperature is low. --Bowlhover 23:44, 12 February 2006 (UTC)

I could start a debate with you, but I have a feeling we'd just be debating semantics. Condensation occurs when the air is saturated and there are condensation nuclei present. Whether the air is saturated is determined by temperature (i.e.- the saturation vapor pressure/mixing ratio is a function of temperature). At lower temperatures, condensation doesn't occur because condensation is any easier/harder at that temperature, but rather because at that temperature the saturation vapor pressure is lower than it is at higher temperatures. A good answer I found about this is located here. Be sure to read the whole thing- while the first sentence supports your point, the first sentence of the fourth paragraph indicates that it's pretty much the same thing. EWS23 | (Leave me a message!) 03:30, 13 February 2006 (UTC)

Well, it's much harder to make a blanket statement about how clouds effect temperatures during the day. Without any other variables, simply putting clouds into an environment will cool that environment because the clouds reflect sunlight back into space before reaching the ground. However, where I live in Seattle, Washington, cloudy days are often warmer than clear days because we are in a warm, marine air mass from the Pacific Ocean rather than a cold, arctic air mass from Canada. The arguments you make have a lot of other variables that it would probably be counter-productive to address here, but have to do with more clouds due to heat (convection) or more clouds due to cold temperatures (a common cause of fog). EWS23 | (Leave me a message!) 18:58, 12 February 2006 (UTC)