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The reason behind the non- formation of dewdrops on a cloudy night is the non-attainment of below dew point temperatures. It happens as the outgoing longwave radiation (OLR) is not allowed to escape due to cloud cover.

My question is: How the clouds do this, exactly? Do they absorb OLR or they just reflect it back to earth therby not allowing the earth temperature to go below dew point?

My second question is about the formation of dew drops on a clear night. Why do the greenhouse gases that are implicated for global warming allow OLR to pass through to space? Do the greenhouse gases somehow become transparent to OLR during nighttime on clear nights?

Spencer
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HARVEER RAWAT
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  • Dew does form on a cloudy night on surfaces that are cool enough. Advection of warm, moist air over cold surfaces as just one example. –  May 23 '20 at 17:08
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    I think greenhouse gases already prevent most OLR passing directly to space, but clouds will absorb and re-emit IR at lower altitudes and more effectively than clear atmosphere; more comes back down at us than with clear sky. – Ken Fabian May 24 '20 at 02:04
  • @Ken Fabian Which of the two processs namely absorption and reflection is dominant? – HARVEER RAWAT May 24 '20 at 03:39
  • it is not only clouds that has this effect,as soon as you put an object that reflect infra red radiation over the ground like a car, a tree and many othe objects dew stops forming under it,dew forms on the object but not on the ground below.on a clear night frost can form even if the temparature never drops below 0C if the ground is not covered,if the ground is covered the temparature need to be below 0C for frost to form. – trond hansen May 24 '20 at 05:40
  • @HARVEERRAWAT - My understanding is it is absorbed, then re-radiated within the atmosphere (equally up and down), not reflected. – Ken Fabian May 24 '20 at 22:09

2 Answers2

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Clouds absorb thermal radiation and re-emit it, so that some of it comes back towards the Earth (Behind the Forecast: How clouds affect temperatures). During the day this warming effect is balanced by the cooling effect of blocking out the sunlight. They don't strictly speaking stop the formation of dew, but they make it less likely.

In the words of NASA's Climate for Kids pages, "It’s sort of like clouds are wrapping Earth in a big, warm blanket."

The 2nd question is about the effectiveness of greenhouse gases, and why we still get dew at night with all the extra greenhouse gases (GHGs) in the atmosphere. There are two parts to this. Firstly, the GHGs are not as effective as clouds when it comes to blocking the transmission of thermal radiation to space: they absorb in spectral bands related to the vibrational modes of the molecules in the gases, and there are gaps in these bands ("windows") which allow radiation to escape to space.

Secondly, dew formation depends not only on the temperature at night, but also on the near-surface humidity. The increase is GHGs is, as we know, warming the Earth and this also leads to an increase in the amount of water vapour in the atmosphere, because warmer air can carry more water. To get dew formation we need a change in the temperature between night and day -- and GHGs don't contribute directly to this.

There is an indirect effect which we see in nights getting warmer faster than days. There are several recent studies looking at the vapour pressure deficit (VPD) and showing an increase due to GHGs (e.g. Ficklin and Novick) -- VPD is a measure of the dryness of the air and an increases means less dew (though I have not found a reference that give direct evidence of reductions in the amount of dew). This drying of the night-time may also contribute to reduced harvests (Sadok and Jagadish).

M Juckes
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Water vapor is an extremely potent greenhouse gas. But, like all greenhouse gases, water vapor is transparent to some portions of the thermal infrared, and even where absorptivity is relatively high, a thick column of water vapor is needed to absorb almost all of the incoming thermal radiation. Liquid water and ice are much better, and much less selective absorbers of thermal radiation. The following graph shows the absorption coefficient for water vapor (green), liquid water (red) and ice (blue), from 667 nm to 200 μm.

[Graph of water sorption coefficient for water vapor (green), liquid water (red) and ice (blue), from 667 nm to 200 μm.[1] Source: https://en.wikipedia.org/wiki/File:Water_infrared_absorption_coefficient_large.gif

Note that at around 10 μm (the peak of 290 kelvin thermal radiation), liquid water and ice are much stronger absorbers than is water vapor. The absorption coefficient for liquid water and for water ice is about 70000 reciprocal meters. This means that a typical cloud droplet (10 to 15 micrometers across) absorbs one half to two thirds of incoming infrared radiation that impinges upon the droplet. While clouds are mostly air and water vapor, the tiny portion of a cloud that is in the form of small droplets of liquid or small ice particles makes clouds a very potent thermal blanket.

David Hammen
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