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SkS Analogy 3 - The Greenhouse Effect is Like a Cloudy Night

Posted on 9 May 2017 by Evan, jg

Tag Line

The greenhouse effect is like a warm, cloudy night.

Elevator Statement

  • At night clouds trap infrared radiation emitted from the ground, similar to greenhouse gases, and re-emit some of the absorbed radiation back to the ground.
  • More nighttime cloud cover means more trapped heat, and warmer temperatures near the ground, just as more CO2 in the atmosphere means more trapped heat, and warmer temperatures.
  • Because clouds are big and thick, their radiation-trapping effect is felt immediately, within a single night.
  • Because CO2 is diffuse, its effect is felt slowly, over many decades.
  • Increasing the concentration of CO2 in the atmosphere is like increasing the cloud cover at night: both warm the Earth by trapping infrared radiation.

Climate Science

The greenhouse effect describes the trapping of energy by Earth’s atmosphere: infrared radiation from the ground is absorbed by gases in the atmosphere such as CO2, H2O, CH4, and others. Although the greenhouse effect is active 24/7, it is most apparent at night. This is because with no background solar radiation, nighttime warmth occurs mostly by greenhouse gases and clouds grabbing and storing some of the infrared radiation emitted from the ground that is trying to make it to outer space. This is partly why nighttime temperatures have been steadily increasing as greenhouse gases increase: more greenhouse gases implies more heating.

Everything radiates infrared radiation, but the amount emitted depends on its temperature. Because outer space is at a background temperature of about -270ºC (i.e., 3ºC above absolute 0), it emits essentially no radiation. The upper atmosphere is also much colder than the ground, so infrared energy absorbed high in the atmosphere is only weakly re-radiated back to the ground. On a clear night, therefore, the ground emits radiation to space and the upper atmosphere, but receives very little in return. The greenhouse gases in the atmosphere absorb more infrared radiation than they re-radiate back to the ground.

To see for yourself how this works, if you have a car parked outside with one side facing a house, and the other side facing an open field, and if the air temperature is about 2-3ºC (such as on a cool, Spring night), you will observe the following on a calm night with no clouds. If there is nothing covering the windshield of your car, it will become frosted, because as it radiates energy upward to outer space, it cools. Because it radiates more energy upward than it receives back from either outer space or the upper atmosphere, the windshield actually gets colder than the surrounding air. The same happens for the windows facing the big open field, because if the trees and buildings on the other side of the open field are short enough, then the side windows also effectively “look” at outer space on the other side of the field. However, the windows facing the house will not frost over, because although they transmit radiation to the house, the house is warm and radiates a lot of energy back to the window, keeping it warm.

Frosted window, CO2 cloud warming effect

Interaction of CO2, clouds, buildings, and infrared-radiation (graphics by jg).

On a cloudy night all of this changes, because the clouds radiate a lot of energy back to the ground, so that any windows that are looking up or to the side at the sky essentially just exchange energy with the relatively warm clouds. This prevents the windows from cooling to below the ambient temperature, keeping them unfrosted.

This object lesson with the windows of a car illustrates the kind of radiation transfer that is occurring between the ground, outer space, and the upper atmosphere, and allows you to see a clear example of how the greenhouse effect works: on a night when the air temperature is about 2-3ºC, with no clouds the windshield on your car becomes frosted, on a cloudy night it stays unfrosted.

Whereas clouds are like a transient, visible blanket, CO2 and other greenhouse gases are like a permanent, invisible cloak. Increasing greenhouse-gas concentrations increases the warmth of this invisible cloak, trapping more infrared radiation trying to make it out to space, keeping us warmer than we may prefer.

NOTE: if you have a very clean windshield, supercooled water may form on your windshield instead of frost. That is, clean water can cool to below 0ºC without freezing. But if you turn on your windshield wipers, the supercooled water will instantly freeze, demonstrating that the windshield is below 0ºC, even though there may be liquid water instead of frost.

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Comments 1 to 13:

  1. Re Elevator Statement: It is very convenient, but rather misleading, not to address the effect of clouds during daylight, only 50% of the time!


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    Moderator Response:

    [JH] Please keep it civil and do not shout at others by using all-caps.

    The use of all-caps is prohibited by the SkS Comments Policy. Please read the policy and adhere to it.

  2. dudo39, the article is an analogy about how the greenhouse effect acts similarly to clouds at night; it is not a treatise on the overall effect of clouds. SkS has addressed that topic as well:

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  3. The point of the analogy is to illustrate the greenhouse effect, which is a process whereby infrared radiation is trapped by greenhouse gases. The greenhouse effect can therefore be seen more clearly at night in the absence of the sun, when only infrared radiation is acting. There is no intention of minimizing the importance of clouds during the day. It was just not the point of the analogy.

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  4. I used to backpack in the Sierra Nevada mountains.  In the summer on cloudless nights, above 9,000 ft, it was definitely more comfortable to sleep next to a tree rather than out in the open, even if it somewhat robbed your view of the amazing stars.  The tree served as an infrared blocker, and definitely kept me warm and cozy compared to lying out exposed under the stars.

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  5. ubrew12, interesting story and comment. Whereas UV blocker actually does block UV rays from reaching us, I think you will agree that what you mean by "block" was the tree returning as much infrared radiation to you as you were sending to it. I know this is a subtle point, but it will help people understand what controls climate change to understand that what is often viewed as "static" is really a game of give-and-take that is in balance. If you get as much as you give, the system appears to be static. Sit under a tree or out under the stars and you are giving up the same amount of infrared radiation, but under the stars you are getting much less in return than the trees have to offer.

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  6. steveingbg, a simple mention would be sufficient for me. The link does not give a definitive statement, say as to what is the net effect, of an increase in cloud coverage, on the thermal balance of the biosphere. I would say that clouds act like a greenhouse gas by blocking infrared radiation from the earth day and night, and reflect [and block?] incoming solar radiation [during the day, of course]: the key point is to determine what is the net effect of clouds on the thermal balance....

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  7. Dudo39,

    Did you read the intermediate tab at the link you were given?  It states that it is difficult to exactly determine the effect of clouds.  It is most likely that the effect of clouds will be a positive feedback (possibly a large positive feedback) and that it is very unlikely that there is a large negative feedback from clouds.  There is a small chance that clouds have a small negative feedback.   A more recent lecture is attached that I did not view but probably addresses your questions.

    Overall climate feedbacks are positive.  It is very unlikely that clouds will bail us out and cause warming to be small.

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  8. michael sweet, I stated "The link does not give a definitive statement....": which word don't you understand?

    Your statement is iffy and indefinite....

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  9. Dudo39,

    I am sorry, I thought that you understood how to read a scientific report.

    It is very rare for a scientist to make a definitive statement as you request.  There is always the possibility that new data will be uncovered that results in something unexpected, even though that possibility is very low.  Instead, scientists often speak in terms of probabilities.  In the Climate field, lay people have objected to numerical descriptions of data (for example saying there is a 95% chance something will happen) so the terms likely (>66%), very likely (>90%) and extremely likely (>95%) are used (IPCC definations).

     It is difficult to get an exact  value for cloud feedback so research continues on this topic.  In simple terms, for clouds the data indicate that clouds are not a large negative feedback.   It is most likely that clouds are a small positive feedback.  Clouds will not prevent overheating caused by AGW.  Clouds might make warming worse.  A scientist would not make absolute claims about clouds because the research is not yet done.  

    Many things in life are not definite.  If I go for a drive in my car I might not come back.  We have to make the best decision we can with the information we have.

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  10. michael sweet, it is not a matter of "understanding how to read", its is a metter of having sufficient knowledge and understanding on the subject matter before even attempting to solve a problem or get some answers.

    Statistics do not provide solutions: they may indicate how to express an educated guess or opinion [neither one is a fact].

    So, it appears to be quite evident that when it comes to what is the net effect of water vapor, the fat lady has not sung as yet.

    Yes indeed, we have to make the best decision we can with the information we have, which to me, it does not mean to make facts out of opinions to justify the decision.

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  11. dudo39 - Your claim that "Statistics do not provide solutions: they may indicate how to express an educated guess or opinion [neither one is a fact]" is quite incorrect. 

    Statistics in science state that the evidence points to a particular result with some uncertainties due to measurements, predictability, available observations, etc. That's not expressing an opinion, your opinion won't change the evidence one bit. 

    To quote Philip K. Dick, "Reality is that which, when you stop believing in it, doesn't go away." Evidence is not opinion, and not a guess. 

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  12. KR, thats precisely my point: statisctics do not provide a solution.

    As to P K Dick's quote, I may add that believing does not explain a thing in science

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  13. Isn't this example a little more than an just an analogy? A cloudy sky implies higher relative humidity. Water is a greenhouse gas, which will absorb infrared radiation and return some of it to the ground. I think most people would expect a more gradual temperature drop at sundown in humid climates than in dry climates, even without cloud cover. The only difference seems to be that clouds are visible but greenhouse gasses are not. Isn't the same effect noticeable during a total solar eclipse? In other words, isn't there a more gradual temperature drop during a solar eclipse in humid areas as compared to dry areas? How measurable is the effect of 45% more CO2 and 124% more CH4 in the atmosphere on the rate of temperature drop? If we compare the rates of temperature drop we measure now during a solar eclipse with the rates measured before the Industrial Revolution would we notice a difference? I guess I'm asking for a quantitative estimate of the difference. Given that an eclipse moves so quickly over the earth's surface, I would expect that the radiative effects would dominate convective effects. A total solar eclipse will cross the United States in August. Could this present a teaching moment on the greenhouse effect or is the effect too small? 

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