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

Posted on 29 March 2022 by Evan, jg

This is a reposting of an SkS analogy first posted May 2017. The first 13 comments are from the original posting.

Tag Line

The greenhouse effect is demonstrated by frost forming on a car window on a clear night.

Frosted window, CO2 cloud warming effect

Interaction of CO2, clouds, buildings, and IR radiation. At about 2°C air temperature, on a clear night frost forms on car windows. On cloudy nights no frost forms. (graphics by jg).

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 2ºC (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.

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 2ºC, 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 18:

  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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  14. Its intriguing reading a thread of comments like this. Because reading them in their entirety its obvious dudo39 is just highjacking the thread to push an agenda. The article was obviously using clouds at night an an analogy only, and said quite clearly "Although the greenhouse effect is active 24/7, it is most apparent at night, " but dudo39  still rambles on @1 about the article not addressing clouds during the day and then the rest of the comments posted drift on from that, and the main issue gets forgotten. 

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  15. I wrote this in my climate change dream journal at 12:24 am Ogden Utah Time. Light pollution makes it hard to see the stars and I thought the less stars the easier is to make a connect the dots picture — the clouds are the peaceful alternative — and I thought what about a sky polluted no stars shown through and I couldn't make a connect the dot picture that would be a very sad time for me. And a very sad world.

    So, I think that when an entire town decides not to drive a car for a week in a row and the stars get brighter and we all go for a walk it can be a very good thing.

    And now I'm promoting Take Back the Night brought to us by the Women's Center at WSU because ever since they helped me when I needed it I made a vow to take back the night whenever I can until we no longer have to.

    But have I just been off-topic, political or ad hominem in my rhetoric?


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    Moderator Response:

    [BL] This does seem to be rather off topic.

  16. "Because CO2 is diffuse, its effect is felt slowly, over many decades."

    Yes like about 700 to a thousand of them, beginning with farming and grazing by domestic animals?

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    Moderator Response:

    [BL] Off-topic deleted.

    Several times recently, people have started talking about agriculture, etc., in threads where it is off-topic.

    If people want to discuss veganism and/or agriculture, there are two possible threads here at Skeptical Science that might be more suitable. You can also use the Search box to find suitable posts.

    If you are to comment on those threads, make sure to read the original post, any following comments, and make sure that your comments are on topic.

  17. This is a very good discussion of radiant energy transfer. I have only a couple picky technical distinction quibbles with it. I do appreciate your providing a commonly observed example of radiant energy. Very similar to your example is the function of smudge pots in fruit orchards on cold, clear nights. It is not the heat of the smudge pots that keeps the fruit from freezing.
    You say that since “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.” All energy that is absorbed high in the atmosphere will be re-radiated, half upwards and half downward. More importantly, as CO2 increases, a smaller amount of IR energy is radiated toward space. This is because more energy that would otherwise escape directly is absorbed and half of it gets re-radiated downward. Without greenhouse gases (GHG), the energy doesn’t get absorbed and all of it goes toward space. Thus the “trapping” occurs by not letting as much energy that exits toward space out the top of the energy system.
    Clouds don’t store (accumulate) energy unless the cloud is getting warmer. Cloud droplets either reflect radiant energy or absorb and re-radiate it back toward the Earth. Also, greenhouse gases don’t absorb more radiation than they receive, other than any slight imbalance which is manifested as a temperature change. This brings the temperature of GHGs in equilibrium with the surrounding atmosphere. GHGs re-radiate energy of specific wavelengths back to the ground. As greenhouse gas concentrations increase, more energy is returned to the ground, warming it. To bring the system back into balance with a warmer ground, more IR is emitted by the ground and all wavelengths. Some of it will be at wavelengths that are transparent to IR and will escape to space. All of this is observable using an atmospheric radiation model. The key is the energy balance. Be mindful of the understanding of stored or accumulated energy.
    Forgive me for dwelling on esoteric distinctions. I appreciate the opportunity to practice writing about them. I hope that trying to understand them could prompt others to think about them also.

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  18. Charlie_Brown@17, thanks for your heat transfer lesson.

    I certainly agree with you that the system must be in equilibrium, not storing nor generating energy.

    Regarding what happens at the top of the atmosphere, I'm not sure it is as simple as you state, nor do I want to get into a discussion that may take me over my head. Although I understand geometrically that radiation is emitted uniformly in all directions, in the direction towards space molecular density is lower than in the direction towards Earth. Therefore, radiation directed towards Earth tends to get intercepted more than the radiation that is directed towards space, which has an easier time escaping the atmosphere. This cools the upper atmosphere, because the more CO2 present the more infrared radiation is created and the more heat escapes to space. Therefore, one of the fingerprints of global warming is that the lower atmosphere warms, and the upper atmosphere cools. Read here for a NASA description of this effect.

    I am reminded of the sun. Most people know that from the surface of the sun the sunlight takes only a little over 8 minutes to reach Earth. But how many people know that radiation created at the center of the sun takes about 100,000 years to reach the surface of the sun, because of the density of the inner sun. So there is the added complexity of the density of atmosphere's that also affect radiation heat transfer.

    And this is all way beyond the intent of the analogy. I appreciate your comments and I will try to sharpen up the comments in the analogy to make sure they do not mislead.

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