Out of the Blue

By: Lee Grenci , 3:58 PM GMT on December 13, 2012

Yesterday I gave you the sense that people often take shortcuts when they try to explain why nighttime temperatures can plummet on clear, calm nights with relatively low dew points. By way of review, any explanation that incorporates "a lot of radiational cooling" sends the wrong message because radiational cooling is greatest around the time of the daytime maximum temperature (radiational cooling is proportional to the fourth power of absolute temperature). That's why I prefer "nocturnal cooling," or, more precisely, "net radiational cooling," which indicates that the ground runs an energy deficit at night (radiational cooling exceeds the amount of energy absorbed by the ground).

Another "shortcut" that bothers me is the popular explanation for a blue sky. I think most of us were taught that the sky is blue because air molecules preferentially scatter blue light (in all directions). Hogwash! Before I reveal a more scientific explanation that doesn't take any shortcuts, let me first set the stage by examining the spectrum of energy emitted by the sun. This image displays the spectrum of electromagnetic energy according to wavelength, which is the distance from crest to crest (or trough to trough). Focus your attention on the rather limited range of visible light, whose wavelengths vary between 0.4 and 0.7 micrometers. For the record, a micrometer (or micron, if you prefer), is one-millionth of a meter.

Unlike similar figures in most textbooks, there isn't any specific color assigned to a specific wavelength...for good reason!


The spectrum of skylight measured by pointing a spectrophotometer at a cloudless sky away from the direct rays of the sun. Courtesy of A World of Weather: Fundamentals of Meteorology

To see what I mean, check out the spectrum of skylight (light from the sky) as a function of wavelength (above). I measured this spectrum by pointing a spectrophotometer at the blue sky (away from the direct rays of the sun). The screaming message from this spectrum of skylight is that all the wavelengths of visible light comprise what we see as a blue sky. In short, there really isn't any pure blue light in nature. Granted, this visible spectrum dramatically peaks in the shorter wavelengths, but, as you can see, there is no single wavelength associated with light from a blue sky.

Alas, "in nature" is an important qualifier here because we can rightfully consider a blue laser, for example, as a source of pure blue light. Don't let that dilute my message, folks. The spectrum of light from a blue sky contains all the wavelengths of visible light. Do a Google search on blue light and you'll find that there are a lot of references to 0.475 micrometers (or some other equivalent units). Just looking at my measurements from the spectrophotometer should convince you that assigning a specific wavelength to a primary color is not cool. Heck,the spectrum of skylight doesn't even peak at 0.475 micrometers.

My spectrophotometer measurements of skylight pave the way for disputing statements such as "Oranges are orange because they only reflect orange light." I'm sorry to say that such statements you probably learned in school are blatantly false. Indeed, check out my measurements of the spectrum of light from an orange (my wife thinks I'm nuts). Yes, the spectrum of light from an orange includes all the wavelengths of visible light. To be fair, your teachers never had a trusty spectrophotometer to make the correct scientific point.

To me, the most compelling aspect of skylight is that, despite its spectrum not peaking in the "blue" (0.475 micrometers according to some Web sources), we perceive skylight as blue. What's up with that?

Here's the REAL story. Air molecules (mostly oxygen and nitrogen) scatter all the wavelengths of visible light emitted by the sun (revisit the spectrum of skylight above). This "assortment" of wavelengths of visible light from a cloudless sky enter our eyes and get processed (integrated) by our brains. As children, we learned to call this brain-integrated spectrum of visible light, "sky blue" (or simply "blue").

Make sure you understand what I'm saying here. Technically speaking, our eyes don't see the sky as "blue." Rather, they merely act as collectors of light (skylight, in this case). Only our brains truly "see" the blue sky. If you're queasy about this explanation, I point out that we sometimes dream in color despite the absence of light striking our retinas.

The bottom line here is that the popular explanation of air molecules preferentially scattering blue light is not correct. Air molecules scatter all the wavelengths of visible light (albeit unevenly, as you observed above). Moreover, such an explanation removes our brains from the entire process. As I stated earlier, we perceive skylight as blue. Not to tell students (or readers) this irrefutable fact is to deprive them of knowledge about how their brain processes visual stimuli.

Here endeth the lesson.

Lee Grenci


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

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17. bappit
7:48 PM GMT on December 15, 2012
Diffuse sky radiation in Wikipedia:

"The sunlit sky is blue because air scatters short-wavelength light more than longer wavelengths. Since blue light is at the short wavelength end of the visible spectrum, it is more strongly scattered in the atmosphere than long wavelength red light. The result is that the human eye perceives blue when looking toward parts of the sky other than the sun.[1] The color perceived is similar to that obtained by a monochromatic blue of wavelength 474–476 nm mixed with white light, i.e., an unsaturated blue light.[2]"
Member Since: December 31, 1969 Posts: Comments:
16. WunderAlertBot (Admin)
9:05 PM GMT on December 14, 2012
24hourprof has created a new entry.
15. tgmail
8:37 PM GMT on December 14, 2012
Quoting 24hourprof:


People who are self-confident don't mind being tested. Indeed, they are pleased when their errors are pointed out to them (lest they continue to propagate them). Most of us are wrong some of the time. Learning science (or anything) is an iterative process...we take in ideas and adjust our own knowledge accordingly.


As a former student of Mr. Grenci, I will tell you he will test you and challenge you, but you will learn, retain and excel. Great experience.

Lee, it's great seeing you here again, sir. I very much look forward to your posts and discussions. Hope all is well and you are enjoying your retirement. Have a wonderful holiday season...

T. Frost, Class of 2012
Member Since: December 31, 1969 Posts: Comments:
14. Lee Grenci , Retired Senior Lecturer and Forecaster
6:08 PM GMT on December 14, 2012
Quoting ChillinInTheKeys:


Well those would definitely have an impact on our weather. As far as the link that you provided I get this: "The requested article is not currently available on this site."


Try the link now. (pdf file)
Member Since: December 31, 1969 Posts: Comments:
13. Lee Grenci , Retired Senior Lecturer and Forecaster
2:29 PM GMT on December 14, 2012
Quoting 1911maker:
Might you be able to provide energy numbers for what low clouds radiate down and up, also what the ground is radiating out etc.

Seeing real numbers (even if they are just scaled/proportional) would help me grasp the way the system is working.

On the color of the sky, as you go up in elevation the sky "looks bluer" then at lower elevation. My understanding is that more blue spectrum light is available because it has not been absorbed/scattered by the atmosphere.

Is that correct?

Might you have a graph of the incoming light (visible spectrum) at the top of the atmosphere and a graph at say sea level for Los Angeles.

Might be I asking for more info then you want for a short blog.

Good stuff , thanks.


Thanks. Of course, each day is different, depending on the type of clouds, their altitude, thickness, etc. But, if you follow observations at one of the SURFRAD stations, you'll get a better quantitative feel for the numbers.

Hope this helps.
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12. ChillinInTheKeys
2:28 PM GMT on December 14, 2012
Quoting 24hourprof:


Many thanks.

The first that comes to mind is the impact by contrails (visible satellite image of the SE United States).

Here's a convincing paper on a study after September 11, 2001, that attempts to quantify the impact of condensation trails (pdf file):

http://journals.ametsoc.org/doi/pdf/10.1175/1520- 0442%282004%290171123%3ARVIUDT2.0.CO%3B2

I have my own strong beliefs on the role humans play in climate change, but, as a weather forecaster, I plan to stick to topics related to weather (and not climate change). Thanks for understanding.





Well those would definitely have an impact on our weather. As far as the link that you provided I get this: "The requested article is not currently available on this site."
Member Since: December 31, 1969 Posts: Comments:
11. Lee Grenci , Retired Senior Lecturer and Forecaster
2:22 PM GMT on December 14, 2012
Quoting ChillinInTheKeys:
Good afternoon Lee and welcome to WU. I would love to see you weigh in sometime on the topic of weather modification as it seems to pop up on a regular basis on Dr Jeffs blog thanks to one persistant blogger. I know that weather is definitely modified by man through many of our actions but I would love to here what you think about it being modified intentionally on a large scale. Thanks, Paul.


Many thanks.

The first that comes to mind is the impact by contrails (visible satellite image of the SE United States).

Here's a convincing paper on a study after September 11, 2001, that attempts to quantify the impact of condensation trails (pdf file):

I have my own strong beliefs on the role humans play in climate change, but, as a weather forecaster, I plan to stick to topics related to weather (and not climate change). Thanks for understanding.



Member Since: December 31, 1969 Posts: Comments:
10. theshepherd
1:29 PM GMT on December 14, 2012
"Make sure you understand what I'm saying here. Technically speaking, our eyes don't see the sky as "blue." Rather, they merely act as collectors of light (skylight, in this case). Only our brains truly "see" the blue sky. If you're queasy about this explanation, I point out that we sometimes dream in color despite the absence of light striking our retinas."

As in, if a tree falls in the forest when nobody is present, it doesn't make a sound?

Likewise, if you close your eyes, does the sky turn black?

Your not from the south are you?


Just joshing ya, Sir.
:)))

Great, informative, entertaining reading.
Welcome to the land of WU.

Straight talkers welcome.
:)












Member Since: December 31, 1969 Posts: Comments:
9. 1911maker
6:32 PM GMT on December 13, 2012
Might you be able to provide energy numbers for what low clouds radiate down and up, also what the ground is radiating out etc.

Seeing real numbers (even if they are just scaled/proportional) would help me grasp the way the system is working.

On the color of the sky, as you go up in elevation the sky "looks bluer" then at lower elevation. My understanding is that more blue spectrum light is available because it has not been absorbed/scattered by the atmosphere.

Is that correct?

Might you have a graph of the incoming light (visible spectrum) at the top of the atmosphere and a graph at say sea level for Los Angeles.

Might be I asking for more info then you want for a short blog.

Good stuff , thanks.
Member Since: December 31, 1969 Posts: Comments:
8. ChillinInTheKeys
6:07 PM GMT on December 13, 2012
Good afternoon Lee and welcome to WU. I would love to see you weigh in sometime on the topic of weather modification as it seems to pop up on a regular basis on Dr Jeffs blog thanks to one persistant blogger. I know that weather is definitely modified by man through many of our actions but I would love to here what you think about it being modified intentionally on a large scale. Thanks, Paul.
Member Since: December 31, 1969 Posts: Comments:
7. Lee Grenci , Retired Senior Lecturer and Forecaster
5:52 PM GMT on December 13, 2012
Quoting originalLT:
I love to read, "discussions" between two learned people. Both seem to make good points and rebuttals. A real, "university" atmosphere here!


People who are self-confident don't mind being tested. Indeed, they are pleased when their errors are pointed out to them (lest they continue to propagate them). Most of us are wrong some of the time. Learning science (or anything) is an iterative process...we take in ideas and adjust our own knowledge accordingly.
Member Since: December 31, 1969 Posts: Comments:
6. originalLT
5:20 PM GMT on December 13, 2012
I love to read, "discussions" between two learned people. Both seem to make good points and rebuttals. A real, "university" atmosphere here!
Member Since: December 31, 1969 Posts: Comments:
5. Lee Grenci , Retired Senior Lecturer and Forecaster
4:55 PM GMT on December 13, 2012
Quoting STLweatherjunkie:
I posted this just before your new post and decided to repost on this thread instead.

Welcome to the community Lee, 6 in/hr snowfall rates sound fantastic indeed!

I have a problem with your pet peeve though. Nocturnal cooling occurs regardless of clear skies and light winds, which is why I frequently reference ideal radiational cooling conditions in my forecast discussions. (I am a student agricultural forecaster for AgEBB at the University of Missouri) "If the truth be told, the greatest radiational cooling occurs around the time of the daytime maximum temperature." If you can be a stickler so can I, any cooling or warming refers to the direction and magnitude of the net radiation flux present at that time. You are referring to outbound long wave radiation as radiational cooling, but this is only half of the problem. If there is still incoming solar radiation at peak temperature then the net radiative flux is not minimized. Radiational cooling will actually be maximized at sunset, when the only radiative component is outbound and the surface temperature is maximized. I use the phrase ideal radiational cooling conditions to describe a particular set of atmospheric conditions that "nocturnal cooling" neglects. In some cases strong warm air advection will cause nocturnal temperatures to actually rise, which would create a hole in your "nocturnal cooling" term. I understand that every object with a temperature emits radiation and this is what makes you cringe about the term. However, when used properly I believe it does a better job at describing the atmospheric conditions present than the "nocturnal cooling" term, which does not imply anything about the atmospheric conditions (except the sun has set).

"Yes, there is radiational cooling, but the amount of radiation from the atmosphere is relatively small." I assume this is a typo since the atmosphere is merely the medium through which radiation passes rather than the frame of reference used. Additionally, this statement does not consider the presence of clouds, which can seriously alter the radiation balance.


I think I make all of the stipulations you seem to be talking about...radiational cooling and radiation from the atmosphere. So, no, that is not a typo. A clear atmosphere radiates at a blackbody temperature of roughly 250 Kelvins, so you are mistaken about that. Your point indicates to me that your understanding might not be complete.

Honestly, saying "ideal radiational cooling" just isn't correct. It is controlled by temperature alone. To state otherwise flies in the face of the Stefan-Boltzmann Law. I don't see how you get around that. I would argue that "ideal radiational cooling" takes place around the time of the maximum daytime temperature.

I appreciate your note, but there is no such thing as "ideal radiational cooling." You'll not find that anywhere in the literature. Your position is the one I'm was trying to rectify.

Hope this helps.

P.S. I also use "net radiational cooling," which I avoided trying to make it palatable. I use this term in the first graph of today's blog. If you read closely on my first blog, I use "nocturnal cooling" to describe cooling on a clear, calm night, so temperature advection cannot a part of the equation in this context. Thanks again.

P.P.S. I also made your point about clouds.
Member Since: December 31, 1969 Posts: Comments:
4. TheCloudyskies
4:36 PM GMT on December 13, 2012
Yes . That is the reason I signed up . I really want to learn .
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3. STLweatherjunkie
4:21 PM GMT on December 13, 2012
I posted this just before your new post and decided to repost on this thread instead.

Welcome to the community Lee, 6 in/hr snowfall rates sound fantastic indeed!

I have a problem with your pet peeve though. Nocturnal cooling occurs regardless of clear skies and light winds, which is why I frequently reference ideal radiational cooling conditions in my forecast discussions. (I am a student agricultural forecaster for AgEBB at the University of Missouri) "If the truth be told, the greatest radiational cooling occurs around the time of the daytime maximum temperature." If you can be a stickler so can I, any cooling or warming refers to the direction and magnitude of the net radiation flux present at that time. You are referring to outbound long wave radiation as radiational cooling, but this is only half of the problem. If there is still incoming solar radiation at peak temperature then the net radiative flux is not minimized. Radiational cooling will actually be maximized at sunset, when the only radiative component is outbound and the surface temperature is maximized. I use the phrase ideal radiational cooling conditions to describe a particular set of atmospheric conditions that "nocturnal cooling" neglects. In some cases strong warm air advection will cause nocturnal temperatures to actually rise, which would create a hole in your "nocturnal cooling" term. I understand that every object with a temperature emits radiation and this is what makes you cringe about the term. However, when used properly I believe it does a better job at describing the atmospheric conditions present than the "nocturnal cooling" term, which does not imply anything about the atmospheric conditions (except the sun has set).

"Yes, there is radiational cooling, but the amount of radiation from the atmosphere is relatively small." I assume this is a typo since the atmosphere is merely the medium through which radiation passes rather than the frame of reference used. Additionally, this statement does not consider the presence of clouds, which can seriously alter the radiation balance.
Member Since: December 31, 1969 Posts: Comments:
2. Lee Grenci , Retired Senior Lecturer and Forecaster
4:20 PM GMT on December 13, 2012
Quoting TheCloudyskies:
Thank You . Interesting read


Many thanks! Man, you had it read within minutes after I posted it. Good to know there are hungry learners out there.
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1. TheCloudyskies
4:04 PM GMT on December 13, 2012
Thank You . Interesting read
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About 24hourprof

Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.