Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
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.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.