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Name: Johny
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Question:
Ten years ago while I was living in Ajijic, Mexico I witnessed lightning flashes in three colors, red, green, and blue. Ajijic is at five thousand feet altitude and I was living in a house higher up in the surrounding mountains. I had just gone to bed, around ten or so as a thunder storm came in and I noticed the strange colors. I got back up and watched this truly wonderful colorful display which lasted thirty to forty minutes. I was at the same altitude as the rain clouds so it was like being in the middle of the storm. The colors were vividly bright and very distinct. The green I could understand as possibly reflection from the green foliage, the blue as possible reflection from the night sky, but the red I have no guess as to its origin. I have never heard or read anyone else describe colored lightning. Is this a known phenomenon or did I witness something truly unusual?


Replies:
Bob,

I am sorry that I could not respond sooner. I was involved in a field project.

The color of lightning can be affected by scattering and absorption in so many ways, that I had to hedge a bit in answering the question on the Argonne NEWTON web site. We also try to keep our answers as simple as possible for the readers, who most often are school children, but even if they are not, they usually do not have the technical background to understand the physical underpinnings of our answers in too much detail.

Having performed studies of lightning chemistry, I would have to discount ionization as a significant factor in the colorization of lightning, even in a city area, as one web page speaks of. There has to be a significant concentration of particular chemicals that create colors during ionization; those concentrations do not occur even in highly polluted air. The exception occurs in the upper parts of the atmosphere where certain chemical species dominate more than in the lower atmosphere and high solar wind energy can ionize them creating the aurora, or intense energy outflow from the top of thunderstorms can ionize the molecules, producing blue jets and red sprites.

Lightning light in the troposphere is white, and includes all solar visible wavelengths, no matter what the temperature of the lightning. The temperature of lightning actually varies little from the weakest to the most energetic of strokes and is equivalent to or higher than the temperature of the surface of the Sun.

The colorization of lightning (and of the sky) is primarily due to absorption/reradiation/reflection and scattering.

Absorption is material dependent; different elements and molecules are better absorbers of some wavelengths than others.

Water droplets and water vapor are good absorbers of the red wavelengths.

Oxygen and ozone are excellent absorbers in the ultraviolet wavelengths.

Terpenes and other biological products are generally aerosol size (and are often lumped in with aerosols) and do not scatter light (see the discussion of absorption/re-radiation/reflection below). Again, I was being simplistic in my Newton answer.

Rayleigh scattering applies only to objects (air molecules primarily), that are smaller than the wavelength of visible light; the larger the wavelength the less scattering the object can cause. Furthermore, the larger the object, the longer the wavelengths that it can effectively scatter. Therefore, short wavelengths like blue are scattered most in the atmosphere (since it is composed mainly of tiny nitrogen and oxygen molecules) and the red end of the visible spectrum is scattered least, under normal circumstances. If larger scatterers, in large amounts, are in the atmosphere, they may scatter green and yellow wavelengths (these scatterers may be too large to scatter blue; the blue would be scattered by the smaller scatterers normally found in the air). These larger scatterers could be very small aerosols or large molecules (such as from a volcano) and leave a yellow or reddish cast to the sky, as occurred after the eruption of Krakatua and Tambora; this would be quite rare.

However, aerosols (which often contain water and if so can absorb red wavelengths) are usually larger than visible wavelengths and therefore absorb and reflect all wavelengths of light equally (this is not technically scattering, although it is often called that; it technically involves absorption and re-radiation, or reflection).

I hope that this clarifies my statement about aerosols scattering both blue and yellow wavelengths; the wavelength(s) scattered or absorbed/re-radiated/reflected by air molecules and other components is dependent on the sizes of the components present.

Hail is larger than solar wavelengths (and is solid too), so it absorbs/re-radiates/ reflects all wavelengths of light equally, much like dust, soil, and smoke particles do, so it does not affect the colorization of lightning.

When the humidity is very high, water droplets are present, and if large enough, also tend to absorb/re-radiates/reflect all wavelengths, both preferentially absorb and not re-radiate red wavelengths; the result is a washed out, pale blue sky.

Air molecules scatter the short wavelengths (blue) the most and long wavelengths (red, yellow) least. That is why a clean, clear sky appears blue - the blue wavelengths have been scattered primarily and thus we see them. In an aerosol and or water vapor laden sky, visible light wavelengths have been scattered more or less equally, resulting in a pale blue or whitish sky.

At sunset, most of the blue wavelengths have been scattered out by the large amount of atmosphere that the light has to pass through. If the amount of water vapor in the air is very high, the reds have been absorbed and the Sun appears to be yellow. If the amount of water vapor in the air is low, the setting Sun appears to be red. The Sun, at whatever stage, appears red (if there is not too much water vapor) through smoke or heavy pollution, as the blue wavelengths have been scattered out.

Inside a large thunderstorm, much of the blue wavelengths are simply extinguished by the clouds (you do not see these well unless you can observe them scattered in a clear sky) and the huge amount of water vapor and liquid water absorb the red wavelengths, leaving a green tinge to the cloud.

This could result in lightning being tinged green or perhaps even blue if the yellows are absorbed or scattered also. Otherwise, the color of lightning is going to be affected by the amount of various scatterers and absorber/re-radiator/reflectors present. Most of what I said about the cause of the colors of lightning in my Newton answer were as accurate as possible, considering the observational information given, but, as you can see from the discussion above, I can only conjecture for the most part.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


Mr. Cook,

My name is Bob Swanson, assistant weather editor with USA TODAY. I sent an email yesterday about colored lightning that may or may not have reached you (the Argonne email interface gave me an error upon submission, so I am not sure if you got the text or not).

I am hoping you can help me better understand the mechanisms that occur to create colored lightning. Id like to illustrate this process in an upcoming weather focus graphic (runs daily in print on our weather page as well as online -

http://blogs.usatoday.com/weather/weather_focus_graphics/index.html

I read an Ask a Scientist answer of yours to regarding colored lightning, to which you attributed the colors to scattering and absorption of certain wavelengths --

http://www.newton.dep.anl.gov/askasci/wea00/wea00279.htm

In the answer you state that rain drops and water vapor absorb the red wavelengths. Since the drops and water vapor are vastly different in size, what is the mechanism by which both can be frequency-specific absorbers for red light? You also state that aerosols scatter the blue wavelengths I assume this is standard Rayleigh scattering and that the aerosols are small compared to the wavelength of the incident light. If the aerosols were larger, Mie scattering would predominate and would not show a dramatic wavelength dependence. Further on in your answer, you mention that aerosols were scattering yellows and blues, again I assume this is Rayleigh scattering at work. I do not quite get the next step where reds are absorbed and yellows scattered leaving blue is there another scattering mechanism that allows for this?

Sorry to ask so many questions, just want to make sure I have the science clear enough in my head to be able to put into a graphic and put into print.

Thanks for any and all help you can provide in helping me better understand colored lightning.

Kind regards,

Bob Swanson
Assistant Weather Editor
USATODAY.com


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