thread van december 05/12/2002 van de lightning list:
van: Elin Roberts
Hello there...
I am currently researching for a public show about electricity and lightning in particular. It has brought a few questions to mind, and I thought you out there may be well qualified to answer some of them.
My main question at the moment is about the colour of lightning. In most pictures I've seen it tends to be blue or purpleish in colour. Is this a trick of the camera, or is it really this colour? If so, what causes the colour of lightning?
I hope you can help me,
Elin
van: Kyle Stevens
Hey Elin,
I have learned that lightning color can be changed by rain. Depending on whether it is raining between you and the lightning gives off a different color because the rain scatters light. I don't know how acurate this statement is but I believe it to be right. Normally lightning is a white color and can be changed if the light passes through rain jus like a Rainbow I guess. Lightning color on photos can be a trick of the camera (like a filter or something) or it can be the type of film used that makes it come out bluish. I guess it's both. Lightning can appear different colors but can also be changed in photos by film and filters. hope this helps!..lata.
van: Harald Edens
Rain (and hail) consists of big particles which do not scatter one wavelength of visible light any more than the other, so the scattered light still appears white. The rainbow is caused by refraction and dispersion of light in raindroplets at a very specific angle (~138 and 129 degrees from the light source), and does not contribute to lightning color, as lightning is not a point light source. However, _transmission_ through water and ice gives a blue color. So there is probably some color shift of lightning towards the blue due to rainfall and hail, but due to transmission, not scattering. Also significant are air humidity, dust content and distance. Rayleigh scattering makes distant lightning (near the horizon) look yellow, orange, or red (by Rayleigh scattering, like the low sun); dust usually makes the lightning look brownish.
Lightning actually is purplish to blue colored but is usually so bright that it appears just white for the eye. However, daytime lightning happens on a bright background (as opposed to nocturnal lightning); your eyes are adapted to a high light brightness so the lightning does not bleach out your retinae too much and the actual color is revealed. That is why daytime or evening lightning appears to have more color than nighttime lightning.
As for photography, photographs are never really reliable when it comes to lightning colors. The blue and purple/magenta color you see on a (slide) picture (taken at night) has most likely more to do with film reciprocity error (the Schwarzschild effect), which is different for every color layer in the film. E.g. towards longer exposures for most slide film types, the yellow coupling layer with the blue pigment is more sensitive than the other layers, so the film color shifts toward the blue.
Elin, to answer your question more compactly don't trust lightning color on photos too much. The intrinsic color of lightning (spectrally) is violet to blue indeed, by the black-body radiation at around 20,000 K (although the lightning channel temperature varies widely). But the blue or magenta which many photos show is usually due to film reciprocity error (a coincidence). Air humidity, rainfall, hail, dust and the optical thickness
of the air between you and the lightning affect the color of lightning,
so it is difficult to tell the atmospheric conditions from lightning color alone.
Harald
van: Richard Orville
The colors of lightning are determined by the excitation emissions from singly ionized nitrogen and oxygen, and from neutral nitrogen and oxygen. All colors of the spectrum are represented in lightning.
One of the strongest line emissions occurs from hydrogen that is derived from the dissociation of water vapor. The strongest hydrogen emission if from
H-alpha at 6563 Angstroms (red region). Hydrogen
beta (green region) is also present, but it is Stark broadened and therefore faint. It occurs at 4861 Angstroms.
Distant lightning will appear red for the same reason that the setting sun appears red; the short wavelengths are scattered out of the the light beam that eventually reaches your eye.
Richard Orville
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