Last week I was walking along a beach, around midnight
looking out to sea and observed a distant thunderstorm - It was too far
away to hear the thunder. The lightning flashes were distinctly yellow
rather than the normal blue-white color. I asked others from the local
town and villages about it and they also confirmed that it was a strong
yellow color. Can you explain why this was, and why lightning is
normally blue-white and not some other color?
You basically explained why the color was yellow
by saying that the thunderstorm was far away.
Just as the moon appears yellow when near the
horizon, the shorter wavelengths of the light
from the lightning were scattered out by the
atmosphere, leaving the longer wavelengths
(yellow, red) to be seen. The sun, when
right on the horizon, sometimes appears red
because there is sufficient water vapor,
aerosols, and other pollutants to scatter
out even yellow light. The lightning, being
both far away and in proximity to a lot of
water vapor and water droplets (over the ocean
and associated with a storm) was seen as yellow.
As far as the color of lightning, even when
seen close, natural lightning usually appears
somewhat yellowish. I have only seen the
blue-white color that you describe when I
witnessed a man-made spark, and particularly
when it was in pure air or another gas,
having no contaminants. The contaminants
in natural air scatter the short wavelengths,
leaving a pale yellow tinge to the light.
Having performed lightning research both for
natural and triggered lightning and large
sparks outdoors, I have to say that I have never
seen pure white lightning outdoors.
I hope that these explanations help.
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Update: June 2012