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> Does anyone out there know why the sky often turns green before
tornadoes?
Interesting question, with which I am not very familiar. But there were
some discussion about that topic about three years ago in this mailing
list. Perhaps I add some messages here.
-VeikkoM-
Date: Thu, 11 May 1995 12:35:16 -0400
From: pepin%cfa4.DECNET_at_cfa.harvard.edu
Subject: RE: green storm clouds
Darryn Schneider's oral folklore about the green clouds
squares with my own boyhood experience in the U.S. Midwest.
Not so much green clouds, as a green cast to the sky was
considered a sure portent of a "twister" (tornado)-- which
was the scourge of that area. These tornados often demolish
portions of small towns and farm, and cause loss of life
every year.
I have no meteorological explanation for the color,
except that it may be a scattering phenomenon of the
crystal or droplet size in the clouds concerned. Perhaps
our Finnish analysts have some clue to the mechanism.
Over and out,
Barlow Pepin
Associate Editor Sky & Telescope
Date sent: Mon, 15 May 1995 09:44:01 -0600
From: tyler_at_MARIAH.PLK.AF.MIL
also seen green skies during severe weather. The first time it happened,
it was coincident with a broadcast tornado warning, and my father
observed
that folks who'd lived there longer than us equated green skies with the
threat of a tornado. Although I wasn't much of a young scientist at the
time (I didn't keep a notebook of my observations), I do seem to recall
that "a green cast to the sky" is more accurate than "green clouds."
David Tyler
USAF Phillips Laboratory
tyler_at_plk.af.mil
Date sent: Tue, 16 May 1995 23:31:33 -0600
From: Alister Ling <watcher_at_FREENET.EDMONTON.AB.CA>
Subject: Green skies
The association of green clouds with severe weather is a "solid" one. I'd
be suprised if there wasn't a comment on this topic in Weatherwise
magazine, since they frequently deal with questions from readers.
In general, my view is that the severe weather clouds that are green (and
I too have pictures) are quite low, and simply reflect (backscatter) the
green vegetation at ground level. Anyone seen green clouds above yellow
fields? I'm not sure I can buy the color via scattering off right sized
droplets hypothesis, since droplets are vastly larger than the wavelength
of light, and we enter Mie scattering range. Typically large particles
scatter white light. In a severe thunderstorm, the droplets are going to
be sized all over a alrge range, tending to imply that the resulting
scatter would be white. Of course, if the terrain below is green, then the
cloud will take on that sheen.
Alister.
Date sent: Wed, 17 May 1995 13:32:19 +0300
From: Timo Nousiainen <tpnousia_at_CC.HELSINKI.FI>
Subject: Re: Green skies
The explanation that the green colour would have its origin in the
reflection
from the ground below seems reasonable. In a scattering process the
wavelength
of incident light is unchanged, so the only way the (single) scattering of
light
could cause colours is that the refractive index of a scatterer is quite
strongly the function of wavelenght, so that different colours would tend
to scatter in different angles. This, of course, cannot explain large
regions of sky being green. Also, water droplets do not scatter light
this way.
Another explanation that might work is that there are inpurities in
droplets, so that they not only scatter, but also absorb light. Actually,
the
impurities doesn't have to be IN the droplets, it may also be within them.
Immediately comes into my mind if wind blown grass or green leaves could
do the job... However, scattering of light by water droplets cannot be
the key, be the droplets small or large, irregular or spherical.
What comes to Mie theory, it is valid for homogeneous and isotropic
spheres of any size. It is also one only ways to calculate the
scattering from particles that are neither very small or very large
compared to the wavelength.
To summarize, it seems to me that the colour must be due to absorbing
inpurities in droplets or with them, if the origin is not green incident
light, reflected from surface. Of course, all the factors may be
present. Again, correct me if I'm wrong.
Date sent: Wed, 17 May 1995 16:32:47 +1000
From: Darryn Schneider <das112_at_rsphy1.anu.edu.au>
Subject: Green skies
Because it has been many years since I was able to
regularly see these large storms (and it was only a curiosity
to me then), I'll try to not let my imagination fill in details.
Yes, I agree the green is more of a cast to the sky not associated
with an individual cloud. But on the other hand, I also remember
it being well defined in the sky, framed by very dark cloud regions.
I don't think it is due to backscatter as by this time it is usually
quite dark because the sun is obscured by the heavy cloud. And I
wouldn't call most of this land very green anyway. I've
never seen this in the east (opposite the sun)
I think more reliable observations by a more experienced person
is need to give some clues. I'm going to give the weather bureau
in Brisbane a call and see what they say
Darryn Schneider
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