Green Clouds
From: Darryn Schneider
To: Multiple recipients of list METEOPTIC
Subject: green storm clouds
Date: Wed, 10 May 1995 17:27:32 +1000
The question I have for the list is about a regular
summer event in south east Queensland (Australia). I grew
up on a farm in this area and during the 'storm season',
summer, usually a large storm builds up during the day over
mountains to the south (Macpherson range) and then they
move up along a range parallel to the coast (The
Great Dividing range) in the late afternoon.
The result is often flash floods, not much damage
because the buildings have to be made for it, and
a very spectacular lightning display. The storms
move from the south west of most of the population
and it can go almost completely dark, and once the
storms have passed bright sunshine again and
a drop of temperature of 15 deg. cent.
The local folklore is that if the storm clouds are
green then there will be hail. The clouds do
get very green sometimes, lightish grass green even.
Does anyone have any ideas about why this happens ?
There is almost always hail somewhere in these
storms. Hail can be disastrous to crops and I can
imagine a strange phenomena like a green sky being
linked with ruin. Are the locals right ? Is
the green clouds linked with hail ?
I'd love to hear peoples theories.
Thanks.
Darryn Schneider
p.s. These are serious rain storms. I can remember often being
dumped on by 3inches of rain in 30minutes
From: Owen Hertzman
Subject: Re: green storm clouds (fwd)
Date: Thu, 11 May 1995 14:09:55 -0300
Darryn,
your post came to me indirectly, so if you've already received
answers from others I haven't seen them.
About greenish clouds and hail, this phenomenon is also well known in the
U.S. Plains states, where thunderstorms are common. Indeed green
clouds are sometimes used in hail warnings. The green colour may come
>from preferential scattering of certain wavelengths of light by hail of
a certain size. In some thunderstorm situations the hail can be quite
monodisperse (in size) and because of the vagaries of MIE scattering and
geometric optics, it ispossible to get a lot of green light coming from
certain cloudss. I know of no actual reference which explains this in
detail. OH
________________________________________________________________
Owen Hertzman
Dept. of Oceanography
Dalhousie University
________________________________________________________________
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From: Makela Veikko
Subject: (Fwd) RE: green storm clouds
Date: Fri, 12 May 1995 12:06:13 +0200
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.
By the way, we have received some interesting photographs
at Sky & Telescope, prompted by the Atmospheric Symphony
article in the May issue. Some very fine crepuscular rays,
tales of "moonbows" also.
"...we missed the lunar eclipse due to a time zone confusion,
but saw something I hadn't heard of before: a moonbow. We
were on top of a mountain, with fog banks rolling in and out
around us, and at one point the fog built up on one side
only, and opposite from the moon. That was when we saw the
moonbow -- a softly glowing semicircle on the fog bank. We
couldn't distinguish any colors, and after a while the fog
receded, taking the moonbow with it.
Regards,
William Nelson (Los Alamos, New Mexico, USA)"
Over and out,
Barlow Pepin
Associate Editor Sky & Telescope
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From: David Tyler
Date: Mon, 15 May 1995 09:44:01 -0600
Content-Transfer-Encoding: 7bit
tatus: R
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@plk.af.mil
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From: Alister Ling
Subject: Green skies
Date: Tue, 16 May 1995 23:31:33 -0600
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.
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From: Darryn Schneider
Subject: Green skies
Date: Wed, 17 May 1995 16:32:47 +1000
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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From: Timo Nousiainen
Subject: Re: Green skies
Date: Wed, 17 May 1995 13:32:19 +0300
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.
- Timo Nousiainen
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From: Russ Sampson
Subject: Re: green skies
Date: Tue, 23 Jun 1998 10:38:24 -0600
I just returned from Iqaluit on Baffin Island. While landing, the aircraft
flew over the still frozen Frobisher Bay. The wind and Sun had removed most
of the snow from the surface of the ice and the colour appeared a vivid
green, much like the colour of glacial lakes. The next day I was hiking on
the tundra just outside the town. There was low cloud covering most of the
sky. When I reached the top of a small hill overlooking the bay I could
easily see that the bottom of clouds over the bay were green. The clouds
over the land showed more neutral colouration (grey or gray brown). I took
lots of photos of the effect including sectional panoramas showing the
neutral colouration over the land and the green over the ice. I just sent
my slides to the processors.
I believe the Inuit use this general effect to look for open water, land or
ice-flows while in their kayaks. I think the common name for the effect is
cloud maps or cloud blink.
I have also seen and photographed, green light reflected off agricultural
crops onto the bottom of low cloud although the effect was very subtle.
Russell (Russ) D. Sampson
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From: Russ Sampson
Subject: Re: green skies
Date: Wed, 24 Jun 1998 07:30:00 -0600
A couple of curious observations of green clouds associated with severe
weather appear to suggest that reflectance of ground colouration (i.e.
vegetation) may not be the complete explanation. During one tornadic event
I witnessed, the green colouration appeared to come from a very small
section located at the side of the cloud. In another instance the green
colour appeared in a small opening in the low cloud giving the appearance
that I was looking up into the interior of the cloud. In both cases the
green colour was surrounded by neutral gray. Also, in each case the green
colour was associated with a very particular section of the storm, a bright
area in the advancing arcus cloud. In the green clouds I witnessed on
Baffin Island (apparently caused by reflected light from the sea ice) there
was no selectivity in the location of the green colouration. In other
words, ALL the clouds appeared green.
Russell (Russ) D. Sampson
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From: "Jan O. Mattsson"
Date: Mon, 3 Aug 1998 11:19:08 +0200
Colours of clouds, rain and fog
It has been interesting to read the reports mailed to the members of the group
of meteorological (atmospheric) optics on cloud colours and related phenomena
(the reference list below). By this letter I wish to contribute with some own
observations.
Some years ago I noticed in a situation with thunder cells that the dark and
low-levelled base of one of the cumulonimbus clouds had a greenish tint. As the
cloud was situated over a wooden area (pine) I thought that its special colour
tone was a result of light diffusely reflected from the wood (scattered light).
In the same wooden area (southern Sweden) I have several times also observed
that heavy downpours have a weak greenish colour probably due to diffuse
reflection of light from the wood. Maybe, has such an effect contributed to
the greenish tone of the cumulonimbus cloud mentioned above. The cloud
developed thunder and rain but no tornadoes.
Another observation was made on March 22, 1981 at about 2.30 p. m. in
southern Tunisia over the steppe and semi-desert south of Medenine. Scattered
cumulus mediocris clouds were formed over the area. The bases of the clouds
in or near the zenith had all a reddish tint. Closer to the horizon the cloud
bases became more violet, probably an effect of more blue light being involved.
I was and am convinced that the reddish tint was due to light diffusely
reflected from the red soil of the steppe and the semi-desert.
A third observation of related nature was made on May 23, 1973. About
9 a.m. I was driving on the motorway from Malmö to Lund in southern Sweden.
It was a morning with medium dense to thin fog (probably a combination of
advection and radiation fog) covering the area. In this time of the year
the numerous fields with rape on the plains of southernmost Sweden were in
bloom. Over the rape fields the fog was coloured yellow with a weak tendency
towards green as a result of diffuse reflection of light from the flowers.
On "the sky" it was easy to find out the directions to nearby rape fields
without directly seeing them. I guess that yellow light has the property of
penetrating fog rather easily. Sometimes I believed that the fog became
thinner ahead along the road and than the sunshine was coming through.
However, in all these cases the fog was still there and instead the
above-mentioned effect was in action. In the afternoon two days later I
noticed that also haze was coloured in the same way, however, much weaker.
These observations have convinced me that the colour of the ground
surface and its vegetational cover during some circumstances strongly
influences the colours of the clouds and the boundary air layers.
Lund August 3, 1998
Jan O. Mattsson
Department of Physical Geography
Lund University
Soelvegatan 13, SE-223 62 LUND
Sweden
References (e-mails on green skies and related phenomena sent to the
meteorological (atmospheric) optics group)
Pepin, Barlow 11 May 1995
Tyler, David 15 May 1995
Ling, Alister 16 May 1995
Nousiainen, Timo 17 May 1995
Schnyder, Darryn 17 May 1995
Kolan, Amy 8 June 1998
Makela, Veikko 9 June 1998
Smith, Jonathan A. 9 June 1998
Sampson, Russell 23 June 1998
Sampson, Russell 24 June 1998
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From The MAD Scientist Network
Re: Why does the sky turn green preceding a tornado?
Area: Earth Sciences
Posted By: Nezette Rydell, forecaster,National Weather Service
Date: Mon Sep 1 12:30:17 1997
Area of science: Earth Sciences
ID: 871935318.Es
Message:
A greenish cast to the sky has occasionally been described in association
with severe thunderstorms producing large hail and/or tornados. This
coloration is usually attributed to scattering of light by ice particles,
the hail, in the cloud. Other suggestions for possible sources of the green
color include reflection of light from green foilage (trees, leaves,
vegetation) on the ground. Some skeptics claim the green color exists o
nly in the eye and mind of the beholder.
I have seen green thunderstorms on two occasions. Both times the thunderstorms
produced very large hail (greater than two inches in diameter). One one
occasion, small tornados were also present. Research conducted during
the summer of 1995 during the VORTEX project measured the spectra of green
and non-green clouds from the ground and from the air. The results indicate
the color comes from the thunderstorm itself.
The working hypothesis is that the path of light through the water or ice
(the cloud particles) must be long enough to allow sufficient absorption
of light at the red end of the light spectrum. This would require massive
clouds (ie severe storms) absorbing lots of red light (sunlight late in the
day is "redder" having traveled through a greater distance of atmosphere).
The presence of hail may aid in additional scattering in the green portion
of the spectrum, or may only be a by product of the conditions that produce
the green thunderstorm.
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From:http://wwws.wfu.edu/wfunews/releases/040497p.htm
Weather Expert to Explain Tricks of Nature's Light
By Wayne Thompson
(910) 759-5237
Posted April 4, 1997
Craig Bohren, distinguished professor of meteorology at Penn State
University and winner of the American Meteorological Society?s
Louis J. Battan Award for his book, "Clouds in a Glass of Beer,"
will present a program Thursday, April 17, at Wake Forest University.
etc etc etc
While other scientists have speculated that the thunderclouds themselves
are not green but are framed against a backdrop of green airlight,
Bohren believes the clouds themselves may appear green to the eye.
If the light is sunlight reddened at sundown, Bohren says that the
blueness of clouds from water could cause a subtle shift in the perceived
color of the clouds from blue to green. Since most clouds are so thin,
and have so few particles absorbing water that the light they transmit
is not markedly colored, Bohren believes only the most massive storm
clouds are large enough to absorb enough water to shift the color of
sunlight and produce the phenomenon of "green thunderstorms."
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