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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
e-mail adress: Jan.Mattsson_at_natgeo.lu.se
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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