WFU Physics Colloquium
Title:
"Green Thunderstorms"
Speaker:
Professor Craig F. Bohren
Department of Meteorology, Pennsylvania State University
Time:
4 PM, Thursday, April 17, 1997
Place:
Room 101, Olin Physical Laboratory
All interested persons are cordially invited. Refreshments will be
served at 3:30 PM in the lounge.
Abstract:
Green thunderstorms are observed occasionally, yet they have received
little scientific attention. Fraser suggested that thunderstorms
themselves are not green but are the dark backdrop for green airlight
near sundown. Greenness is a consequence of reddened sunlight
illuminating selective scatterers along the observer's line of
sight. Bohren's alternative explanation is that green thunderstorms
may be a consequence of the intrinsic blueness of clouds because of
selective absorption by pure water, liquid or solid. Most clouds are
so thin that the light transmitted by them is not markedly colored
because of selective absorption. Only the most massive clouds--large
both vertically and horizontally-are thick enough to shift the color
of incident sunlight upon transmission. If that incident light is
sunlight reddened at sundown, the transmitted light can be
perceptually green. These two explanations do not exclude one another
but allow for multiple causes, including those not yet identified. A
recent field program undertaken in Oklahoma and Texas with colleagues
at the University of Oklahoma (Bill Beasley and Frank Gallagher)
captured the first spectra of green thunderstorms, providing objective
evidence of their existence. These spectra indicate that greenness is
not a sudden turning on of a green lamp in the sky but rather a subtle
shift in perceived cloud color from blue to green. These measurements
also indicate that contrary to reports of the "sky suddenly
turning bright green", the shift from blue to green is
accompanied by a reduction in luminance, which is to be expected from
both the Fraser and Bohren theories. This seemingly frivolous research
attracted the ire of Newt Gingrich--and look what happened to him.
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PUBLIC LECTURE
"All That's Best
of Dark and Light"
Professor Craig F. Bohren
Department of Meteorology
Pennsylvania State University
8 PM
Thursday, April 17, 1997
Room 101, Olin Physical Laboratory
Understanding the world we perceive visually requires blending
psychology with physics. If we were equipped only with a knowledge
of optics, our predictions about what humans perceive
would be inaccurate or even ludicrous. This talk is an overview
of brightness perception and physical mechanisms for producing
brightness differences in nature, such as those between clouds
and snow and between wet and dry objects. Everyday life is rich
with these and other examples, accessible to everyone yet which
often escape notice. This talk is intended to be an eye-opener
--both literally and figuratively
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About our speaker:
Craig F. Bohren is Distinguished
Professor of Meteorology at the Pennsylvania State University. His PhD
(in physics) is from the University of Arizona. He is the co-author
(with Donald R. Huffman of Buckeyball fame) of Absorption and
Scattering of Light by Small Particles as well as the author
of two popular scientific books, Clouds in a Glass of
Beer (for which he received the first Louis J. Battan Author's
Award of the American Meteorological Society) and What Light
Through Yonder Window Breaks?, and the editor of Selected
Papers on Atmospheric Scattering. In addition, he recently completed
the manuscript of a textbook on atmospheric thermodynamics
(co-authored with Bruce Albrecht) to be published by Oxford University
Press, and is under contract with VCH to write to a book on
atmospheric optics. He contributed the chapter on scattering by
particles to the Handbook of Optics and the chapter on atmospheric
optics to Vol. 12 of the Encyclopedia of Applied Physics. In 1988 he
was elected a Fellow of the Optical Society of America for
"outstanding contributions to radiative transfer and atmospheric
optics". During the academic year 1987-88 he was Visiting
Professor of Physics and Astronomy at Dartmouth College, in 1993 was
the Selby Fellow of the Australian Academy of Sciences and in the fall
semester of 1994 was the O. R. and Eva Mitchell Distinguished Visiting
Professor in the Physics Department at Trinity University in San
Antonio, Texas.
Bohren lives in splendid isolation on a large
tract of land on the south slope of Mt. Nittany in Centre County,
Pennsylvania where he trains two Golden Retrievers and an Australian
Shepherd to fill his home with tasteless trinkets transformed into
treasures by virtue of being won in canine obedience trials. He is a
member of the Board of the Mount Nittany Dog Training Club, instructs
in basic obedience classes, trains dogs for tracking, and is a
frequent visitor to local nursing and retirement homes with his
dogs.