Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site utastro.UUCP
Path: utzoo!watmath!clyde!floyd!harpo!seismo!ut-sally!utastro!bill
From: bill@utastro.UUCP (William H. Jefferys)
Newsgroups: net.physics
Subject: Re: Clouds
Message-ID: <39@utastro.UUCP>
Date: Mon, 12-Mar-84 15:40:06 EST
Article-I.D.: utastro.39
Posted: Mon Mar 12 15:40:06 1984
Date-Received: Wed, 14-Mar-84 07:23:09 EST
References: <17385@sri-arpa.UUCP>
Organization: UTexas Astronomy Dept., Austin, Texas
Lines: 31

> Alternatively, why does the sky never appear yellow or green?  After
> all, the sunset is red because of loss in energy of the photons.  Why do
> they have to lose energy all the way to red - why aren't sunsets ever
> green?

Actually, there is a seldom-seen phenomenon called the "green ray" or
"green flash" that appears under the right conditions just as the last
edge of the Sun disappears.  It appears as a flash of emerald light,
sometimes shooting up from the horizon.  I myself have never seen it; 
according to M. Minnaert (*The Nature of Light and Color in the Open Air*,
Dover, 1954, pp. 58-63) the best place to see it is at sea, either from
the deck of a ship or from shore.  He says: 
	"There can no longer be any doubt as to the explanation of the 
green ray.  The sun is low, so that its white rays have a long way to 
travel through the atmosphere.  A great part of its yellow and orange 
light is absorbed by the water vapor, the absorption bands of which lie 
in this spectral region.  Its violet light is considerably weakened by 
scattering, and there remain, therefore, red and green-blue, as can be
seen by direct observation.
	"Now the atmosphere is denser below than above, so that the
rays of light on their way through the air are bent; and this bending
is somewhat slighter for red light, and somewhat stronger for the
more refrangible blue-green rays.  This causes us to see two sun discs
partially covering one another, the blue-green one a little higher, the
red one a little lower..."
-- 

	Bill Jefferys  8-%
	Astronomy Dept, University of Texas, Austin TX 78712   (USnail)
	{ihnp4,kpno,ctvax}!ut-sally!utastro!bill   (uucp)
	utastro!bill@ut-ngp			   (ARPANET)