Subject: Re: Colors Date: Mon, 25 Jan 1999 09:03:30 +0200 From: Bob Fosbury Organization: ST-ECF To: "Deborah A. DeMania" Dear Deborah, What you describe in your second paragraph is right. If you look at the SUN through increasing thicknesses of atmosphere, it will gradually get redder as more and more of the blue light gets scattered away. The (rather subtle I admit) apple green comes when you look at the BLUE SKY through a layer of shadowed atmosphere. Because the sky is so blue (the white curve in my sketch), when you start peeling away the blue from that, you end up with a shallow peak in the spectrum which moves from blue through green to yellow and orange as the thickness increases (the pink lines in my sketch). When this peak lies in the green part of the spectrum, you can see a colour which could be described as apple green but which you might call a 'slightly non-orange yellow'. Because the peak is so shallow, the colour is not very vivid and so hard to describe. The greenish tints you describe during storms are due to this effect - here you are looking at distant clear blue sky through the shadow cast by a big black cloud. You get the same effect during total eclipses of the sun where you are sitting in the middle of a shadow. Because my photograph of Rayleigh's Fish does not involve these shadows, the sequence of colours here is more analagous to looking at the Sun during sunset than to looking at the sequence of colours in the sky. I'll dig through my collection of photographs to see if I can find a convincing green sky. You cold try: http://ecf.hq.eso.org/~rfosbury/home/lars/Twilight_index.jpg - ignoring the clouds which are illuminated directly by the reddened Sun and looking just at the clear sky near the horizon. But I agree this is not a very clear example. Rgds, Bob... -- R A E Fosbury (Bob) Space Telescope - European Coordinating Facility rfosbury@eso.org, http://ecf.hq.eso.org/~rfosbury/ Tel:+49 89 320 06 235 (o) +49 89 609 9650 (h) Fax:+49 89 320 06 480