evans@mhuxt.UUCP (crandall) (02/16/84)
YAPPP (Yet Another Physics Prelims Problem): Everyone knows why the sky is blue. Explain why clouds are white and sunrise/sets are red. steve crandall mhuxt!evans
cwb@cbneb.UUCP (Bill Brown) (02/17/84)
I don't know why clouds are white, but I believe sunsets are red for the same reason that the sky is blue: the blue light gets scattered away.
mbs@ecsvax.UUCP (02/20/84)
Diffraction. 'nuff said. Michael Smith East Carolina University Dept. of Physics Greenville, NC mcnc!ecsvax!mbs
bill@utastro.UUCP (02/21/84)
Clouds are white because of *diffraction*? No, they
are white because they do not selectively absorb or
reflect any particular color, but rather reflect all
visible wavelengths with about equal efficiency. In the
parliance of stellar atmospheres, they are grey bodies. The
atmosphere as a whole, as has been pointed out, is an efficient
scatterer of blue light (hence blue skies), and leaves
in the red (hence red sunsets). When the clouds are pink,
that is because they are reflecting the red light that
falls on them after the blue has been removed.
--
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)dub@pur-phy.UUCP (Dwight U. Bartholomew) (02/21/84)
Rayleigh scattering, 'nuff said.
Shinbrot.WBST@PARC-MAXC.ARPA (03/08/84)
re: Everyone knows why the sky is blue. Explain why clouds are white and sunrise/sets are red. 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? - Troy -------------------- ------------------------------------------------------------
bill@utastro.UUCP (William H. Jefferys) (03/12/84)
> 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)
Craig.Everhart@CMU-CS-A.ARPA (04/13/84)
Just in case the backlog of mail doesn't catch this one. Bill Jefferys' answer about the Green Flash verges on correcting some earlier mis-statement about sunsets being red because the photons lose energy passing through the atmosphere. Not as I understand it! The atmosphere preferentially scatters the higher-energy (more blue) photons to the side (90 degrees from the original direction of travel), and preferentially allows lower-energy (redder) photons pass through undisturbed. Other media will do this, also; my college E&M professor showed this to us with a flashlight beam through milky water. This phenomenon alone will never produce other than reddened sunsets and blue skies--and as a bonus, explains why the sky is the richest blue when you look at the part 90 degrees away from the sun. The Green Flash needs the atmospheric absorption in order to work, not just scattering. (As Jefferys' submission explains.) I just couldn't bear to let people think that sunsets were red due to ``energy loss of the photons passing through the atmosphere.''
mullen%NRL-CSS@sri-unix.UUCP (04/13/84)
From: Preston Mullen <mullen@NRL-CSS> Thanks to Bill Jefferys for citing a reference on the green flash; I've long wanted an explanation of this phenomenon. I saw one under just such conditions a few days before Christmas, 1977. I was in a boat crossing from St. Kitts to Nevis (in the Caribbean). Another person in the boat saw it too. It lasted about one second. It didn't "shoot up from the horizon" in any extreme sense, but had just about the shape you would expect from covering all but a thin edge of the sun with the horizon. It was beautiful, a perfect Christmas present. P.S. Although I lived in the Caribbean for a couple of years and also spent some months in the South Atlantic near the equator, I never saw another one. Guess I was usually on the wrong side of the island at sundown. (end of message)
Scheuer.Wbst@Xerox.ARPA (04/13/84)
Excuse the delay in responding but the Physics mail off the ARPANET was delayed a month at Xerox. I have seen the green flash. However, the explanation given by Minnaert is WRONG. The green flash is only observable when there is little turbulence in the atmosphere and the western horizon is sharp. As the sun sets, there is a point at which the photosphere is completely obscured by the horizon (including the effect of light bending by the atmosphere) and the chromosphere (a 2000 km layer @ 4500 degrees K between the photosphere @ 6000 degrees K and the corona @ 1,000,000 degrees K) is still visible. At this instant the green flash can be observed. It is due to the Calcium K line which is prevelant in the chromosphere. The motion of the earth quickly moves the chromosphere below the horizon, limiting the duration of the flash. Don't blink. You will miss it. By the way, the procedure of blocking out the photosphere is used in the coronagraph to study the corona during the day at all major solar observatories. My view of the green flash occurred while I was working at Sacramento Peak Observatory in Sunspot, N.M., which is at 9500 feet just outside of Alamagordo and where the western horizon is formed by the mountains surrounding Las Cruces (70 miles away). The conditions there are near-perfect. Mark Scheuer Xerox Corporation Webster, New York 14580
bill@utastro.UUCP (William H. Jefferys) (04/21/84)
Mark Scheuer sent me this note before posting. Since I am responsible for
the original posting of Minnaert's explanation of the Green Flash, I
ought to comment.
Minnaert may well be wrong. I simply relayed his explanation. However,
I do have a problem with Mark Scheuer's explanation, since it relies
upon the "K" line of (singly) ionized Calcium. Since the wavelength
of that line is in the near UV, how can it produce a *green* flash?
Mark, if you get this and have a reference for this theory (which I
would be very interested in learning more about), could you post it?
Thanks!
--
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)