Date: Mon, 14 Apr 1997 21:40:07 +0200 From: Stefan Arentz <stefan.arentz@luna.net> To: "Jordan K. Hubbard" <jkh@time.cdrom.com> Cc: isp@freebsd.org Subject: Re: Some advice needed. Message-ID: <19970414214007.22084@blah.rotterdam.luna.net> In-Reply-To: <3968.861022514@time.cdrom.com>; from Jordan K. Hubbard on Apr 04, 1997 at 05:55:14AM -0700 References: <Pine.BSF.3.95.970414030635.27997U-100000@mail.MCESTATE.COM> <3968.861022514@time.cdrom.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Apr 04, 1997 at 05:55:14AM -0700, Jordan K. Hubbard wrote:
> > Hmm, besides not being an Intel, what is the main differences
> > between the AMD and the Intel?
>
> Geeze, Vince, you're doing it again - asking everyone here questions
> you could easily find out for yourself. VISIT THE FRIGGIN' AMD WEB
> SITE ALREADY! :-)
>
> You're filling up my mailbox with this "daddy, why is the sky blue?"
> line of questioning! :)
Well, that's easy Jordan :)
The sky's blueness is due to a litany of factors. The most important
of these are a phenomenon known as scattering, our atmosphere's
composition, and the sun's spectral output.
Scattering, simply defined, is when light hitting a volume of air is
reflected out of the air, as opposed to being absorbed or just passing
through. The light bounces off of individual air particles in random
directions, scattering, and making that air appear the color of the
light being scattered. Preferential light scattering means that some
colors of light scatter better, while other colors tend only to pass
through.
Lord Rayleigh, a 19th century British physicist, first explained
preferential light scattering. (Sir Isaac Newton first established the
phenomenon's existence.) Common air particle diameters are 1 micron
(micrometer), .1 micron, and .01 micron. (Keep this in mind when
reading the following statement.) "Rayleigh's Law indicates that the
light energy scattered per unit volume of air containing particles
smaller than .1 micron is inversely proportional to the 4th power of
the wavelength of the illuminating radiation (Schaeffer 155)":
lambda 14E1scattered = lambda 24E2scattered
More simply stated, the amount of light that bounces around in our air
increases as the wavelength (which is related to color) of the light
decreases. Below are some common wavelengths of light and their
perceived colors.
COLOR WAVELENGTH (In um)
UV 0.30-
violet 0.40
blue 0.45
green 0.50
red 0.60+
IR 1.0++
wavelength chart
Applying Rayleigh's Law, consider the scattering of red and blue
light. Plugging into the equation blue light, with a common wavelength
of .45m, and red light, with .75m being common, we get a ratio between
the scattering of the two of (.44)-1:(.74)-1, or about 10:1. This
means that if a volume of air is lit by blue and red light
simultaneously, the amount of blue light scattered (reflected) to the
eye will overpower the red by a factor of ten, effectively drowning
out the latter. (See illustration next page.)
Lastly, the sun's spectral output has a significant effect on the
sky's color. ". . . Almost the whole of the sun's output is in waves
of length between .17 microns and 4 microns. . . . The maximum
intensity of sunlight is in radiation of wavelength about .5 microns.
The human eye responds only to waves between .4 and .7 microns, so
that the peak intensity occurs in the middle of the visible range, the
blue-green part of the solar spectrum (Sutton 7). . . ." Taking into
account that the sun's brightest light is blue-green, blue's
preferential scattering is magnified.
What would make the sky other colors, and when? Several key factors
might turn its color, including more or less atmosphere, changes in
the sun, more dust content, and more water vapor.
First, consider the normal changes you see in the sky's color
everyday. ". . . When the sun's rays come more slantingly through the
layers of the atmosphere [at sunrise or sunset], othersred, orange,
yellow, greenare scattered at the horizon (Stetson). . . ." More
atmosphere would tend to reduce this effect, while less atmosphere
would make those bands wider. Also, the albedo ("... The fraction of
the solar radiation that is reflected [back] . . ." (Neiburger 56))
would vary with the amount of atmosphere. If the atmosphere was
thicker, more light would be reflected back into space, making the sky
darker; if it was thinner, more light would be allowed in, resulting
in a brighter sky. This factor is not likely to change, however, as no
credible proposal for any significant change in atmospheric volume has
been proposed.
Eventually, the sun will degenerate into a white dwarf, at which point
its energy output will drop steadily until it becomes a black dwarf,
giving off no energy. Though neither will occur for billions of years
to come, these two events will put a serious damper on the blueness of
the sky.
Realistically, we are having the greater effect on color changes. The
sky is whiter with increased dust content, and other pollution makes
it both whiter and more like the color of the pollution.
. . . Small particles like molecules scatter a larger proportion of
the short-wave radiation, the blue and violet light, than the
longer-wave yellow and red light. That is the reason the sky is
blue in the absence of haze or smog. Haze, fog, and smog contain
larger particles, which scatter more nearly equally in all
wavelengths. When they are present, the sky tends to be white,
particularly near the horizon, except when smog with absorptive
properties gives it a yellow or brownish color. (Neiburger 56)
A large asteroid colliding with the earth could stir up enough dust to
occult the sky (as has been proposed to have caused the dinosaurs'
fate). Also, widespread nuclear war, accompanied with widespread
fallout, might alter the sky's color. The biggest contributor to this
factor right now, though, is pollution, which tints the skies above
industrial centers around the world.
A rise in the amount of water vapor in the atmosphere would both make
the sky whiter and more prone to rainbows. ". . . Humid air and bright
sunlight sometimes break up into all colors to form a rainbow"
(Stetson). If global warming turns out to be an actual issue, and the
conjecture concerning melting polar caps is true, then the water
content of our atmosphere would increase greatly, and the sky's color
would probably change as just described.
Would plants and humans survive if it (the sky) were a different
color? Yes and no.
The temperature prerequisites for global warming and increasing water
vapor (the White Sky problem) pose problems in themselves, numerous
problems about which books have been written. These temperatures would
have to being going up at least several degrees, which could have many
devastating effects. For now, suffice it to say that ". . . The water
vapor in the air is . . . all-important for the maintenance of surface
temperature (Sutton 17). . . ."
Plants require only respiration (and water) to live. Plants do,
however, need light to grow. If the sunlight were blocked (the Black
Sky problem), new plants could not grow, and the old ones that die
would not be replaced. Additionally, some plants incorporate infrared
or ultraviolet color schemes into their patterns to attract insects
and promote pollination. If the sky passed no IR or UV rays to them
(the No-IR/UV Sky problem), they could not reflect these colors and
perform those functions. Luckily, none of these cataclysmic
possibilities seems likely.
Lastly, and most importantly, is the occurrence of skin cancer in
humans (the Too-Much-UV Sky problem). Tied in loosely with global
warming, ". . . Ozone is opaque to all solar radiation of wavelength
less than .3 microns, and no ultraviolet light of wavelength shorter
than this is received at sea level (Sutton 9). . . ." If too much
ozone is depleted, or people spend too much time on the beach,
entering UV rays can cause skin cancer, the most prevalent cancer
among U.S. men and women ("Cancer"). "Only about half of the output of
the sun can be seen by us, but we feel the radiation in the solar beam
over a much wider range of wavelength" (Sutton 7).
Why did God make the sky blue? The factorsour planet's position, our
atmosphere's composition, preferential scattering, our sun's output,
our eye's spectrum, our oceans' regulation, etc.balance perfectly to
land blue as the color. The possibilities of it being any other color
and working correctly are remote. The manner in which the sky is blue
is very important to our survival. Is this all just coincidence, or
does all this point to His divine hand? Why is the sky blue? He simply
made it that way; everyone knows that blue is the ideal color.
Works Cited
"Cancer (disease)." The Software Toolworks Multimedia Encyclopedia.
1992 ed.
Neiburger, Morris, et al. Understanding Our Atmospheric Environment.
San Francisco: W.H. Freeman and Company
Schaeffer, Vincent J. and John A. Day. A Field Guide to the
Atmosphere. Boston: Houghton Mifflin, 1981.
Stetson, Harlan T. "Atmosphere." The New Book of Popular Science, vol.
2. 1988 ed.
Sutton, O.G. The Challenge of the Atmosphere. New York: Harper &
Brothers, 1961.
------
Copyright 1996, by Chris Campbell <ccampbell@tng.net>. All rights reserved.
http://www.spacecoast.net/users/dcampbell/chris/whyblue.htm
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?19970414214007.22084>