On
25/12/2003, Kirsten posted:
The following was asked on a home ed list I am on. With the
permission of the questioner I now post the questions here on her
behalf:
"Snow is white for a reason. But I don't know it. Does anyone? The sea
is blue because it reflects the sky. Why doesn't snow reflect the sky.
And why are plants mostly green? Is it because chlorophyl reflects only
green
and absorbs the other colours in the light spectrum? White and black
aren't colours, are they? So does that mean that snow reflects all the
colours of the spectrum? Is that why clouds are white? Why are some
clouds white and others grey, and others dark grey? Hmmmm. "
Podargus
replied:
The sea is not blue from
reflection. The sea is blue because water is blue, and there is a
lot of it in the sea.
Zero Sum responded:
> The sea is not blue from
reflection. The sea is blue because water is
> blue, and there is a lot of it in the sea.
>
You don't expect to get away with that, do you?
Water does not appear blue when it comes from the tap. The only
blue water I have ever seen was dyed.
Peter
Adderly answered:
Although I'm no expert, I think
snow appears white because, like clouds and steam, we see it on a macro
scale.
I think also that if you could
look at a snowflake at a microscopic scale, you would see not only
reflections of ambient light but also myriads of tiny rainbows from
internal refractions from the crystalised water.
But what colour is snow on a dark
moonless night?
White clouds are white when they
are lit directly by the sun. Grey clouds are such when they are in the
shade of other clouds. It's much the same as those little road
reflectors. Without direct illumination, they appear just white (or
whatever colour), but when hit by direct illumination, they really
shine.
The sea is blue, yes, but the
reflection is much more direct, that is from the sky above.
Snow or clouds are reflecting
light from up, down, and all around.
What I don't understand is why the
sea is SO blue. Even on a crappy cloudy day, it can still appear much
more blue than the sky, so there's probably another better explanation
out there.
Black/White - GREAT question.
Although it's technically correct
to say black and white are not colours, I reckon it's very misleading.
White is composed of light of every colour (or frequency), whereas
black is the absence of light. Sort of all or nothing. It's confusing to say that black and
white aren't colours, because artists, photographers, printers, and
most of the rest of us do so in daily life. They're descriptive terms
which have common understanding in our world, and our language.
To say they're not colours assumes
some technical understanding of light frequencies, and to do so out of
context is simply being a smartarse. Next time someone says that, drag
out a small tube of black or white paint
and ask them "what colour is
that?".
You could also respond by asking
if grey is a colour or not. Mix in a tiny bit of red or blue into grey
and ask for a scientific colour definition. Of course there are quite
simple explanations for all these possibilities, (eg texture) but such
questions laugh at the difference between scientific and common
descriptions. But makes for some fascinating discussion for kids. (see
note below)
The green question is pretty much
on track.
Sorry if this is coming in at an
over simplistic level, but I take it your are looking for analogies to
pass on to kids', always brillant, questions. Wonderful line of
questions.
But so many questions about
colour, it prompts me to suggest a book I found many years ago.
It's called: "Colour - why the
world isn't grey", by Hazel Rossotti, published by Pelican (as a
paperback)
ISBN 0 14 02.2201.4
It's a lay book, written in clear,
technically correct, and reasonably simple terms, but certainly not a
kids book. If you're
home teaching and have at least secondary education you'll find it a
fascinating read. Quite illuminating in fact ;-)
(note below): Interesting to note
the difference between the RGB and CMYK colour systems.
The RGB (red, green, blue) in
display systems such as TV and computer screens where the three
coloured light beams ADD together to produce all other colours. Thus to
produce white, the three colours are combined - to produce black, they turn the beams off.
Of course remembering that the unlit screen is black (or close to it).
The CMYK (cyan, magenta, yellow,
and black) system is used for printing colours onto white paper.
Therefore you need an extra "colour" of black to produce darker shades.
But in this system the coloured inks are used SUBTRACTIVELY; that is, just like when
you mix artist's paints.
But a little searching on the net
will find some much more graphic examples of the above.
Kirsten replied:
> Black/White - GREAT question.
> < snip>
> Next time someone says that, drag out a small tube of black or
white paint
> and ask them "what colour is that?".
> You could also respond by asking if grey is a colour or not. Mix
in a tiny
> bit of red or blue into grey and ask for a scientific colour
definition. Of
> course there are quite simple explanations for all these
possibilities, (eg
> texture) but such questions laugh at the difference between
scientific and
> common descriptions. But makes for some fascinating discussion for
kids.
> (see note below)
Actually you can mix a mid grey with red, blue and yellow paints - if
you get the proportions entirely right - it's one of the challenges
given to 1st yr colour students at art school - I know, coz I woz one
many moons ago;-)
If you look carefully at black pigments you usually can see all colurs
reflected off the grains of pigment - not that my eyes can see that
finely anymore, but I could when I was much younger.
Then again the same can be said for white pigments :-) BUt that
probably has more to do with refraction within the pigment grains...
> Sorry if this is coming in at an
over simplistic level, but I take it your
> are looking for analogies to pass on to kids', always brillant,
questions.
> Wonderful line of questions.
Actually the person who asked them was an adult, but one who is always
questioning things - it's the way she learns stuff...
> But so many questions about
colour, it prompts me to suggest a book I found
> many years ago.
> It's called: "Colour - why the world isn't grey", by Hazel
Rossotti,
> published by Pelican (as a paperback)
> ISBN 0 14 02.2201.4
Cool, thanks for the reference. I'll pass on your answers. We may well
be seeing the lady in question on this list in the next month or so.
Toby
Fiander commented:
Snow is less white than it used to
be (watch the wrap):
http://www.smh.com.au/articles/2003/12/23/1071941730953.html
http://news.bbc.co.uk/1/hi/sci/tech/3333493.stm
The original report by James
Hansen and Larissa Nazarenko is called, Soot Climate Forcing Via Snow
And Ice Albedos. I
can't find it on line. The Proceedings of the National Academy of
Sciences which is supposed to be the source of the article has both an
online section and archives and it is in neither as at today (Boxing
Day).
However, there is at least one
background article (750kb) for the work, which describes (among other
things) a technique for estimating atmostpheric carbon:
http://www.pnas.org/cgi/reprint/100/11/6319.pdf
The snow phenomenon could explain
quite a lot and would mean some different constraints on fossil fuel
use in the future from those we
have heard about to date. But I think the important thing is that
changing the albedo of snow back to something like what is was before
requires technology which is probably within reach. In constrast,
major reductions in greenhouse gas emission do not seem to be within
reach.
Podargus wrote:
> > The sea is not blue from
reflection. The sea is blue because water is
> blue,
> > and there is a lot of it in the sea.
> >
> > Podargus
> Tantalising. More please?
> Deep!
>
> Peter
And
> You don't expect to get away with
that, do you?
>
> Water does not appear blue when it comes from the tap. The
only blue water
> I have ever seen was dyed.
>
> Zero Sum
Yes I do expect to get away with it.
Water is blue because it absorbs light at red and yellow
wavelengths, but reflects blue. The ocean is normally
bluest in deep water away from land. This is because it is generally
cleanest in such areas. Other colours of
the ocean are from 'pollutants of one kind or another. I.e. Green
from chlorophyll, both from phyto plankton and leakage of same from
them into the water; turquoise from coccolithophores.
To see blue water in the comfort of your own home-
Obtain 1 WHITE bucket. Fill with CLEAN water. If your local
water supply is not clean enough wait for rain and collect in a clean
manner.
Snow, frozen water, is also blue. However clean snow reflects all
wavelengths. Like other water it can be 'contaminated' by micro
organisms such as algae etc..
Peter
Macinnis posted:
At 14:11 25/12/03 +0800, Kirsten
wrote:
>The
following was asked on a home ed list I am on. With the permission
>of the questioner I now post the questions here on her behalf:
>"(1)Snow is white for a reason. But I don't know it. Does anyone?
> (2)The sea is blue because it reflects the sky. Why doesn't
snow reflect the sky.
> (3)And why are plants mostly green? Is it because chlorophyl
reflects only green and absorbs the other colours in the light spectrum?
>(4)White and black aren't colours, are they? So does that mean that
snow reflects all the colours of the spectrum? Is that why clouds are
white?
> (5)Why are some clouds white and others grey, and others dark
grey? Hmmmm. "
1. Snow is white because it
reflects almost all of the light shining on it, which comes from the
sun -- light from the sky is a minor component. We see that
lisght as white -- but we also see the light from inxandescent bulbs as
white, but look at a colour shot taken under a tungstenm bulb without
flash. Our eyes adjust, but film does not.
2. The sea is not blue
because it "reflects the sky". It appears blue because of a
complex of effects involving selective reflection and absorption.
3. Plants are indeed green
-- well, the photosynthetic parts are -- because chlorophyll absorbs
energy at certain wavelebgths and reflects the rest. The absorbed
wavelengths are used as energy to drive photosynthesis.
Plants of other colours may well
have other pigments in the leaves.
4. White and black ARE
colours, in exactly the same way that orange is a mix of yellow and
red. White is a mix of colours, and what appears white or grey or
black in one context can be seen as another if the surrounds are changed to SERIOUS black or white, or a
differenty shade of grey.
5. Clouds appear white when
they reflect or transmit most of the light and grey or black when they
absorb a significant proportion. You never see black clouds when you
are looking down on them from an aircraft flying in blue skies.
Do not trust your eyes: they will
lie to you :-)
Zero Sum wrote:
On Fri, 26 Dec 2003 09:53, Podargus wrote:
> Yes I do expect to get away with
it.
For some indeterminate value of "get away with", perhaps. OTOH
you have now explained yourself so clearly you did not "get away with
it".
> Water is blue because it absorbs
light at red and yellow wavelengths,
> but reflects blue. The ocean is normally bluest in deep water away
> from land. This is because it is generally cleanest in such areas.
> Other colours of the ocean are from 'pollutants of one kind or
another.
> I.e. Green from chlorophyll, both from phyto plankton and leakage
of
> same from them into the water; turquoise from coccolithophores.
Nolo Contendre. Any S.C.U.B.A. understands this but most
people do not understand it.
> To see blue water in the comfort
of your own home-
>
> Obtain 1 WHITE bucket. Fill with CLEAN water. If your local water
> supply is not clean enough wait for rain and collect in a clean
manner.
Methinks it would take a rather huge bucket. I have been unable
to observe this phenomena in a white swimming pool. That may be just me.
{Podargus replied:
Nope. Any white bucket, and clean water. Even Lismore
water is usually clean
enough;-)}
> Snow, frozen water, is also blue.
However clean snow reflects all
> wavelengths. Like other water it can be 'contaminated' by micro
> organisms such as algae etc..
I think prismatic effects override.
Paul Williams responded:
This is correct. If one has ever dived to any depth (in clear water),
one can slowly see that red and yellow colours slowly disappear as
these wavelengths are absorbed - eventually leaving only blue. (Well
eventually leaving nothing if one could go deep enough)
Blue light is reflected off the sea more than red and yellow - it is
less absorbed than the longer wavelengths.
Nevertheless, at sunrise and sunset one can clearly see that the ocean
does reflect these longer wavelengths. One can also see that
reflections of buildings, trees, ships etc. do not have a discernable
blue tinge. It seems to me that the preferential reflection of blue is
insignificant.
The (clean) deep sea appears bluest on bright cloudless days.
This is due to reflection of the blue sky.
It appears to me that this is the most important reason for the sea
appearing blue.
One can see on a full moon night that all wavelengths are reflected - I
have not detected a blue tint to the moonlight as it reflects off the
sea.
Tamara Kelly
wrote
Mmm...what if one was to shine different coloured lights on the bucket
of water in a darkened room? (cellophane paper might suffice)
Would this be a better test?
Toby Fiander responded:
> Nope. Any white bucket, and
clean water. Even
> Lismore water is usually
> clean enough;-)
Lismore's water is treated these days and is probablybetter than it
used to be. Prior to filtration, it was too tannin stained to show any
blue, I think. From time to time also had some clay in it, which
made it pinker colour.
Both the tannin colour and the clay are removed by treatment, so there
is a better chance. Casino's water is also well treated with a
plant that probably has spare capacity so, it ought to be good enough.
I think I only see in about 16 colours (on that's on a good day), and,
consequently, I can never see the subtle blue colour in a white bucket,
but I know others who say they do.
Chris
Forbes-Ewan posted:
> White
clouds are white when they are lit directly by the sun. Grey
> clouds are such when they are in the shade of other clouds.
According to an answer to a
similar question sent to Scientific American, clouds look grey because
they are very thick (from bottom to top). See below for the
complete explanation
Reference:
http://www.scientificamerican.com/askexpert_question.cfm?articleID=000350D0-D08A-1C71-9EB7809EC588F2D7&catID=3
Why do clouds turn gray before it
rains?
Jason Warren,
Roanoke, VA
Richard Brill, a professor at
Honolulu Community College, gives this answer:
It is the thickness, or height of
clouds, that makes them look gray. Clouds are made of tiny droplets of
water or ice. They are formed when water vapor condenses within pockets
of rising air. Under the right conditions, the air continues to be uplifted, causing the
cloud to build higher and higher.
The tiny water droplets and ice
crystals in clouds are just the right size to scatter all colors of
light, compared with the smaller molecules of air that scatter blue
light most effectively. When light contains all colors, we perceive it
as white.
When clouds are thin, they let a
large portion of the light through and appear white. But like any
objects that transmit light, the thicker they are, the less light makes
it through. As their thickness increases, the bottoms of clouds look
darker but still scatter all colors. We perceive this as gray. If you
look carefully, you will notice that the relatively flat bottoms of
clouds are always a little grayer than their sides. The taller the
clouds become, the grayer their bottoms look.
Answer originally posted January
24, 2000.
Answer posted on July 21, 2003
Peter Adderley replied:
But then again if you talk to someone like Keith Bigg, he'll tell you
that the whiter the cloud, the dirtier it is.
Keith stated this at my father's wake in 1992. He worked closely with
my dad on cloud-seeding experiments in the 50's and 60's at CSIRO.
Another interesting aspect is at:
http://www-das.uwyo.edu/~geerts/cwx/notes/chap09/seeding.html
"Experiments in various areas of mainland Australia all showed an
unexpected decline over 3-6 years in the effectiveness of cloud seeding
an area. The reason is not clear. Keith Bigg proposed that it arises
because silver on vegetation, from silver iodide used in previous
seedings, which multiplies the formation of bacteria which themselves
act as nuclei in rain formation, confusing comparisons with rainfalls
from unseeded, control areas (2). In other words, rainfall gradually
increases in the control areas because of persistent contamination from
the seeded areas, reducing the superiority of the seeded areas.
However, there is no explanation of how the bacteria could reach cloud
level. Also, there has been no apparent increase in rainfall in the
'control' areas."
Yet another interesting document can be found at:
http://www.deh.gov.au/water/publications/cloud-seeding/index.html
Interesting for me reading stuff on cloud-seeding. It's like a who's
who of my father's career. Good to see Prof Nori Fukuta (see
liquid CO2 method in the DEH doc) is still alive and well. We
used to take him fishing and snorkelling when he lived over here
several decades ago.
I remember his infectious enthusiasm, and his laugh.
Jim
Edwards wrote:
To 'observe this phenomenon in a
white swimming pool' take a deep breath and submerge yourself at the
shallow end then open your eyes and look towards the deep
end. The further away you look, the deeper blue the water
appears.
Obviously no reflection of the sky
is involved here.
The January picture on my New
Scientist 2004 calendar shows a fluted iceberg which is white
where the light is reflected off it but blue where the light is
transmitted through it, demonstrating that ice is also blue.
This all goes back to a discussion
some may remember where we debated why the sky is blue, the
consensus being that air molecules selectively scatter light at
blue wavelengths. I remember suggesting at the time that oxygen
is blue and this explains why the sky (on Earth) is blue and so
is water, whereas the sky on Mars is orange because its
atmosphere is mostly CO2. Has anyone seen any research on the
size of atoms in a transmitting fluid and the wavelength of the light
transmitted?
Kevin Phyland wrote:
This question (which I have asked myself on occasion having seen a
number of "green storms" over the years) has been debated quite
vigorously over recent years...
The consensus view (not necessarily correct mind you) goes as
follows:
[1] Storms containing large hail (which most green storms reputedly
have) require enormous thicknesses of cloud, as high as 20km+ on
occasions.
[2] These thicknesses of cloud (as somebody else remarked) containing
vast amounts of both liquid water and ice crystals preferentially
absorb blue and (for some reason I can't recall) other wavelengths,
leaving only greener parts of the visible spectrum.
[3] However, green storms are routinely reported without hail (large or
otherwise), lending support to the idea that it may just be the
enormous thickness of these nasty storms, OR that hail (clearly present
in these storms at SOME levels) are still responsible for some sort of
scattering that produces green.
[4] Last I heard, the jury was out but majority opinion in the
meteorological world is that it is purely the thickness of cloud and
quantity of water...
Don't know if this helps or not...
Gerald
Cairnes commented:
I recollect being taught that the
blue of the sky was due to blue light scattering by O2 and O3 and that
this had an effect on the colour of the ocean. It's a long time ago and
I haven't time right now to refresh my memory other than to point to
some links brought up by Google. These tend to confirm the above.
During overcast weather the sea looks grey to green
to me ....?
I saved a number of links then
found that I had lost the global address but Google the following and
this should produce some results including a Teachers site by NASA
which looks very interesting.
Webastroed.html
"Sky color"
"Ozone spectra"
"Understanding sky color and
phenomena"
Peter Adderley replied:
> [1] Storms containing large hail
(which most green storms
> reputedly have) require enormous thicknesses of cloud, as high
> as 20km+ on occasions.
Has to be for the cycle to be created.
> [2] These thicknesses of cloud (as
somebody else remarked)
> containing vast amounts of both liquid water and ice crystals
> preferentially absorb blue and (for some reason I can't
> recall) other wavelengths, leaving only greener parts of the
> visible spectrum.
Much closer here, but as most water/ice/light effects tend to be blue,
but whyso green?
Maybe it's a reflection phenomenon - as in, when there's not much light
filtering down from above, and you have something pretty solid up
there, could it be a diffused reflection from from the ground, which is
generally green in colour, rather than absorption?
> [3] However, green storms are
routinely reported without hail
> (large or otherwise), lending support to the idea that it may
> just be the enormous thickness of these nasty storms, OR that
> hail (clearly present in these storms at SOME levels) are
> still responsible for some sort of scattering that produces green.
Sounds a bit vague. It may be hail up there, but could have melted by
the time it hits the ground.
It's a long way to fall through warmer air. I reckon it's amazing that
we get hail at all.
> [4] Last I heard, the jury was out
but majority opinion in the
> meteorological world is that it is purely the thickness of
> cloud and quantity of water...
Hmm, sheer quantity of water may be the culprit.
I don't buy into the cloud thickness idea, as green clouds usually have
no different illumination than others.
There's a cycle of condensation, falling (due to weight), updraughts
from associated thermals, more condensation and refreezing, falling,
and then more updraughts which continue the cycle many times as the
hailstones grow.
This produces enormous amounts of stored water aloft. As I
understand it, there is a catastrophic point when the updraughts cannot
support the weight, and down it comes. So maybe it's got more to
do with the the overall mass of water, and we simply make a wrong
connection that it's the hail that looks green.
> Don't know if this helps or not...
Yes it does. Many thanks Kevin.
Glad for your comments.
May ask anothery?
When it really pelts down during a thunderstorm, there's usually a lot
of lightning.
Is the lightning associated with greater conductivity through the
raindrops, or is it simply due to the huge potential difference between
the clouds and the ground?
Is this "voltage" potential difference increased by the falling rain,
and therfore causing the lightning? ie does the falling rain carry
potential charge?
So, is there more lightning during hailstorms than from just heavy
thunderstorm rain? ie Is there a difference between rain and hail
in the transferrance of potential charge? (Admit I'm making assumptions
in my last question.)
Would appreciate links.
Kevin
Phyland answered:
hehehehe...you sure get into the
nitty-gritty man! This question is another thorny one....:))
Last I heard, lightning requires a
mix of all three phases of water, but large hail forms in a way in
which ice pellets may come into contact with other ones in such a way
(hand-waves here) that charge gets separated... (if you wanna muck
about with this comment - join the club!)
Storms containing large hail (on
average) do have lotsa lightning, as do storms with torrential
downpours... however, not ALL hailstorms or flooding storms do...why
some do and some don't (as with why some supercells produce tornadoes
and others don't) is not yet known...
The whole field of atmospheric
electrification is widely debated...
Again, I'm sure that I've
generated more heat than light with this reply...I've been trying to
get definitive answers for both your questions Peter for a
while...<shrug>....we live and hopefully we learn...
and..,
Forgot to comment on this:
<Hmm, sheer
quantity of water may be the culprit.
I don't buy into the cloud thickness idea, as green clouds
usually have no different illumination than others.>
Clouds are actually composed of
liquid water albeit very small droplet size...so the idea that clouds
of that enormous height have only small amounts of liquid water is
either a misunderstanding on my part or yours....
Thunderstorms which extend to
stratospheric heights have enormous amounts of water present...my
theory would be that droplet-size has a great deal to do with both
scattering and absorption...
The illumination of clouds at
ground level in storms is almost totally filtration...
Paul Williams responded:
> Why do clouds containing hail have
a distinctive green colour?
>
> Peter
This is not always the case (green colour - hail) but when it is, this
is my rather shaky hypothesis:
The greenish storm centre in question is lit from within by electrical
activity producing a similar phenomenon to "Rayleigh scattering" (which
produces blue skies via scattering of sunlight)? So we may
perceive a bluish centre if not for added sunlight effects. Considering
(perhaps erroneously) that many if not most violent hail
storms occur in the later afternoon, we may perhaps add to our palette
the more predominate yellowish rays of the lowering sun - resulting in
a sort of tumbling mixture of blues and yellows which some may like to
call green...
But then again I may be just talking through my hat :-)
Jim
Edwards wrote:
Nicola Tesla was obsessed with
atmospheric electricity, he was sure that given the funding to continue
his research he could tap this phenomenon to generate unlimited
supplies of free energy. Of course, when he mentioned this to his
patron, Westinghouse, who was making squillions from Tesla's invention
of alternating current generation, the magnate abruptly decided that he
would no longer fund Tesla's research. So Tesla died in poverty
and all his research papers were commandeered by the US
government. No-one has since been able successfully to reproduce
Tesla's experiments. I wonder why?
