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THIS
ARTICLE IS THE
COPYRIGHT OF
THE AUTHOR
A (very) rough guide to
physics involved in answering the question “why
there is snow on mountains”
Written by Wayne Naylor
A snowball will melt if it is
taken near to a fire and the nearer it gets
the faster it melts, but climb a mountain
(getting closer to the sun, which is very
hot), you get colder and snow doesn’t melt,
WHY?
Fires produce light, hot
gasses and radiation which warms the air
around it, when you take some snow into this
area of radiation, gas and heat it will
start to melt back into water and eventually
evaporate. If you move away or into a
shadow the melting process slows down as the
gases and heat diffuse (thin out) with
distance. In contrast, the heat and hot
gases produced by the sun pass through the
vacuum of space, so there is no direct
warming of the earth’s atmosphere (or your
lump of snow).
The sun releases (it does
“not“ create) energy due to gravity and
pressure forcing hydrogen nuclei to fuse
together to form a helium nucleus (well not
exactly, but near enough for this) . As the
new helium nucleus has less mass than the
combined hydrogen nuclei it gets rid of the
extra mass in the form of energy. Each
second the sun converts around 680 million
tons, (that’s a “lot” of atoms) of hydrogen
in helium and so… well lets just say it goes
“bang”. This electromagnetic energy
radiates outwards in various forms such as
heat, x-rays, light, gamma, radio waves,
etc.
One form energy can be
converted into another, so when this
“Sun-Light” comes into contact with the
earth it warms the ground, oceans and
everything else and this energy is converted
and re-radiated upwards as heat which (like
the fire), warms the air. There is more air
closer to the surface than higher up so, as
you climb a mountain, the warming effect is
lessened. When you’re far enough away from
the earth’s surface the warming effect will
be so reduced that the air is either the
same temperature or lower than the snow and
the snow will not melt.
Now all this is all well and
good but:
Why
is snow cold and fire hot?
What
is the difference between snow and ice?
Why
is there more air lower down?
Where
do snow and water go to (and where
did they come from in the 1st
place)?
Why
is it colder in a shadow?
Why is a fire hot and snow
cold?
Fire (in the above example)
is a exothermic chemical reaction (that is
it releases energy) which occurs when a fuel
(normally containing carbon atoms) is heated
to a high temperature and it comes into
contact with an oxidizing agent, normally
oxygen from the air (oxygen itself does not
burn as nothing can oxidize oxygen). It
does “not”, as is commonly thought, turn
matter into heat energy, it simply releases
the energy stored within the matter. Ok it
does this very quickly sometimes but it’s
still not creating or destroying anything.
–On this point, when
the universe was created there was a certain
amount of matter/energy created and it is
still here, and this “exact” same amount
will be around until the end of time. The 1st
law of Thermodynamics (conservation of
energy) states the energy/matter can not be
destroyed (although Einstein tell us they
are interchangeable) –
Everything is made up of atoms which are
continuously moving around, the application
of heat causes molecules to move around more
than normal and, at a high enough
temperature, atoms will break free,
releasing high amounts of light radiation
and infrared energy (infra means below or
before – so infrared is light below red in
the spectrum). We see the light radiating
out and the protons (in the infrared energy)
causes the air around the fire to heat up
(producing more infrared energy). Our
nerves can detect the increase in
temperature caused by the released energy
which we feel as
heat. This is why fire is hot.
Snow is water that is cold,
but in this case we’re talking about snow
feeling cold – which very different from a
lack of heat (don’t ask, it’s complicated).
So, snow is cold and your hand is hot. Heat
will (without a heat pump) always pass from
a hotter object to a colder object. This is
one of the very few general laws of the
universe that everyone agrees on (the 2nd
law of Thermodynamics). When you touch the
snow some of the kinetic energy (of the
atoms) in your hand is transfered to the
snow. This added energy relaxes the
molecular bonds of the ice crystals allowing
some of them (near the surface) to flow and
the snow starts to melt. Heat is a form of
energy and your hand has just lost some
(into the snow) so your hand cools. This is
why snow is cold.
The 1st law (basically) says that
you get out what you put in. As energy can
not be destroyed the total energy that a
system can produce is equal to the energy
put in (eg. petrol into a car, a cheese
pasty into you or the Big Bang itself).
But, as anyone who has ever run out of
petrol will know, whilst this may be
“technically” possible, it just doesn’t work
in the real world. You can’t start a
machine, then run it from its own exhaust –
that would be a perpetual motion machine and
these simply ain’t possible. This is due to
the 2nd law which states that
heat/energy will “always” move from a hot
object to a cold so some of the heat/energy
will flow out of the system (entropy) and it
will eventually run out of fuel. QED;
perpetual motion ain’t possible. The 2nd
Law of T explains everything from why your
chips go cold to answering “will the
universe end one day” - the answer to that
by the way is dependent on whether it is a (en)closed
system, ie. Is there anything outside the
universe and this is a religious question so
I’m going nowhere near that one. The 3rd
law is all to do with the way matter reacts
at (or just above) the temperature of
absolute zero and is a bit mad!!
QED
= “Quot Erat demonstrandum” = “which was to
be demonstrated” (or simply = “so there you
go”)..
What is the difference
between snow and ice?
Ice is made when liquid water
freezes and snow is made when airborne
moisture (gas) freezes.
Dihydrogenoxide (that’s water
to you and me) is unlike almost all other
matter as it’s volume (size) increases when
it freezes, even though the mass (the amount
of matter) stays the same. This is due to
the molecule consisting of two hydrogen
atoms (slight negative charge) and one
oxygen atom (slight negative) which, when
bonding attract the oppositely charged ends
of other molecules (like magnets) forming
hexagonal (six sided) spherical crystals
which have lots of empty space inside (9%
less dense than water – which is why ice
floats) so the size of body of water/ice
increases.
Snow is not (as is often
claimed) frozen rain, that is sleet.
Airborne moisture will freeze (coalesce)
around airborne particles such as evaporated
sea salt, bacteria, pollen, etc. forming
hexagonal crystals which grow larger as more
moisture coalesces around it. As the
corners stick out from the sides of the
crystal (at a molecular level this is a vast
distance) any airborne moisture hitting the
crystal will arrive at the corners more
often than the sides, so they grow faster.
As all six corners are in the same packet of
air/moisture they grow the same, forming an
elegant and symmetrical six pointed star.
Fluffy snow is made when, through friction
with air molecules, the crystals thaw
slightly making them sticky so they adhere
to other crystals. Snow is white because
the accreting process does not line up the
molecules in any regular manner, so any
light (and some other types of energy)
hitting it is scattered in all directions –
like lots of small mirrors (which is why
snow doesn’t melt on a warm day) with all the
colors mixed up and this is seen as white as
no part of the spectrum is focused enough to
give it a color.
Ice, on the other hand, is
normally made from simply frozen water with
all the molecules in regular slots and so it
is
translucent (not transparent)
as light does not reflect
from the lined up molecules in the same way
as snow. Light passes through the ice and
then back again off whatever is behind it
(which may be just more ice in a glacier).
Of course the light’s photons do mess about
a little on the way through which gives the
blurred
translucent effect. If they
did go right through without inter-reacting
with a least some of the water then it
really would be transparent.
As the light passes through
the ice the lower (red) part of the spectrum
is absorbed so the eventual reflected light
is bluer - which (contrary to what “that
bloke in the bar” will tell you), is almost
exactly opposite from the reason the sky is
blue. The deeper the ice the more red is
absorbed and the bluer the ice.
Test it for yourself -
Put a glass of water in the fridge. As the
temperature decreases the water will shrink
until, at around 4c, it starts to enlarge as
the molecules start to bond into crystals.
Of course if you open the fridge door to
have a look you will increase the
temperature and so affect (by allowing warm
air into the fridge) the process you’re
meant to be studying – this is a brilliant
example of the Uncertainty Principle, which,
contrary to what Hollywood would have you
believe, has nowt what-so-ever to do with
Jurassic epoch dinosaurs.
Why is there more air lower
down?
It’s all to do with gravity.
Basically everything attracts everything
else (no, don’t go there!). The amount of
stuff (matter) in a thing is called the Mass
and the more Mass there is, the more that
thing will (according to Einstein) distort
the space around it, causing other stuff to
move towards it.
The pulling force is weakened
by distance, so the further apart two lots
of “stuff” are the weaker the pull (it’s not
really a “pull” but that space-time is
curved so mass “falls” towards other mass,
like a having a mattress with a heavy object
in the center then rolling a small ball over
the mattress). If you double the distance
between things the pull becomes 4 times
weaker (Newton’s Inverse Square Law).
Imagine everything has a cloud around it,
the more mass the thing has, the bigger and
denser the cloud. This cloud pulls other
things towards its centre. As you move away
from the centre the cloud thins and the
pulling power become less strong. So
everything is falling towards everything
else but of course the bigger the thing
(more Mass), the more it will pull. The
earth (at just under 6.9 Billion Trillion
metric tones), compared to a molecule of air
(weighing almost nowt or about 29AMU’s =
just over twice that of a carbon atom), is
pretty big, so it pulls the molecule towards
it more than the molecule pulls the earth so
the air is pulled downward towards the
centre of the Earth’s mass. This means that
the density of air (the pressure) becomes
lower the further you are from the centre of
the earths Mass (higher up).
Test it for yourself -
Drink a cup of tea in a café at the base of
a mountain and then another cup in a higher
café – the cup of tea higher up will be
colder.
This is due to the weight
of air pressing downwards forcing the water
molecules to form into a liquid. Some of
the molecules have a higher than average
kinetic energy (energy caused by their
motion) and are able to escape the molecular
bonds holding the liquid together and they
escape from the surface as gas (evaporate).
By heating the liquid the average energy is
increased so more of the molecules can
evaporate. When the pressure of the air
(pressing down on the surface) and that of
the evaporating gas is equal, the water will
boil as, at this point, the molecules have
enough energy to evaporate not just from the
surface but also from inside the body of the
liquid. This forms bubbles of gas which
rise to the surface and escape (boil).
Obviously if there is less air (like there
is at altitude) pressing down on the surface
of the water then less heat is required to
increase kinetic energy before the two
pressures (air pressing down and the gas
escaping from the surface) become equal, so
the water boils at a lower temperature
(approx. 1c for each 350m).
Where do snow and water go to
and where did they come from?
In any body of water (from a
single raindrop to an Ocean) the molecules
are constantly moving and some will escape
from the surface – evaporate. So, sooner or
later, even without the addition of extra
heat, if no extra molecules are added, the
whole body will evaporate. Of course if
extra heat is added, the average energy of
the molecular structure (of the body of
water) is increased and the process is
speeded up and the water disappears faster.
Where water comes from is a
“very” long story. Each time you drink a
glass of water you are swallowing one of the
oldest things in the universe - somewhere
between 15 and 20 Billion years old. The
hydrogen atoms you have just gulped down
were created within the very stages after
the Big Bang itself. 1st came
gravity and other forces next came heat and
then came the lighter elements – one of
which is hydrogen.
Only slightly less amazing is
the other “bit” of the water molecule – the
oxygen. Stars “burn” by fusing hydrogen
into a heavier element, helium, which in
turn is converted into another even heavier
element, carbon and so on, each time
converting one element into something
heavier (oxygen to silicon, etc) until it
reaches Iron. This conversion of one
element into another normally produces
energy and it is this that (basically)
pushes outwards against the gravity (weight)
of the star (geeky people love the phrase
Hydrostatic Equilibrium that is used for
this). Iron does not produce energy when
converted, it consumes it, so there is
nothing to push outwards and the whole lot
goes hurtling downwards towards the center
of the star. If a star is “big” (many times
larger than our Sun) the center of the star
may collapse forming a neutron star and, in
less than a second, the rest of in-falling
solar matter hits this – well lets just say
it goes bang! Not only producing a brief
flash of light, that sometimes outshines the
whole galaxy, it also throws of all the
other bits – including the oxygen – which
billions of years later, ends up in your
kettle.
So next time you make a
cup of tea, savor it – it took a extremely
long time to make. Oh and don’t forget that
“everything” is made of atoms, including you
- 63%Hydrogen, 25%Oxygen, 9.5% Carbon,
1.5%Nitogen, .5% other weird stuff – so
happy birthday. You, well some bits of you
anyway, have just turned 15billion years
old.
If heat comes from the ground
why is it colder in a shadow?
Our nerves are better at
detecting “changes” in temperature than just
simple heat (I’m not talking about too much
“heat” by the way). Whilst you are in a
shadow your nerves are detecting the ambient
temperature. But step out of the shadow and
some of the “sun-light” (energy) that warms
the earth also hits you (giving you a sun
tan), and even some of the air molecule
floating around causing them to move around
faster and so “bump” into each-other more
often producing heat energy, your nerves
detect this temperature difference and
you’ll feel warmer and get a tan.
Test it for yourself -
Jump into a hot shower and it will feel very
hot, but quite soon you’ll get used to it.
This is because you will be warmed up by the
hot water so your nerve ending are not able
to detect such a large temperature
difference.
Test it for yourself.
Sit in the sunshine and your body will react
by producing pigments such as Eumelanin
(brown) and Phaeomelanin (red), these
pigments are able to absorb more UV
and so reduce skin damage. The longer you
sit in the sun the more pigments are
produced and the browner you get – or the
redder you get, which is “very” funny.
Bibliography
-
Some bloke in the pub
-
QI
-
FHM – High Street Honeys
section 2007 & 2005
-
Wayne’s 1st
law = Understanding is better than
recital.
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