Download A Quick Look at the Atmosphere and Climate

A Quick Look at the Atmosphere
and Climate
The thin, light blue line around the Earth is about all there is of the Atmosphere.
Because of the confining pressure of the gas above, the bottommost part of the atmosphere (the troposphere) is
the most densely packed. We have seen that air is compacted by immersion in water, but it is also compacted by
immersion in itself. As you get higher – up in the mountains or in an airplane – the confining pressure is lower
because there is less air above you. In very high mountains it can be difficult to breathe, and “mountain sickness”
can result from lack of adequate oxygen. Airliners that fly above a certain level have pressurized cabins to ensure
adequate breathing air, which does not exist outside the airplane. If the cabin pressure is lost, little masks
attached to pressurtized oxygen drop out of the overhead area so you can keep breathing. (What, didn’t you
listen to the flight attendant?)
1000+ km
Thermosphere – Very high temperature from absorbance of solar radiation.
(But very little heat because of the near non-existence
of air molecules to hold it!)
~80km
Mesosphere -- Generally very cold and very thin air.
~50 km
Stratosphere – Relatively warm air because absorbance of solar radiation creates abundant ozone, which absorbs much energy.
~12 km
Troposphere
-- ~90% of the air is in the troposphere. This is the only layer thick enough to breathe (at the bottom) and it is the layer in which all weather takes place.
Ground Surface
~30km
99% of air!
The Sun radiates electromagnetic energy across the entier spectrum, from high-energy/short
wavelength cosmic and x-rays to low energy/long wavelength radio waves.
Most of these wavelengths do not reach even the top of the troposphere because they are
absorbed and ionize atoms in the upper layers – particularly in the “ionosphere” (most of the
thermosphere and the upper part of the mesosphere). This is what creates ozone, for example.
Ozone is particularly good (but not perfect) at absorbing UV radiation, so only a little of that gets
to the troposphere.
Visible light (almost all wavelengths), about half of the Infrared (heat – IR) and a little UV are all
that reach the troposphere, but that layer is almost perfectly transparent to them, so they pass
right through and reach the surface. (Clouds, dust, and gasses absorb, on average, about 20% of
it, and reflect about 25% of it back to space, so on average about 55% of the total incoming energy
actually makes it to the ground).
That 55% that gets to us is about half visible light and about half IR, with a tiny (but potentially
dangerous) smidgeon of UV. About 5% of that is reflected from the Earth’s surface, leaving about
50% for us.
This energy is absorbed by the Earth and re-radiated. All of what is radiated is heat – IR.
Air is transparent to light, but not to IR. The various gasses in the atmosphere
are very good at absorbing IR, which is why only about half makes it through
the atmosphere coming in.
Some of this IR passes through the troposphere but most is retained, warming
the air when the sun is actually shining. Of course, when the sun goes down
the absorbance and re-radiation stop, but enough heat remains in the
atmosphere to keep us from freezing to death before the sun comes back!
Some gasses, CO2 in particular, are much better at absorbing the heat than
others – they are less transparent to it. These are called greenhouse gasses.
The dominant gasses in the atmosphere, N2 in particular, retain some heat but
not nearly as mush as the greenhouse gasses.
Glass is also transparent to light but not to heat. Light coming through the
closed windows of a car is absorbed by the interior, re-radiated as heat, which
can then not get back out. It builds up inside making the car hotter and hotter
until its exterior radiates heat at the same rate as light is absorbed.
Obviously the amount of heat the atmosphere absorbs will depend upon how
much light the Earth below it absorbs. So why is it hot at the equator and cold
at the poles?
The diagram below shows a part of the same sheet of black
construction paper taken with light from a source beyond it
taken at four different angles. When light reflects off a surface
the angle at which the reflected light rays reach our eye is the
same as the angle of incidence. Notice that the paper looks
less black as the angle of incidence decreases. Why is this?
15°
15°
30°
30°
60°
60°
90°
You have certainly noticed another manifestation of this phenomenon driving down the
road. Far off ahead of you there appears to be water on the road – a mirage. No matter
how fast you drive that “water” stays the same distance away from you.
Most people think this has something to do with the hot pavement, but it is simpler than
that. (The hotter the pavement the more shimmery the “water”, but that’s not what
creates the illusion in the first place.)
Next time you see this notice that as a car approaches you from the other side of the
“water” you see a blob of whatever color the car is crossing the “water”. The same thing
happens as you approach a sign – you get a blob of yellow if it’s a caution sign or red if it’s
a stop sign. This is a clue about what’s happening and what you are seeing.
What causes this illusion?
As the angle of incidence of light on a surface gets smaller the amount of light reflected gets greater. There is, in
fact, a direct correlation between the two – a mathematically definable relationship.
The “water” that you see on the highway ahead of you is really a reflection of the sky. The poor, blobby reflections
of approaching cars or distant signs is the best observational evidence supporting this fact. The edge of the
“water” is at that distance ahead of you where the angle of incidence is such that there is 100% reflectance of the
light off the surface. 100% of the light striking the road and coming to your eye at that angle of incidence is
reflected.
~100% reflectance
Similarly, the paper in the earlier slide looked lighter and lighter as the angle of incidence declined because you
see more and more of it reflected at the lower angles.
Minimum amount
of reflectance
(90°)
More
reflectance
(60°)
Even more
reflectance
(45°)
A lot of
reflectance
(30°)
~100%
reflectance
(~0°)
We can semi-quantify this relationship with a simple experiment. We could
do it better with a more controlled one.
With a light meter that reads in lux (picture at left) in a dark room I did a very
rough experiment, the result of which results are shown below.
Notice that the meter reads higher as the angles of incidence and reflection
decrease – more light is being reflected.
(Shining the light directly on the light meter from the same distance as the
other readings were taken gives a value of about 350 lux so not much of the
light is reflecting off the black surface even at low angles.)
31 lux
64 lux
15°
15°
30°
30°
9 lux
60°
60°
4 lux
90°
So what? What does how much light a surface reflects have to do with
climate?
Well, reflected light doesn’t really have anything to do with it, but ask yourself
this: what happens to the light that isn’t reflected?
A certain amount of light leaves the flashlight. Some of it reflects off the
surface and returns to the light meter. Clearly the amount is not the same for
different angles of incidence/reflection so something else must be happening to
the light – happening in inverse relationship to the reflectance.
What might that other thing be?
Remember this experiment?
What happened to the missing 426 lux
that didn’t reach the sensor through
the red filter?
Similarly, what happened to all the
red light that didn’t reach the
balloon and my hand 3m down in
the water?
Again, similarly, why does the balloon look red? Sunlight is white, not red. When we
look at the balloon all we see is red. What happens to the other colors in the sunlight?
In every case the light is absorbed – it excites the molecules of the substance
absorbing it and is then radiated by those substances as heat.
And remember – the more light something absorbs the more heat it radiates. A
white car interior doesn’t get as hot as a black one if it’s closed up and in the sun.
The white reflects most of the light (which passes back through the glass as easily
as it got in in the first place) and the black absorbs it and converts it to heat (which
does not escape through the glass.)
So let’s propose that the temperature piece of climate may have something to do
with absorbance. A place that absorbs more light is able to radiate more heat into
the atmosphere, where it can be stored as heat in gas molecules. A place that
absorbs less light will create less heat to radiate into the atmosphere for storage.
Ray
path
Ray
path
Remember our
flat- Earth
hypothesis?
Angle of
incidence = 90°
Angle of
incidence = 60°
Ray
path
Albemarle Is.
(no shadow)
New Iberia, LA
(.5775m shadow)
Angle of
incidence = 45°
Equator (0°)
Chippewa Falls, WI
(1m shadow)
45°
30°
Center of Earth
Angle of
incidence = 0°!
15°
At every latitude the amount of
heat generated is intermediate
– there is a gradient of
temperature between the two
extremes.
60°
90°
Angle of
incidence = 90°
The higher angle of incidence at
the Equator means that less
light is reflected and more
absorbed. More absorbance
means more heat to transfer to
the air.
The lower angle of incidence at
the poles makes for more
reflectance and less
absorbance. This means there
is less heat generated and
radiated to the air.
30°
Angle of
incidence = 45°
Angle of
incidence = 60°
Albemarle Is.
New Iberia, LA
Chippewa Falls, WI
Equator (0°)
This is why the climate is hot at
the Equator and cold at the
poles.
Angle of
incidence = 0°!
More
Reflectance =
More
Absorbance =
Colder Air
Warmer Air
North Pole
Here is the point more
graphically, in two senses.
Max Absorbance of Light =
Max Radiation of Heat
Amount
Albemarle Is.
New Iberia, LA
Chippewa Falls, WI
90° (pole)
Equator (0°)
Latitude
Min Absorbance of Light =
Min Radiation of Heat
0° (equator)
More
Reflectance =
More
Absorbance =
Colder Air
Warmer Air
North Pole
While we’re playing with balloons, think
about these. Why do they fly?
You’ve been told why – because warm air
rises. But why does warm air rise?
As we add energy to air the molecules
have two ways of dealing with the
additional energy. As we discussed earlier
the electrons can jump up a shell, but
heat will not do this – its energy is
inadequate.
What is added in this case is, actually, just heat. A gas burner and fan are used to heat the air
at what will eventually be the bottom of the balloon and blow it into the envelope. As the air
inside gets warmer the balloon begins to rise and finally takes off. What’s happening?
As the air gains heat its molecules absorb that heat, causing them to move
faster. The electrons spin faster, the atoms get more “jiggly” and they push
against each other more, forcing more space between them. You can’t see it,
but cooler air leaks out the bottom of the balloon because there is no room for
it as the warmer air expands. That’s right – the warm air begins rising even
before the balloon inflates and gravity forces it upward in the same way it
forces a boat to rise when you step out of it – by pulling colder air beneath it.
In this case though the balloon envelope keeps the warm air from
Going very far and the open bottom lets the cooler air escape.
Eventually there is less air (fewer air molecules) in the balloon
than outside of it! The air inside is, in other words, less dense than
The air outside. Now as gravity pulls the denser outside air beneath
the balloon it also forces the
balloon to rise.
The result of the differential
heating of the atmosphere is
that it undergoes convection.
The air would rise where it was
warmest and sink where it was
coolest, moving from one end of
that system to the other to
replace the air “ahead” of it.
Slightly cooler air would be
flowing in to replace rising warm
air at the Equator, slightly cooler
air flowing in to replace that,
and so on back to the pole.
Coldest Air
Sinks at
Poles
Gravity keeps the air
from escaping to
space. At the same
time, as the air rises to
the top of the
troposphere the
pressure decreases
and the air cools – like
the air coming out of a
spray can.
Warmest Air Rises at Equator
The convection would work as
shown – transferring excess heat
at the equator to the poles.
(Much would radiate into space
from the winds aloft as well.)
Equator
If the Earth did not rotate there
would be a single convection cell in
each hemisphere, moving air from
the equator to the poles and back
like this:
90° N
~60° N
Because the Earth does rotate the
Coriolis effect causes the winds to
follow a different path. They are
deflected to the right in the Northern
Hemisphere and to the left in the
Southern. By the time they have
crossed about 30° of latitude they
have turned to move almost parallel
to the latitude lines.
This is what causes “prevailing
winds”. At a given latitude the wind
is more likely to blow from one
particular direction rather than
others.
POLAR FRONT
WESTERLY
WINDS
~30° N
EASTERLY
Equator
(0°)
DOLDRUMS
(TRADE)
WINDS
~30° S
WESTERLY
WINDS
~60° S
POLAR FRONT
90° S
HIGH
90° N
Cold Desert
LOW
~60° N
The deflection also established
three convection cells instead of
only one.
Where air rises the atmospheric
pressure tends to be low and
where it sinks the pressure tends
to be high.
Rainy belts go along with low
pressure zones, deserts with
high pressure zones.
HIGH
LOW
HIGH
Warm Desert
~30° N
Equator
(0°)
Hot Tropical Rainforest
Warm Desert
~30° S
LOW
Cool Coniferous Forest
(Taiga)
~60° S
Cool Coniferous Forest
(Taiga)
Cold Desert
90° S
HIGH