Hello.
This is Richard Collins, your
instructor in ATM101 Weather
and Climate of Alaska at
the University of Alaska,
Fairbanks.
Welcome to Unit 3,
Energy and Radiation.
Today, I want to highlight
the fundamental ideas that we
will be exploring in Unit 3.
The sun is the primary source of
energy and life on our planet.
This is a fact that's
being appreciated
by humans for a very long
time and it's particularly
appreciated by humans living
in Fairbanks in the winter.
Early solar observatories
were built around the world
to keep track of the seasons
to ensure the crops would
be planted in time to mature and
be harvested before the winter.
Today, much of our
interest in the sun
is focused on harvesting
solar energy directly.
The map shows the
distribution of solar energy
around the world.
This represents the
amount of solar energy
that would be received
at the surface
of the earth over a day.
And this data has been
averaged over several years.
What we see very clearly is
there's a strong variation
with latitude.
As we go from the
equator zero degrees
to the poles at 90 degrees
in both hemispheres,
we see that the amount of solar
energy reaching the surface
of the earth falls off.
If we look in longitude
at a given latitude,
we also see variations.
And these are mainly
associated with cloudiness.
In cloudy regions
like Southeast Asia,
there is less solar
radiation reaching the ground
than in regions that are
clear skied like the Sahara
Desert of North Africa.
The overarching
question is, what
are the flows of energy into
and out of the Earth atmosphere
that maintain a
habitable planet?
There are several
topics here, but there
are three overarching topics.
One is the fundamental
physics of radiation.
The second one is
the relationship
between the geometry of
the Earth and the sun,
and how sunlight
falls on the Earth.
And as the Earth moves around
the sun, how that varies
and how we have seasons.
And the last
overarching topic is
how different components of
the Earth atmosphere system
interact with radiation,
whether their gases, clouds are
the surface of the Earth itself.
Radiation is energy that travels
as electromagnetic waves.
Electromagnetic
waves are waves that
have coupled electric
waves and magnetic waves.
They're the mechanism by which
energy can travel in a vacuum.
Waves are described by their
wavelength, the distance
between their crests.
They can also be described by
the corresponding frequency,
which is inversely
proportional to the wavelength.
So when we have short
wavelength waves,
we have high frequency waves.
When we have long
wavelength waves,
we have low frequency waves.
Invisible light,
we have a spectrum,
a range of wavelengths
from 0.4 microns where
we have purple
light to 2.7 microns
where we have red light.
The short wavelength
light is associated
with the high frequencies.
The long wavelength
with low frequencies.
And the energy of the light is
proportional to the frequency.
And so the short
wavelength light
has higher energy than
the long wavelength
light, which has lower energy.
There is a wide variety of
electromagnetic radiation
in terms of wavelength spanning
a vast number of wavelengths.
I want to highlight
just two parts
of the electromagnetic
spectrum-- ultraviolet
radiation, which
is higher energy
radiation than
visible radiation that
is on the blue-purple
side of visible radiation,
and the infrared radiation,
which is longer wavelength,
lower energy radiation on
the red side of the spectrum.
I'd also point out that
we're using radiation here
as energy that radiates,
not to be confused
with radiation as nuclear
radiation, which is particles
emitted by nuclear processes
that can cause cancers
and other health effects.
All objects emit radiation
based on their temperature.
Warmer objects will emit more
brightly than colder objects.
And they will also emit
at shorter wavelengths.
Here, the warmest
object emits in the blue
more brightly than the
colder object in the red.
We have temperatures here in
kelvin, 4,000 to 6,000 kelvin.
We'll come back and talk
about the specifics of kelvin
in a minute.
When we look at the sun, which
is at a temperature of 6,000
kelvin, it emits with a peak
wavelength of 0.5 micrometers
and is quite bright at
about 1750 on this scale.
When we look at the Earth
at 288 kelvin, much colder,
it is emitting at 10
microns, a longer wavelength,
and much dimmer at
a scale of 25 here.
The question of kelvin
comes to the question
of how we measure temperature.
Kelvin is considered an absolute
temperature measurement,
because when the
temperature is 0 in kelvin
minus 273 degrees Celsius,
there is no energy emitted
by the body, and it
is an absolute measure
based on energy, whereas
zero degrees Celsius is just
the freezing point of water.
The other thing I
want to point out here
is that we now have the idea
that there is energy coming
to the Earth in the visible
and there is energy leaving
the Earth in the infrared.
And there is a balance
between the two.
For the Earth to sit at
a constant temperature,
there must be a balance
between the energy coming in
and the energy coming out.
But if the Earth is to stay
at a constant temperature
over the course of a
year, then the energy in
and the energy out must equal
so at the end of the day
the Earth temperature
doesn't get
colder or warmer on average.
And we'll be seeing more
of this in the next unit.
The way in which the
sun heats the Earth
is based on the height or
altitude of the sun in the sky.
When the sun is directly
overhead, a beam of light
makes a small spot
on the ground.
And therefore the sunlight
is quite concentrated.
As the sun falls in
the sky from A to B,
then that spot spreads out.
The energy of the
sun hasn't changed.
But the footprint of
the sun has changed,
the sunlight has become
less concentrated,
and therefore the
ground is warmed less
because the sunlight
is more diluted.
Therefore as we look at
latitudes on the Earth,
if we go from the equator
to higher latitudes,
we see that at the equator
the sunlight is shining
directly down on the ground.
At higher latitudes,
we're shining
at a slant angle that spreads
the light out on the ground.
And therefore,
inherently, there's
going to be less
solar energy reaching
the ground per unit
area at high latitudes
than at low latitudes.
To add insult to injury, beams
of light at high latitudes
travel through a slant
path through the atmosphere
that's a longer path than
beams of light at the equator.
And so there's more chance for
solar energy to be absorbed
or to be lost in clouds.
So this also works against the
high latitudes, if you will,
so that the beams of
light are diminished more
and then they're
spread out more.
And this is the major reason
for why solar radiation varies
with latitude on the planet.
The textbook and other
textbooks you've seen
do a very nice job of showing
pictures of the Earth moving
around the sun.
I just wanted to highlight
some specific numbers
about the seasons using
Fairbanks as an example.
We see that at the equinoxes
March and September
the sun is about 25
degrees above the horizon.
And sunlight is spread out
on the ground about 1.4 times
as much as the sunlight
overhead at the equator.
And the path through
the atmosphere
is about 1.4 times longer.
Not a huge difference.
And midsummer in Fairbanks
it can get quite warm.
At Equinox, the sun is
spread out 2.4 times as much
and the path is 2.4 times
longer because the sun is
lower in the sky.
And then finally
at winter solstice,
the sun is extremely
low in the sky
and the sun is spread
out 38 times as much
and the path through the
atmosphere is 38 times longer.
And in this instance,
the sunlight
becomes very weak in heating
and Fairbanks gets very cold.
And this just highlights how
extreme those variations are
and that the variations
become most extreme when
the angles become small.
And that's why the winter falls
away so dramatically compared
to the summer in terms of
temperatures at high latitudes
relative to the equator.
The amount of sunlight
absorbed in the ground
also depends on how
reflective the ground is.
If we have 50 units of
sunlight hitting the ground
and 20 of them are reflected,
then only 30 of them
go into the ground
to heat the ground.
Of particular interest to us
are variations in reflectivity
between, say, fresh snow, which
has the reflectivity of 95%--
only 5% goes into the ground--
compared to, say, water,
which has a reflectivity
or albedo of 10%.
And so albedo changes can
result in significant changes
in heating.
If snow and ice melt, then the
albedo decreases, less sunlight
is reflected, and more sunlight
is absorbed into the ground.
And we see this every
spring and break up,
that as you clear the
ground, the heating
becomes more rapid as the
dark ground absorbs heat.
It can also setup a
feedback mechanism
where if we warm the
planet, we melt the snow.
And by melting the
snow, we allow more heat
to be absorbed by the planet.
And we can get a runaway effect
or an amplification effect,
where the initial effect
of warming the planet now
gets amplified by
an albedo change.
Similarly, we see that there
are high albedos associated
with clouds.
And so the role clouds
play with albedo
is also very important
in the climate system.
And so it's not just an
issue of the sunlight landing
on the earth at some
angle, it's also
the property of the Earth itself
and how that light is absorbed.
Looking at infrared radiation,
the other side of the equation
is the light emitted
from the Earth.
And here we see that in
the wavelengths where light
is being emitted
from the Earth, we
have methane, nitrous oxide,
oxygen, and ozone carbon
dioxide and water
vapor all have areas
where they absorb close
to one high absorption
of that radiation.
And so this sets up
the greenhouse effect.
What happens is that if the
Earth had no atmosphere,
it would just cool to space.
However, with an
atmosphere, the Earth
is cooling, letting
go infrared radiation.
But that infrared
radiation is picked up
by the neighboring
atmosphere, warms
the neighboring atmosphere, and
the neighboring atmosphere then
re-radiates that radiation
back to the Earth,
further heating it.
It re-radiates some of it
to space, but some of it
back to the Earth.
And so if you will, the
heat released from the Earth
is recycled back into the
Earth, making it warmer
than it would be otherwise.
And the current great
question that we're
interested in in terms
of greenhouse gases
is the fact that concentrations
of carbon dioxide are rising.
Therefore, we expect there to be
more energy in the atmosphere.
And possibly, we expect that
the atmosphere should generally
warm.
The other gas that's important
in the atmosphere in terms
of its interaction with
radiation is ozone.
On the previous slide,
we show that there's
strong absorption due to
ozone in the atmosphere
and the high energy ultraviolet
light that is blocked
is very important.
Because if that light
came to the ground,
it would disrupt
biological functions.
One of the things to highlight
in ozone or any other gas
in the atmosphere is that
there is a chemical balance.
Ozone molecules are in a
cycle with oxygen molecules
and oxygen atoms.
The population of ozone overall
stays relatively stable.
But individual
molecules and atoms
are cycling through different
compounds very quickly.
But the overall
population stays constant.
That overall population
is in a chemical balance.
If we change other
chemicals in the system,
we may get a population
that's smaller or larger.
If we change chlorine
in the atmosphere,
we get a balance where there's
less ozone, and therefore
less absorption of
ultraviolet light
and more ultraviolet
light reaching the ground.
This happens catastrophically
in the ozone hole where
two things come together,
the human release
of chlorine into the atmosphere
and the unique meteorology
event chart to go with
very cold temperatures.
And here's a picture of the
ozone layer in Antarctica.
The solid line in August
is a typical ozone layer
with height, with a peak
at about 17 kilometers.
And that's when there's
low chlorine in Antarctica
before sunlight
returns to Antarctica.
And then with the
return of sunlight,
we see high levels
of chlorine released
and immediately a balance
with destruction of ozone.
And you see the ozone
practically disappears.
And this has been the great
concern about the ozone hole
and global levels of
ozone that chlorine
may have diminished them.
And this is why we banned
CFCs as a global community
to restore the ozone
hole hopefully sometime
in this or the next century.
Clouds are a very important
part of the radiative balance.
Clouds interact with radiation
in two very important ways.
Because they're high albedo,
they reflect sunlight
and they have a cooling
effect during the day.
Clouds are also made of water,
which is a greenhouse gas.
And water interacts
very strongly
with infrared radiation.
And so on cloudy
or humid nights,
where there's a lot of
water vapor in the air,
radiation from the Earth will
be absorbed by the clouds,
and then red-radiated back to
the Earth, warming the Earth.
And so in general, cloudy days
are cooler than clear days,
while cloudy nights are
warmer than clear nights.
Changes in clouds
and changes in albedo
are part of the
largest uncertainty
in how we think the
climate will respond
to changes in greenhouse gases.
If there are changes in
the water vapor cycle
that change the way
clouds work, that
could have a big effect on
maybe making the planet cloudier
and blocking solar radiation,
and maybe counteracting
some of the effects of
greenhouse gas warming.
These remain active research
areas in climate science.
In terms of Alaska, we have some
measurements of solar radiation
and solar energy on
the ground in Alaska
through the seasons
at several sites.
Here we have Anchorage,
Barrow, Fairbanks and Annette--
Annette in Southeast Alaska,
Anchorage South Central,
Fairbanks in interior, and
Barrow on the Arctic north
slope.
You can see that the
concentration of sunlight
at the ground depends on
solar altitude, the length
of the day, and the cloudiness.
And you'd expect crossing
Alaska that the sun
is higher in the sky as we
go south and lower in the sky
as we go north.
The length of day is longer
in Barrow in the summer
than it would be in Annette.
You don't see a lot
of variation here,
and that's partially because
of variation in cloud cover.
And so for example,
Barrow has longer days,
but it has cloudy skies than
Fairbanks and high summer.
Anchorage has the sun
is higher in the sky,
but it's generally
cloudier in the summer.
The other thing just to
highlight in terms of Fairbanks
is that there's less
clouds in March, April
than there is in July, August.
And therefore, we actually
get more sunlight in Fairbanks
early in the summer
than late in the summer,
and we're slightly
skewed because
of the skew in our cloud cover.
So all those elements
are coming together
and we see them in
the data from Alaska.
Closing thoughts and summary.
I can read the bullets here.
I'm just going to reflect
on them generally.
All objects absorb
and emit radiation.
And the amount of
radiation emitted by body
is related to the
temperature and defines
the fundamental
temperature scale
of kelvin, that when read
absolute zero, zero degrees
Kelvin, a very cold temperature.
It's the lowest we can go.
And at that point, there is
no radiation emitted by body.
Latitude is very important in
how the sun heats the ground.
And seasons are caused
by the tilt of the Earth
and more pronounced
at higher latitudes.
There is a balance in the
atmosphere between absorption
of sunlight or visible
radiation and emission
of heat or infrared radiation.
If there was not a
balance, then the Earth
would keep cooling if it was
emitting too much radiation
or it would keep warming
if it was absorbing too
much radiation all the time.
But we end up in a balance in
that the Earth, on average,
stays pretty constant
year to year.
We'll talk about trends in
climate later in the semester.
Different gases
in the atmosphere
interact with different
types of radiation.
The ozone layer blocks
ultraviolet radiation
very effectively and is an
important part of maintaining
life on our planet.
The absorption and re-emission
of infrared radiation
by greenhouse gases is very
important in maintaining
a warmer temperature
than we otherwise
might expect that also
helps life on the planet.
Remember, life on our
planet is tied to the fact
that we have liquid water.
Containing a range
of temperatures
that allow liquid water to
exist is very important.
And finally, as concentrations
of greenhouse gases increase,
we would expect the
atmosphere to warm.
But it doesn't guarantee
the atmosphere would warm.
There are other elements
at play in terms
of feedback in the system
about how albedos might
change if cloudiness changes.
And so it is not an
automatic guarantee
to say that it increases if
greenhouse gases automatically
guarantee increases
in temperature.
And there are also huge
regional effects at play
that we'll discuss
later in the semester.
Thank you.
