Hello.
This is Richard Collins, your
instructor in ATM101, Weather
and Climate of Alaska,
at the University
of Alaska, Fairbanks.
Welcome to unit 4,
Heating the Earth.
Today, I want to highlight
the fundamental ideas
we'll be exploring in unit 4.
These three maps show the
average temperatures in Alaska
for the whole year for
the month of January
and the month of July,
and they highlight
many of the processes we are
considering in this unit.
In the annual
average temperatures,
we see that the state of
Alaska gets generally colder
as we head northward
across the state.
Maritime influence is
visible in the south
and along the west coast
where the climate is generally
warmer than in the interior.
However, maritime
influence isn't
that obvious up on the Arctic
Ocean in the annual average
and in the January average
because the ocean remains
frozen, and we don't have
liquid water contributing
to warming of the land.
In July, you see that the
interior is the warmest part
of the state, and
this is because we
have a continental climate with
low elevation around the river
valleys of the interior.
In the annual mean
and July, we see
that the Alaska Range and the
Brooks Range at high elevation
are amongst the coldest
parts of the state.
However, in January, we see
that there is very cold air
associated with the low
lying valleys in the interior
and the Copper valley.
In this unit, we're
answering the question,
"What are the causes
and consequences
of heat transfer within the
Earth-Atmosphere System?"
We will be looking at
the concepts of heat
and temperature, how
heat is transferred
in the atmosphere, energy
balances in the overall energy
balance of the atmosphere,
and energy balances
and how they vary
with latitude, and how
local temperatures vary over the
cycle of the day and the year.
Temperature is the measure
of the average kinetic energy
of the atoms and
molecules of a substance.
Heat is energy
being transferred.
We can measure temperature
on several scales.
The Kelvin scale is an absolute
scale, where 0 Kelvin--
corresponding to minus 273
Celsius or minus 460 degrees
Fahrenheit--
corresponds to the
temperature when
the atoms and
molecules are at rest
and have no kinetic energy.
This is the lowest temperature
that we can achieve
and it's called absolute zero.
Increasingly, meteorologists do
not use the liquid thermometers
that we see in textbooks.
They use electrical thermometers
where the electrical properties
vary with temperature just as
we're using them in the class.
Thermometers only measure
the ambient air temperature
accurately if they're
properly set up in the shade
and well ventilated.
If not, you're just
cooking the thermometer
in a box and it doesn't
represent the air
temperature around it.
And finally, on the lower
right corner of the slide,
we have the conversion equations
for Fahrenheit, Celsius,
and Kelvin.
Heat is transferred by
conduction, convection,
and radiation.
Conduction, where we
have direct contact
between the hot
and cold material;
convection, where
we have a fluid
between the hot
and cold material,
and the fluid rises in
a convection pattern
as it's warmed and
then falls cooler
and transfers the heat
in a convection bubble;
and finally by radiation,
where the hot object emits
more radiation toward
the colder object, which
absorbs that radiation.
All three processes are
at work in the atmosphere.
And it's important to remember
that heat transfer is always
from the hotter, higher energy
object to the colder, lower
energy object.
We find, again, that water
has unique properties
in the atmosphere, and that
it exists as a solid, liquid,
and gas at atmospheric
temperatures.
When water changes phase,
there is an energy change
as the water molecules become
more tightly bonded as they
go from liquid to solid, or
they become more loosely bonded
as they go from liquid to vapor.
To melt ice, we have
to supply energy
to break bonds without
raising the temperature.
And this is an example
of hidden or latent heat.
When water condenses from vapor
to liquid, heat is released,
and this is an important part
of the energetics of cloud
formation in the atmosphere.
Continuing the idea that water
is special in the atmosphere,
we also find that water
requires more energy
than other materials to warm
and releases more energy
than other materials
when it cools.
Thus, we think of water
as having thermal inertia.
Maritime regions
warm more slowly
than continental
regions in summer,
and cool more slowly in winter.
Thus, the seasonal
range of temperatures
is much lower in a maritime
city like San Francisco
than a continental
city like St. Louis
even though they have the
same average temperature
over the year.
We see similar behavior between
locations in Western Alaska
and interior Alaska.
We can follow the pathways
of solar radiation
and terrestrial radiation in
the Earth atmosphere system.
Averaged over the whole globe,
49% of the incidence sunlight
is absorbed at the surface.
The Atmosphere-Earth combination
has an albedo of 31%,
or 31 units are reflected by the
air, clouds, and the surface.
The surface itself
has an albedo of 14%,
or 57 units of
solar radiation are
incident on the ground and 8
units are reflected to space.
In the terrestrial
radiation, the strength
of the greenhouse effect is
seen in the following way.
Notice that there are
95 units of radiation
emitted by the atmosphere
toward the ground incident
on the ground, and this
compares with 49 units
of direct radiation
from the sun.
We also highlight the role of
two non-radiative processes,
a sensible heat flux associated
with conduction and convection,
and a latent heat
flux associated
with evaporation and
condensation of water vapor.
These combined fluxes contribute
about 30 units of flux.
You can use slide 7 to begin
playing climate scientist
as you think about how
fluxes of radiation
would change as different
elements of the climate system
change.
What happens if the
surface albedo changes?
If the Earth was covered
with white snow uniformly,
that albedo would go to 100%.
What happens when greenhouse
gas concentrations increase?
Then the fluxes associated
with greenhouse gases
should increase and change
temperatures in the system.
You may also think about how
your local region differs
from the average.
How does this picture compare
with Barrow in November?
The table shows
that, in the slide 7,
we have a balanced model
of radiation and energy,
and the Earth's
average temperature
would remain constant
from year to year.
Current climate change
studies indicate
that the balance
is not perfect, and
that the Atmosphere-Earth system
is gaining 0.3 units of energy
a year as greenhouse gas
concentrations increase.
In a system where we're
looking at 100 units,
finding that 0.3
units as an average
over the globe from
real experiments
is one of the greatest
experimental challenge.
We've just looked at
the balance of energy
averaged over the entire
Earth for the entire year.
Now, we look at the
balance of energy
as a function of latitude
averaged over the year.
We see that the tropical
regions receive more energy
from solar radiation than
they lose by emitting
terrestrial radiation.
The polar regions
lose more energy
by emission through
terrestrial radiation
than they receive
through solar radiation.
Thus, the tropical regions
have a surplus over the year.
The polar regions have
a deficit over the year.
If this was left
to its own devices,
the tropical regions would
become hotter from year
to year, and the polar regions
would become colder from year
to year.
And we would get a huge
contrast between the tropics
and the poles.
In reality, energy is
transferred from the warmer
region to the colder region.
And this transfer
of energy is what
sets up the major
weather and storm
systems in the atmosphere,
as well as the major.
Energy balance also explains the
daily and seasonal variations
of temperature.
Each day, the
highest temperature
is found not at noon,
but in mid-afternoon,
while the lowest
temperatures occur not
at midnight, but near sunrise.
The reason for this
is due to the balance
between the solar
energy being absorbed
and the terrestrial
radiation being emitted.
As the sun rises
through the morning,
there is more solar radiation
incident on the ground.
While the ground
warms, we end up
with a net surplus of
radiation and the ground
continues to warm.
As the ground warms
through late afternoon,
the thermal emission now begins
to exceed the solar radiation
absorbed as the sun falls in
the sky in the late afternoon.
And eventually, we go from
a surplus to a deficit,
and the ground begins to
cool at about 4 o'clock
in the afternoon.
After sunset, the
emission continues
and the air cools through the
night, giving us our coldest
temperatures just
before or after sunrise
when we begin to get solar
radiation, again, beginning
to create a surplus of energy.
Seasonal variations
work in the same way,
with a lag where warmest
temperatures are found in July
and coldest
temperatures in January
even though the sun is
highest in the sky in June
and lowest in the
sky in December.
The seasonal variations
are, as expected,
greatest at high latitudes.
It's important to remember that
it's not all about radiation.
Occasionally, transport
of cold and warm air
can define the temperature and
beat the effects of radiation
locally.
The advection of
cold or warm air
can beat local radiative
heating and cooling.
In interior Alaska, warm air
is advected into the interior
and causes winter temperatures
to jump 30 degrees Fahrenheit
during Chinook episodes.
In the lower 48, cold air
transported from Canada
down into the
southern United States
can cause severe
damage to orange crops
in the middle of winter.
Finally, some closing
thoughts and our summary.
Temperature represents the
average kinetic energy of atoms
and molecules in a substance.
And energy transfer can occur
by conduction, convection,
radiation, and latent heat.
Again, we've seen that
water is different,
both in terms of
its phase changes,
which bring latent heat
into the picture of energy,
but also its specific heat
in terms of how it moderates
climate in maritime regions.
The greenhouse effect is
evident in the energy budget
as the infrared
heat flux is larger
than the direct solar
radiation flux in the energy
budget of the Earth.
However, the energy
balance in the Earth
is not balanced at every
point on the Earth,
and it's not balanced
at every time.
If it's balanced
at every time, we
wouldn't have days
get progressively
colder in the fall and
progressively warmer
in the spring.
These energy imbalances
drive our weather,
and the most dominant one
is the energy imbalance
between the tropics and the
poles drives the major pattern
of storm systems on the planet.
Finally, energy imbalance
drive temperature changes
as well in both our daily
and seasonal cycles.
