Chapter 2: Warming
the Earth and the
Atmosphere



Temperature scales
Different modes of heat
transfer
Incoming light and
seasons
Temperature and Heat
Transfer

Kinetic energy is the energy associated with
motion

Thus, what is temperature?

Temperature is the measure of the average
speed of the atoms and molecules

The higher the temperature, the faster the
molecules are moving
Temperature and Density
Example



Suppose we have a parcel of air
 A parcel of air is a balloon-like volume of air
that does not mix with outside air
In the parcel are various molecules moving at a
certain speed, and thus the parcel has a certain
temperature
Now, what happens when we apply heat to the
parcel? What happens to the molecules and to
the temperature of the entire parcel?
Temperature and Density
Example



According to the definition of temperature, the average speed of the
molecules will increase
The increase speed of the molecules will create more distance
between molecules
 Thus, the same number of molecules will occupy a greater
volume
Density is defined as mass/volume (kg/m3)
 So what happens to the density of the parcel?
Temperature and Density
Example

The opposite happens with cold air
 If we cool the parcel, the molecules will move slower and crowd
together. Thus, the parcel would become more dense

So, warm air is less dense than cold air. That is to say, cold air is
“heavier” than warm air. More mass in each cubic meter

What happens if we continue to cool the parcel?

Eventually, it will reach -273°C (-459°F). At this temperature all
molecular motion stops. This minimum movement with no thermal
motion is called absolute zero

What is heat? Heat is the energy transferred between objects due
to temperature differences
Temperature Scales




Absolute zero is the start of the absolute
scale, or Kelvin scale of temperature
No negative numbers, starts at zero
(absolute zero)
Introduced by Lord Kelvin
0 K corresponds to –273 on the Celsius
scale…
Temperature Scales
Celsius scale introduced in the eighteenth
century
 0 degrees is the point at which pure water
freezes (freezing point)
 100 degrees is point at which pure water
boils at sea level
 One Kelvin degree same as one Celsius
degree…so what is the equation?
 K = °C + 273

Temperature Scales






Fahrenheit scale developed in early 1700s
32 degrees is the point at which water
freezes (freezing point)
212 degrees is point at which water boils
0 degrees is simply the lowest temperature
obtained with a ice salt water
Since 1°C is equal to 1.8°F…what is the
equation?
°C=5/9(°F-32)
Fig. 2-2, p. 27
Latent Heat - The Hidden
Warmth


When water vapor become larger water
liquid, this is called change of state or
phase change
When a substance changes phases, heat
is released or taken from the environment
(energy is needed)
Latent Heat - The Hidden
Warmth


What happens when you step out of a shower or
warm pool?
Liquid water on your skin has a certain temperature
(remember what temperature is?)

The fast moving molecules escape and become
vapor. Leaving slower molecules (what has
happened to the temperature of the water?)

Evaporation is a cooling process!! That is, the
energy from the liquid water is “sucked up” into the
water vapor. (What about condensation?)
Latent heat is heat require to change the phase of a
substance. “Latent” because it is “hidden”.

Stepped Art
Fig. 2-3, p. 28
Conduction


Heat transfer between molecules
Different substances conduct differently
(what are good and bad conductors?)
Convection
Heat transfer by moving masses
 Easy for liquids and gases because they can
move easily (why does rising air expand and
cool, while sinking air compresses and
warms?)
 Advection is the
transfer of air prop.
to a different
location

Radiation

Radiation is energy transferred by the sun




Travels in “waves” and is not released until it
is absorbed
Travel at the speed of light
Measured in micrometers (one millionth of
a meter)
Photons are energy
Fig. 2-7, p. 32
Important Radiation Tips


EVERYTHING emits radiation
Objects with higher temperatures emit
shorter wavelengths


Wien’s law (λmax = constant / T). Constant =
2897 μmK What are units of λmax?
Objects with higher temperatures emit
more total radiation

Stefan-Boltzmann law (E=σT4). σ = 5.67 X 10-8
W/m2K4. What are the units of E?
Radiation




We can only see the visible light portion of
the spectrum. Why can we see cool
objects then?
UV radiation more energetic
Infrared radiation less energetic
The electromagnetic spectrum tells us
from which wavelengths an object is
emitting
Radiation




The sun (10,500°F) emits at a lot of
wavelengths (shortwave radiation)
A large portion of the emission intensity
comes from visible light
The surface of earth is cooler (59°F), thus
emits almost all infrared radiation
(longwave radiation)
Radiative equilibrium happens when rate
of emission equals rate of absorption
Fig. 2-8, p. 34
Fig. 2-9, p. 34
Radiation



Earth receives shortwave radiation on half
of the surface, but emits longwave
radiation from every surface
Earth’s radiative equilibrium temperature is
0°F (??)
Blackbody – a perfect absorber and
emitter (absorbs everything and gets rid of
everything)
Radiation



Why is there a difference between
average surface and radiative equilibrium
temperatures?
The answer is selective absorbers
A selective absorber is something that
absorbs selective wavelengths and usually
emits at similar wavelengths. Name some
selective absorbers…

Selective Absorbers and the
Atmospheric Greenhouse
Effect
Gases in the atmosphere
absorb longwave radiation, but
not shortwave radiation well

BOTH water vapor and carbon
dioxide are strong absorbers
of infrared radiation

Ozone absorbs shorter
wavelengths (in stratosphere)
Selective Absorbers and the
Atmospheric Greenhouse
Effect
 Atmospheric window is between 8
and 11 μm where atmosphere
doesn’t absorb longwave radiation


Clouds can close the window
because the molecules are
larger (more clouds, less
radiation leaving)
This is why calm, cloudy
nights are generally warmer
than calm, clear nights
Selective Absorbers and the
Atmospheric Greenhouse
Effect



Molecules absorb radiation,
causing them to vibrate and bump
into neighbors.
This increases the kinetic energy,
and thus what happens to the
atmospheric temperature?
Thus, the absorption of longwave
radiation by water vapor and
carbon dioxide is known as the
greenhouse effect, atmospheric
greenhouse effect, etc.
The Greenhouse Effect Is
Essential


So, the atmosphere is responsible for warming
the surface more
Greenhouse effect is essential to keep the
surface hospitable
Enhancement of the
Greenhouse Effect




Global warming – increase in atmospheric
Carbon Dioxide due to burning of fossil
fuels and deforestation
Other greenhouse gases on rise as well
(CH4, N2O, CFCs)
Current predictions increase temperatures
1.4 to 5.8°C
How can a small increase in Carbon
dioxide increase temperatures?
Enhancement of the
Greenhouse Effect



Positive feedback – a change in a process
that will reinforce the process
Negative feedback – a change in a
process that will weaken the process
For instance, rising ocean
temperatures…move evaporation, more
water vapor (a greenhouse gas), higher
temperatures, rising ocean
temperatures…
Enhancement of the
Greenhouse Effect

Negative feedback may be…rising ocean
temperatures, more evaporation, move
water vapor, more clouds, more sun
reflected to space, lower temperatures…
Warming the Air from Below




Radiation from the sun hits the ground
Conduction warms the surface and air just above
it
Convection allows warm air to move upward and
cool, maybe condense into clouds
Water vapor and
Carbon dioxide
absorb and emit (in
all directions)
infrared radiation
What Can Happen To Solar
Radiation?
Scattering – deflection of light in all
directions (also known as diffuse light)
 Sky is blue because Nitrogen scatters
BLUE LIGHT!
 Also responsible
For red sunsets

What Can Happen To Solar
Radiation?
What Can Happen To Solar
Radiation?
What Can Happen To Solar
Radiation?





Reflected – more light is sent backwards
(toward the source)
Albedo – percent of radiation returning
from a surface (reflectivity)
What has a high albedo? Low albedo?
What is the Earth’s albedo?
What is the Moon’s albedo?
The Earth’s Annual Energy
Balance


The Earth is basically in equilibrium.
Thus, it MUST return to space what it
receives from the sun. So, we can create
a radiation budget
The Earth’s surface and atmosphere are
also in equilibrium, so we can create a
budget there as well
Fig. 2-15, p. 41
Fig. 2-16, p. 42
FIGURE 2.16 The earth-atmosphere energy balance. Numbers
represent approximations based on surface observations and satellite
data. While the actual value of each process may vary by several
percent, it is the relative size of the numbers that is important.
Stepped Art
Fig. 2-16, p. 42
Why the Earth has Seasons



Why does Earth have seasons?
Closer to
sun in
January
23.5° tilt of
axis
Why the Earth has Seasons


Seasons have to do with amount of solar
energy received
Sunlight
intensity
shows that
the equator
experiences
more
intense
sunlight
Why the Earth has Seasons




Longer daylight, more sun reaching the
ground, more energy from the sun
Sun roams from 23.5°N and 23.5°S
(Tropic of Cancer, Tropic of Capricorn)
On the Summer Solstice, June 21 (longest
“day” of the year in the NH), sun is directly
above Tropic of Cancer
At Arctic Circle (66.5°N), sun rise on
March 20 and does set until September 22
Seasons in the Northern
Hemisphere

Still cold because sunlight has to pass
through a lot of atmosphere that absorbs,
scatters, and reflects
Why the Earth has Seasons



Autumnal Equinox – September 22, when the
sun is directly above the equator. NH goes into
Fall, SH goes into Spring. Sun sets at North Pole
Winter Solstice – December 21, when the sun is
directly above 23.5°S. Cold in NH, summer in
SH
Vernal equinox – March 20, when sun is back
over the equator. NH goes into Spring and SH
goes into Fall. Sun rises at North Pole
Local Seasonal Variations
Local Seasonal Variations


Slope of hillsides
Vegetation differences
• Homes can exploit
seasonal variations:
large windows
should face south.
• Trees should be
planted on west side
of house