Warming the Earth and the
Atmosphere
Chapter 2
This chapter discusses:
• Temp and heat transfer
– Conduction, convection, radiation
• Solar radiation, earth energy balance
• The seasons
– Local variations
Temperature and Heat Transfer
Temperature is the quantity that tells us how hot or cold something is
relative to some set standard value.
• Air is made up of billions of atoms and molecules, moving in all
directions, spinning and bumping around. They don’t all move at
same speed.
– Energy associated with this motion is called kinetic energy, the
energy of motion.
– Temperature of air is the measure of its average kinetic energy
– Temperature is a measure of the average speed of the atoms
and molecules, where higher temperatures correspond to faster
average speeds.
– Absolute zero – at this temp the atoms and molecules would
posses a minimum amount of energy and theoretically no
thermal motion
Temperature Scales
•
•
•
•
•
Kelvin Scale - a temp scale
begins at absolute zero (“no
motion”)
Fahrenheit Scale – assigned 32
as the number where water
freezes and 212 where water
boils. 180 equal divisions called
degrees
Celsius Scale – Zero on this
scale assigned to the temperature
at which pure water freezes and
100 to temp where pure water
boils. Divided into 100 equal
degrees
C=5/9(F-32)
K=C+273
Figure 1: Comparison of the Kelvin, Celsius, and Fahrenheit scales
Latent Heat
Change of State (Phase Change)- changes
from gas…liquid…solid (ice)
• The heat energy required to change a substance (water), from one
state to another is called latent heat. (Why do we say latent?)
• Examine a drop of water – evaporating – faster moving molecules
escape most easily and the average motion of all the molecules left
behind decreases as each additional molecule evaporates. Slower
motion suggest a lower water temperature. Evaporation is,
therefore, a cooling process.
• The energy lost by the liquid water during the evaporation can
be thought of as carried away by, and “locked up” within the
water vapor molecules. The energy is in a “stored” or “hidden”
condition and is called latent heat.
• The heat is latent (hidden) in that the temperature of the
substance changing from liquid to vapor is still the same.
Figure 3: Heat energy absorbed and released
This latent heat energy will reappear as sensible heat (that we can feel and measure
with a thermometer) when the vapor condenses back into liquid water. Condensation
(opposite of evaporation) is a warming process.
Latent heat is an important source of
atmospheric energy! Water vapor rising into
the air cools and becomes liquid water and
ice particles…this process releases heat into
the environment
Figure 3: Every time a cloud forms, it warms the atmosphere. Inside this developing
thunderstorm a vast amount of stored heat energy (latent heat) is given up to the air,
as invisible water vapor becomes countless billions of water droplets and ice crystals.
In fact, for the duration of this storm alone, more heat energy is released inside this
cloud than is unleashed by a small nuclear bomb.
Conduction and Convection
The transfer of heat from the hot
end of the metal pin to the cool
end by molecular contact is called
conduction. Molecules in the end
of the pin absorb energy from the
flame and vibrate faster than
those farther away from flame,
energy is eventually transferred
from molecule to molecule to
hand.
The transfer of heat by the mass
movement of a fluid (such as
water and air) is called
convection. Convection
happens naturally in the
atmosphere. (I.e. Thermals –
rising bubbles of air.)
Note: In our atmosphere,
air that rises expands and cools
air that sinks is compressed and warmed!
Figure 5: The development of a thermal. A thermal is a
rising bubble of air that carries heat energy upward by
convection
Radiation
The energy transferred from the sun to your
face on a warm day is called radiant
energy or radiation. The sunlight travels
through the air with little effect on the air
itself. The energy travels in the form of
waves that release energy when they are
absorbed by an object. These are called
electromagnetic waves because they have
magnetic and electrical properties.
micrometer
Figure 6: Radiation characterized according to wavelength. As
the wavelength decreases, the energy carried per wave
increases.
Note different wave lengths, micrometer is 10-6
Photons
Think of radiation as streams of particles,or
photons, that are discrete packets of
energy. UV photons carry more energy
than a photon of visible light. UV photons
produce sunburns, penetrate skin, can
cause cancer.
Figure 8: The sun's electromagnetic spectrum and some of the descriptive
names of each region. The numbers underneath the curve approximate the
percent of energy the sun radiates in various regions.
Solar radiation and Earth radiation
Figure 8: The hotter sun not
only radiates more energy
than that of the cooler earth
(the area under the curve),
but it also radiates the
majority of its energy at
much shorter wavelengths.
(The area under the curves
is equal to the total energy
emitted, and the scales for
the two curves differ by a
factor of 100,000.)
Solar radiation is shortwave radiation. Earth radiation is longwave radiation
Balancing Act – Absorption,
Emission, and Equilibrium
If the earth and all things on it are continually radiating
energy, why doesn't everything get progressively colder?
• The rate at which something radiates and absorbs energy depends
strongly on its surface characteristics, such as color, texture,
moisture and temperature.
• Blackbody – an object that is a perfect absorber (it absorbs all the
radiation that strikes it) and a perfect emitter (emits the maximum
radiation possible at its given temperature). Does not have to be
black in color. Earths surface is nearly 100% efficient and thus
behaves like a blackbody
• Radiative equilibrium temperature – Average temp at which the rate
of absorption of solar radiation equals the rate of emission of
infrared earth radiation.
Selective Absorbers and the
Atmospheric Greenhouse Effect
Many selective absorbers in the environment.
• Snow, good absorber of IR radiation, but
poor absorber of sunlight. Good emitter of
IR energy. At night snow emits more IR
energy than it absorbs (loss of IR radiation
helps cause the air above the ground to
become very cold).
• Others are water vapor and CO2
Figure 9: Absorption of radiation by gases in the atmosphere. The shaded area represents the percent
of radiation absorbed. The strongest absorbers of infrared radiation are water vapor and carbon
dioxide.
Water vapor and carbon dioxide (CO2) are strong absorbers of IR
Radiation and poor absorbers of visible solar radiation.
They radiate a portion of the IR energy toward the ground and act as act
as an insulating layer around the earth, keeping part of the radiation emitted
by the earth from escaping into space.
Insolation
Figure 10a: Sunlight warms the earth's surface only during the day, whereas the surface constantly
emits infrared radiation upward.
Without water vapor, CO2, and other greenhouse gases, the earth's surface would constantly emit
infrared radiation (IR); incoming energy from the sun would be equal to outgoing IR energy from the
earth's surface. Without the greenhouse effect, the earth's average surface temperature would be
-18°C (0°F) .
Figure 10b: With greenhouse gases, the earth's surface receives
energy from the sun and infrared energy from its atmosphere. Incoming
energy still equals outgoing energy, but the added IR energy from the
greenhouse gases raises the earth's average surface temperature about
33°C, to a comfortable 15°C (59°F).
If convection were to suddenly stop - the average
Earth temp would rise about 18F
Figure 11: Air in the lower atmosphere is heated from below. Sunlight warms
the ground, and the air above is warmed by conduction, convection, and
radiation. Further warming occurs during condensation as latent heat is given
up to the air inside the cloud. Most absorption takes place near the surface –
lower atmosphere is mainly heated from below.
At sunrise and sunset, when the white beam of sunlight must pass
through the a thick portion of the atmosphere, scattering by air
molecules removes the blue light, leaving the longer wavelengths of
red, orange and yellow to pass on through
A brilliant red sunset produced by the process of scattering
On the average, of all the solar energy that reaches the earth's atmosphere
annually, about 30 percent (30/100) is reflected and scattered back to
space, giving the earth and its atmosphere an albedo of 30 percent. Of the
remaining solar energy, about 19 percent is absorbed by the atmosphere
and clouds, and 51 percent is absorbed at the surface.
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.
Aurora – caused by charged particles from the sun interacting with the atmosphere.
Solar wind collides with atmospheric gases. Gases get “excited” and emit light
aurora borealis in NH and aurora australis in SH
The stream of charged particles from the sun - called the solar wind distorts the earth's magnetic field into a teardrop shape known as the
magnetosphere.
When an excited atom, ion, or molecule deexcites, it can emit visible light. The electron in
its normal orbit becomes excited by a charged
particle...
...and jumps into a higher energy level.
When the electron returns to its normal orbit, it emits a photon of light.
Aurora Belt
• The aurora belt (solid red
line) represents the
region where you would
most likely observe the
aurora on a clear night.
(The numbers represent
the average number of
nights per year on which
you might see an aurora
if the sky were clear.) The
flag MN denotes the
magnetic north pole,
whereas the flag NP
denotes the geographic
north pole.
Why do we have seasons?
• Earth has an elliptical path around the sun
that takes a little over 365 days
• One spin on its own axis in 24 hours
• Average distance from earth to sun is 93
million miles
• Elliptical path takes us closer to sun in
January than it does in July – Say what?
• Seasons are regulated by sun angle and
the number of daylight hours
The elliptical path (highly exaggerated) of the earth about the sun brings the
earth slightly closer to the sun in January than in July.
Sunlight that strikes a surface at an angle is spread over a larger area
than sunlight that strikes the surface directly. Oblique sun rays deliver
less energy (are less intense) to a surface than direct sun rays.
As the earth revolves about the sun, it is tilted on its axis by an angle of 23.5°. The earth's
axis always points to the same area in space (as viewed from a distant star). Thus, in June,
when the Northern Hemisphere is tipped toward the sun, more direct sunlight and long
hours of daylight cause warmer weather than in December, when the Northern Hemisphere
is tipped away from the sun. (Diagram, or course, is not to scale.)
Land of the Midnight Sun. A series of exposures of the sun taken
before, during, and after midnight in northern Alaska during July.
Note: The sun never sets from Mar 20 – Sept 22.
Equinox
Autumnal (fall) Equinox – Sept 22 – Sun is directly above the
equator. Except for poles, days and nights throughout the world are
Equal length. After this in the NH there are fewer hours of daylight and
The noon sun is slightly lower in the sky.
Winter Solstice – Dec 21 - The shortest day of the year, and the astronomical
beginning of winter in NH.
Summer Solstice – June 21 – Sun is highest in sky in N.H. and directly over
23 ½ N (Tropic of Cancer)
Vernal (spring) Equinox – March 20, once again the noonday sun is
Shining directly on the equator, days and nights are of equal length,
And at the North pole the sun rises above the horizon after 6 long dark
Months.
During the Northern Hemisphere summer, sunlight that reaches the earth's
surface in far northern latitudes has passed through a thicker layer of
absorbing, scattering, and reflecting atmosphere than sunlight that reaches the
earth's surface farther south. Sunlight is lost through both the thickness of the
pure atmosphere and by impurities in the atmosphere. As the sun's rays
become more oblique, these effects become more pronounced.
The average annual incoming solar radiation (red line) absorbed by the
earth and the atmosphere along with the average annual infrared radiation
(blue line) emitted by the earth and the atmosphere.
The changing position of the sun, as observed in middle latitudes in the Northern Hemisphere.
In areas where small temperature changes can cause major changes in
soil moisture, sparse vegetation on the south-facing slopes will often
contrast with lush vegetation on the north-facing slopes.