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Heating Earth’s Surface and
Atmosphere
p
Chapter 2-3
September 6
6, 2009
This chapter discusses:
• Temperature and heat transfer
– Conduction,
Conduction convection
convection, radiation
• Solar radiation, earth energy balance
• The seasons and temperature
variations
This chapter discusses:
• Temperature and heat transfer
– Conduction,
Conduction convection
convection, radiation
• Solar radiation, earth energy balance
• The seasons and temperature
variations
Temperature and Heat Transfer
f
‘‘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
p
g and bumping
p g around. They
y don’t all
all directions,, spinning
move at same speed.
– Energy associated with this motion is called kinetic energy, the
energy of motion
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
Vibration, spinning, moving, bumping around
=> Kinetic energy ≈ Temperature
Temperature Scales
•
•
•
•
•
Kelvin Scale - a temp scale
begins at absolute zero (“no
motion”)
F h
Fahrenheit
h it Scale
S l – assigned
i
d 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
pure water freezes and 100
which p
to temp where pure water boils.
Divided into 100 equal degrees
C=5/9(F-32)
K=C+273
Comparison of the Kelvin, Celsius, and
Fahrenheit scales
Heat
•
The transfer of energy into or out of an object due to temperature
differences between one object to another
•
Heat flows from a region of higher temp to one of lower temp
•
•
Three types of heat-transfer
1. Conduction
2 Radiation
2.
R di ti
3. Convection
Additionally latent heat is very important in the atmosphere
Additionally,
1)) Conduction
• The transfer of heat from the hot
end of the metal pin to the cool
end by molecular contact is called
conduction.
d ti
• Molecules in the end of the pin
absorb energy from the flame and
vibrate faster than those farther
away from
f
flame,
fl
energy is
i
eventually transferred from
molecule to molecule to hand.
2)) Convection
• The transfer of heat byy the mass
movement of a fluid (such as
water and air) is called
convection.
convection
• Convection happens
pp
naturally
y in
the atmosphere. (i.e. Thermals –
rising bubbles of air.)
• Radiator induces convection in a
room.
Note: In our atmosphere, air that rises expands and
Cools air that sinks is compressed and warmed!
Incoming solar energy
The development of a thermal. A thermal is a rising
bubble of air that carries heat energy upward by
convection
3)) Radiation
• C
Carrying radiant energy
• Only heat transfer that can travel through the vacuum
of space (without medium)
• 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
th air
the
i it
itself.
lf Th
The energy ttravels
l iin th
the fform off waves
that release energy when they are absorbed by an
object.
j
These are called electromagnetic
g
waves
because they have magnetic and electrical
properties.
This chapter discusses:
• Temperature and heat transfer
– Conduction,
Conduction convection
convection, radiation
• Solar radiation, earth energy balance
• The seasons and temperature
variations
Electromagnetic Spectrum of Radiation
frequ
uency
Solar radiation
Radiation characterized
according to wavelength.
wavelength
As the wavelength
decreases, the energy
carried per wave increases.
Energy carried
per wave or
photon
micrometer is 10-6
*Radiation laws
•
•
•
All objects continually emit radiant energy
Stefan-Boltzman law
– Hotter object emits more energy
– The rate of radiation emitted by a body is proportional to
the fourth power of the body’s temperature
– E=σT4
– Sun: ~6000K, E=~ 73,483,200 W/m2
– Earth: ~300K, E=~ 459 W/m2 (~0.006% of Esun)
Wien’s displacement law
– Wavelength of maximum emission is inversely proportional
to temperature of a radiating body
– λmax = C/T
– Sun: ~0.483 ɥm
– Earth: ~9.66 ɥm
•
Good absorbers are good emitters
Solar radiation and Earth radiation
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
(Th
d the
th 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,
absorption
emission, and equilibrium
If the earth and all things on it are continually radiating
energy, why doesn't everything get progressively colder?
Absorption = Emission (radiative equilibrium)
•
•
•
Absorptivity, the rate at which something radiates and absorbs energy
depends strongly on its surface characteristics, such as color, texture,
moisture
i t
and
d ttemperature.
t
Blackbody – an object that is a perfect absorber (it absorbs all the
radiation that strikes it) and a perfect emitter (emits the maximum
radiation
di i possible
ibl at iits given
i
temperature).
) D
Does not h
have to b
be bl
black
k
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.
Going to equilibrium temperature
1. Incoming solar radiation is fixed
p
warms the Earth
2. Absorption
3. Radiation from Earth increases as Tearth
increases
4. Tearth reaches a critical temperature,
eventually Absorption = Emission
occures
Radiative equilibrium temperature of the Earth
• A simplest zero-dimensional energy balance model can be
formulated as belows (assuming black-body)
(1 − α ) S / 4 = σ T e
4
Incoming S energy
Earth’s radiation
• In case α=0.3 (albedo), S=1370Wm-2
Te≈255K
• But observed Ts≈288K, ∆T = 33K => greenhouse effect!
Venus, α=0.7,
α=0 7 S=2619Wm-2, Te≈242K Æ Ts
• In case of the planet Venus
≈730K, ∆T = 488K (!)
27 October 2008
Climate Change Modelling
Jee-Hoon Jeong
18
What happens to Incoming Solar
Radiation
1. Reflection and Scattering
2. Absorption by Earth’s surface and
atmosphere
3. Radiation emitted from Earth surface
* Convection and latent heat
1) Reflection and scattering by
atmosphere
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.
*Albedo
Albedo in Earth
Earth’s
s surface
2) Absorption
2) Absorption
- selective absorbers and the
atmospheric greenhouse effect
Many selective absorbers in the environment.
O3, O2, H2O absorb
some solar radiation
CO2 is
i nott a good
d
absorber of solar
radiation, but a good
absorber of Earth’s
radiation
3) Radiation from Earth
Insolation
Sunlight warms the earth's
earth s surface only during the day,
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
f
radiation (IR);
( ) incoming energy from
f
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.
3) Radiation from Earth – IR absorption by atmosphere
Greenhouse effect
With greenhouse gases, the earth's surface receives energy from the
sun and infrared energy from its atmosphere. Incoming energy still
equals
equa
s outgo
outgoing
ge
energy,
e gy, but tthe
e added IR e
energy
e gy from
o tthe
eg
greenhouse
ee ouse
gases raises the earth's average surface temperature about 33°C, to a
comfortable 15°C.
3) Radiation from Earth, IR absorption by atmosphere
Absorption
p
of radiation by
yg
gases in the atmosphere.
p
The shaded area
represents the percent of radiation absorbed
Water vapor and carbon dioxide (CO2) are strong absorbers of IR
Radiation and poor absorbers of visible solar radiation.
Total heat budget
*Convection and latent heat release
If convection were to suddenly stop - the average
Earth temp would rise about 18F (? C?)
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
a
g occu
occurs
s du
during
g co
condensation
de sat o as latent
ate t heat
eat is
sg
given
e up to tthe
ea
air inside
s de
the cloud. Most absorption takes place near the surface – lower atmosphere is
mainly heated from below.
Latent Heat
Change of State (Phase Change)- changes
from gas…liquid…solid
gas liquid solid (ice)
• The heat/energy required to change a substance (water),
from one state to another is called latent heat. (Why?)
When a cloud forms from water vapor
vapor, it warms atmosphere
through releasing latent heat. Namely, additional heat can be
transferred as a form of latent heat.
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
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
crystals. In fact
fact, for the duration of this storm alone
alone,
more heat energy is released inside this cloud than is unleashed by a small nuclear bomb.
Break?!
This chapter discusses:
• Temperature and heat transfer
– Conduction,
Conduction convection
convection, radiation
• Solar radiation, earth energy balance
• The seasons and temperature
variations
Controls of Temperature
•
•
•
•
•
Latitude (colder near the poles
poles, warmer near equator)
Geographic position plus land and water distribution
Ocean currents (warm/cold currents
currents, upwelling)
Elevation (air cools with increased elevation)
Cl d cover and
Cloud
d albedo
lb d
January
July
Why do we have seasons?
• The Earth has an elliptical path around the sun that
takes a little over 365 days - revolution
• One spin on its own axis in 24 hours - rotation
e age distance
d sta ce from
o ea
earth
t to su
sun is
s ~150
50 million
o
• Average
kilometers, varies from 147.3 to 152.1
• Elliptical path takes us closer to sun in January than
it does in July – Say what?
• Seasons are mostly regulated by sun angle and the
number of daylight hours
perihelion
aphelion
The elliptical
p
p
path ((highly
g y exaggerated)
gg
) of the earth about the
sun brings the earth slightly closer to the sun in January than in
July.
=> Minor role in producing seasonal temperature variations
Tilted Earth’s rotation axis
=> Changes
g in angle
g of sun,, daylight
y g length
g
As the earth revolves about the sun, it is tilted on its axis by an angle of 23.5° (inclination of the axis).
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
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.)
The changing position of the sun, as observed in middle latitudes in the Northern Hemisphere.
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.
D il Temperature
Daily
T
t
Variations
V i ti
• Air temperature is a very important weather
element Impacts us every day
element.
day. If it is warm
we don’t always mind the rain, but if its
cold
cold…..Or
Or if it is 40
40°C
C outside…
outside
• A sunny day has its own cycle of warming
and cooling.
• Temperature lag – Sun directly overhead at
noon but noon is not warmest point of a sunny
noon,
day. Why?
What about
precipitation
cycle?
The daily variation in air temperature is controlled by incoming energy (primarily from the sun)
and outgoing energy from the earth's
earth s surface
surface. Where incoming energy exceeds outgoing
energy (orange shade), the air temperature rises. Where outgoing energy exceeds incoming
energy (blue shade), the air temperature falls.
On a sunny, calm day, the air near the surface can be substantially warmer than the air a
meter or so above the surface.
surface Sunlight warms ground and ground warms air very near
the surface of the earth. [EX: Higher Field temperatures than air temperatures. Higher
track temperatures than air temperatures.]
Nighttime Cooling
• As sun lowers, its energy is spread over a larger area,
which reduces the heat available to warm the ground.
g
• Sometime in the late afternoon or early evening, the
earth’s surface and air above begin to lose more energy
th they
than
th receive,
i
th
they start
t t to
t cool.
l
• Radiational Cooling – ground and air above cool by
radiating infrared energy
energy. The ground
ground, being a much
better radiator than air, is able to cool more quickly.
Shortly after sunset the earth’s surface is slightly cooler
th the
than
th air
i directly
di tl above
b
it ((what
h t iis thi
this?).
?)
• By late night or early morning the coldest air is next to
the ground with slightly warmer air above
above.
On a clear, calm night, the air near the surface can be much colder than the air above. The increase
in air temperature with increasing height above the surface is called a radiation temperature
inversion
Radiation Inversion
Strong radiation inversion occurs when the
air near the ground is much colder than the
air higher up.
Ideal conditions:
• Calm conditions – no mixing
• Long nights – more time for radiative cooling
• Dry air,
air cloud
cloud-free
free – clear skies allow maximum
cooling at the surface
Cold air
On cold, clear nights, the settling of cold air into valleys makes them colder than
surrounding
di hill
hillsides.
id
Th
The region
i along
l
th
the side
id off th
the hill where
h
th
the air
i ttemperature
t
is above freezing is known as a thermal belt
Orchard heaters circulate the air by setting up convection currents.
Wind machines mix cooler surface air with warmer air above.
The daily range of temperature decreases as we climb away from the earth's surface. Hence,
there is less day-to-night variation in air temperature near the top of a high-rise apartment
complex than at the ground level.
Air Temperature and
Human Comfort
Air ttemperatures
Ai
t
can ffeell diff
differentt on different
diff
t occasions.
i
15°C in March can feel warm after a long winter, but can
feel cool on a summer afternoon.
Sensible temperature – temperature we perceive
*Humidity, evaporation – we feel much warmer in humid
30°C
30
C than in dry 30°C
30 C
*Wind – windy 0°C vs. windless 0°C
Wind Chill factor – how cold the wind makes us feel
Instrument Shelter
• Why we need this?
• To protect
p
–
–
–
–
Wind
Direct solar radiation
Radiation from sfc
Precipitation (water)
Dif. of Daily,
Dif
Daily Monthly and Yearly
Temperatures
• Daily diurnal range of temperature – difference
between the daily maximum and daily minimum
• Mean daily temperature – average of the highest and
l
lowest
temperature ffor a 24 h
hour period.
i d
• Annual range of temperature – difference between
th average ttemperature
the
t
off the
th warmestt and
d coldest
ld t
months
• Mean annual temperature – average temperature of
a station for the entire year.
Temperature data for San Francisco, California (37°N), and
Richmond, Virginia (37°N) - two cities with the same mean annual
temperature.
Thanks!
Aurora – caused by charged particles from the sun interacting with the atmosphere
atmosphere.
Solar wind collides with atmospheric gases. Gases get “excited” and emit light
aurora borealis in NH and aurora australis in SH
When an excited atom, ion, or molecule de-excites, it
can emit visible light. The electron in its normal orbit
becomes excited by a charged particle...
...and
and jumps into a higher energy level
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
e e you would
ou d most
os likely
ey
where
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
clear.))
The flag MN denotes the
magnetic north pole,
whereas the flag NP
denotes the geographic
north pole.
*Photons
•
Think of radiation as streams of particles, or
photons,, that are discrete packets
p
p
of energy.
gy UV
photons carry more energy than a photon of visible
light. UV photons produce sunburns, penetrate
skin,
ki can cause cancer.
Light has characteristic of a particle as well
as a wave.
wave
‘photon’ represents the ‘particle’ side of light.
‘‘short
h t wave’’ or ‘l‘long wave’’ representt th
the
wave side of light.