Transcript Temperature Scales

Warming the Earth
and the Atmosphere
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Temperature and heat transfer
Balancing act - absorption,
emission and equilibrium
Incoming solar energy
Temperature and Heat
Transfer
Temperature and Heat are NOT the same thing!
•Temperature is the average kinetic energy of a group of particles
(atoms or molecules).
•Heat is a quantity of energy.
•“Heating” is sometimes used to denote a change in temperature –
NOT IN GES 241! Adding or subtracting a quantity of heat energy may
OR MAY NOT result in a temperature change.
Temperature Scales
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kinetic energy, temperature and heat
Kelvin scale
Celsius scale
Fahrenheit scale
temperature conversions
• Every temperature scale has two physically-meaningful
characteristics: a zero point and a degree interval.
Fig. 2-2, p. 27
Temperature as Average
Kinetic Energy
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Atoms/molecules have mass and move at some speed,
thus have kinetic energy:
KE = ½ mv2
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Too many particles to keep track of all those individual
kinetic energies (one for each particle)
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Temperature is the average kinetic energy of all the
particles in a substance
Ideal Gas Law
Ideal Gas Law
p = ρR T
p: gas pressure
T: gas temperature
ρ: gas density
R is a constant for any given mixture of
gases but changes from gas to gas. For
dry air, R = 287 J kg-1 K-1
Phases of Matter
Phases of Matter
Phases and Pressure
• Phase of a substance depends on both
temperature and pressure
• Often more than one phase is present
Phase Changes
• Ionization: Stripping of
electrons, changing atoms into
plasma
• Dissociation: Breaking of
molecules into atoms
• Evaporation: Breaking of
flexible chemical bonds,
changing liquid into solid
• Melting: Breaking of rigid
chemical bonds, changing
solid into liquid
Latent Heat - The Hidden
Warmth
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phase changes and energy exchanges
sensible heat
• Latent heat explains why your skin feels cold when
you step out of a warm shower, and why perspiration
is an effective way to cool your body.
Stepped Art
Fig. 2-3, p. 28
Conduction
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conduction and heat transfer
good conductors and poor conductors
• Why are feathers (down) used in winter parkas?
Convection
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convection and heat transfer
thermals
• Soaring birds, like hawks and falcons, are highly
skilled at finding thermals.
Radiation
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radiation and energy transfer
electromagnetic waves
Wein’s law
Stefan-Boltzmann law
Selective absorption of radiation:
Greenhous Gases
What is light?
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Light can act either like a wave or like a
particle
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Particles of light are called photons
Waves
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A wave is a
pattern of
motion that
can carry
energy without
carrying matter
along with it
Properties of Waves
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Wavelength (λ) is the distance between two
wave peaks
Frequency (ν) is the number of times per
second that a wave vibrates up and down
Wave Speed (c) is the distance one point on the
wave travels in one second
c = λν
Light: Electromagnetic Waves
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A light wave is a vibration of electric and
magnetic fields
Light interacts with charged particles through
these electric and magnetic fields
Wavelength and Frequency
wavelength x frequency = speed of light =
constant
Fig. 2-7, p. 32
Radiation
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electromagnetic spectrum
ultraviolet radiation
visible radiation
infrared radiation
• Moderate amounts of ultraviolet radiation gives
you a healthy-looking tan; excessive amounts give
you skin cancer.
The Concept of Flux
Flux: the quantity passing through a unit area in a unit time
Example: Energy Flux in SI units.
Energy Flux = “the number of Joules of energy passing
through 1 square meter in one second”
Units of Energy Flux: Js-1m-2 = Wm-2
(Chalkboard Example)
Fig. 2-8, p. 34
Stefan-Boltzmann Law
“The hotter the object, the more it radiates.”
F=
4
σT
F: energy flux from body at temperature T (units: Wm-2)
T: temperature of body
σ: Stefan-Boltzmann constant, σ=5.67x10-8 Wm-2K-4
Stefan-Boltzmann Law
• Blackbody flux is the total area under
the curve.
• Fourth power means hot objects are
radiating MUCH more than cool ones
Example: 2 objects, one at 300 K and one at 600 K. One
object is twice as hot, but it radiates 16 times the energy
from each square meter of its surface than the cooler one.
Fig. 2-9, p. 34
Wien’s Law
“The hotter the object, the shorter
wavelength light it emits.”
λmax = C / T
λmax:
Wavelength of maximum blackbody emission (in
microns/micrometers, μm)
C: constant, C = 2898 μm K
T: body temperature in K
Wien’s Law
Example 1: Earth at 288 K
λmax = 2898 μm K / 288 K = 10 μm
10 microns is IR radiation. Earth radiates mostly in the
infrared.
Example 2: The Sun at 5778 K
λmax = 2898 μm K / 5778 K = 0.501 μm = 501 nm
501 nm is visible (green) radiation. The Sun radiates mostly in
the visible (green).
Balancing Act Absorption,
Emission, and
Equilibrium
Selective Absorbers and the
Atmospheric Greenhouse
Effect
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blackbody radiation
selective absorbers
atmospheric greenhouse effect
• The best greenhouse gas is water
vapor.
Particles of Light
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Particles of light are called photons
Each photon has a wavelength and a
frequency
The energy of a photon depends on its
frequency
Wavelength, Frequency, and
Energy
lν = c
 = wavelength ν= frequency
c = 3.00 x 108 ms-1 “speed of light”
E=hν
photon energy
h = 6.626 x 10-34 J s
Atomic Analogy: Child’s Playslide
(Chalkboard)
Energy Level Transitions
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Not Allowed
Allowed
(Hydrogen Atom)
The only
allowed
changes in
energy are
those
corresponding
to a transition
between
energy levels
Chemical Fingerprints
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We can plot those energy transitions as
frequencies (because of E=hν ) and therefore
as wavelengths (because of c=λν)
Each type of atom has a unique spectral
fingerprint
Energy Levels of Molecules
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Molecules have additional energy levels
because they can vibrate and rotate
Energy Levels of Molecules
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The large numbers of vibrational and rotational
energy levels can make the spectra of
molecules very complicated
Many of these molecular transitions are in the
infrared part of the spectrum
So how does the greenhouse effect work
then…
1. Atmosphere contains gases that selectively absorb IR
radiation, but transmit visible light (“greenhouse gases”)
2. Visible light (from the 5778 K Sun) travels through the
atmosphere and is absorbed by the surface.
3. The surface (≈ 300 K) re-radiates that energy upward as IR
light.
1. The IR energy gets absorbed by atmospheric GHGs.
2. The atmosphere radiates some upward to space, but also back
downward to the surface. This energy raises the temperature
of the ground.
The simplest greenhouse effect calculation you can do…
(Chalkboard
)
The Global Energy Budget
Just like a bank budget:
Bank:
Earth Energy:
In = Out + Savings
In = Out + Energy Storage
Energy can be stored in many ways, including
temperature can change.
Working definition of weather: “Weather is the
dynamical way in which the atmosphere
maintains the long-term global energy balance.”
The Bathtub Analogy for Energy Balance
(Chalkboard
)
Incoming Solar
Energy
The Earth’s Annual Energy
Balance
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What happens to the solar energy that
reaches the top of the earth’s
atmosphere?
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What happens to the solar energy that is
absorbed by the earth’s surface and by the
atmosphere?
Scattered and Reflected Light
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scattering
reflection
albedo
• Scattering is
responsible for the
blue sky color.
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
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earth-sun distance
tilt of the earth’s axis
• Earth-sun
distance has
little effect on
atmospheric
temperature.
Seasons in the Northern
Hemisphere
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insolation
summer solstice
spring and autumn equinox
Seasons in the Southern
Hemisphere
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tilt
solstice
equinox
Local Seasonal Variations
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slope of hillsides
vegetation differences
• Homes can exploit
seasonal variations:
large windows
should face south.
Air Temperature
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Daily temperature variations
The controls of temperature
Air temperature data
Air temperature and human comfort
Measuring air temperature
Daily Temperature
Variations
Daytime Warming
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thermals
forced convection
water vapor effects
• Cumulus clouds are markers of convection.
Nighttime Cooling
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radiational cooling
nocturnal inversions
• Inversions tend to occur on clear, calm nights.
Stepped Art
Fig. 3-1, p. 56
Cold Air Near the Surface
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inversions
thermal belts
• Drainage winds: cold air that slides downhill.
The Controls of Temperature
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latitude
land and water distribution
ocean currents
elevation
specific heat
• Average weather conditions in the interior of large continents
are much different than average conditions in coastal areas.
Daily, Monthly and Yearly
Temperatures
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diurnal temperature range
clouds and humidity effects
proximity to large bodies of water
annual temperature range
• Clouds tend to reduce daytime temperatures, but
increase nighttime temperatures.
Fig. 3-11, p. 65