Transcript Chapter 2

Chapter 2
The Energy Cycle
Force, Acceleration, Work, Energy
• The force exerted on an object is the mass of
the body multiplied by the acceleration the force
causes in the body
• Acceleration is a change in speed or a change in
direction of an object’s movement
• Work is done on an object when a force moves
it—work is the distance travelled multiplied by
the force in the direction of that movement
• Energy is the capacity to do work
Four Kinds of Energy
• Kinetic energy is work that a body can do by
virtue of its motion
• Potential energy is work that a body can do as a
result of its relative position
• Electrical energy
• Heat energy
The Air Parcel
• Is a hypothetical balloon-like bubble of air
• Is isolated from its surroundings (the
environment) by an invisible, flexible,
impermeable imaginary boundary
• Mass and energy do not cross the boundary
• The air parcel expands and contracts to
maintain the same pressure as its environment
• The air parcel often has a different temperature
and humidity than its environment
Temperature
• Is related to energy
• Is a measure of the average kinetic energy of a
substance
• Is measured using three commonly used scales
– Fahrenheit, 180° between the melting point of ice
(32°) and the boiling point of water (212°)
– Celsius, metric, 100° between the melting point of ice
(0°) and the boiling point of water (100°)
– Kelvin, metric, Celsius + 273.15°, used in scientific
formulas
Figure 01: The three scales of temperature
Units for Energy and Power
• A calorie is the energy needed to raise the
temperature of 1 gram of water 1°C
• 1 joule (J) = 0.2389 calories
• Power is the rate at which energy is transferred,
received, or released
• 1 watt (W) = 1 joule of energy per second
• Energy flow is measured in watts per square
meter of area (W/m2)
Heating
• Heating is energy transfer from an object at a
higher temperature to an object at a lower
temperature
• The temperature change of an object depends
on
– How much heating is occurring
– The amount of matter in the object gaining energy
– The specific heat of the substance
• The amount of energy required to increase the
temperature of 1 gram of the substance 1°C
Figure T01: The specific heat of a substance is the amount of heat required
to increase the temperature of 1 gram of the substance 1° C
Transferring Energy in the Atmosphere
• Conduction is transfer of energy from molecule
to molecule—it requires touching
– Important at and near the ground
• Convection is warm air rising and cold air
sinking
• Advection or temperature advection is horizontal
transport of energy similar to convection
• Latent heating is energy transferred when water
changes phase
• Radiation is transfer by electromagnetic waves
Figure 02: Heat transfer by conduction
Figure 03: A candle demonstrates convection
Figure 04: Cold air advection
Latent heating
• Latent heating is energy absorbed or released
per unit mass when water changes phase
• Energy absorbed
– Solid to liquid (melting) 80 calories per gram
– Liquid to gas (evaporation) 500-600 calories per gram
– Solid to gas (sublimation) 580-680 calories per gram
• Energy released
– Gas to liquid (condensation)
– Liquid to solid (freezing)
– Gas to solid (deposition)
Figure 05: Phase changes of water
The Energy Cycle
• Starts with the sun, which radiates mainly short
waves on account of its extremely high
temperature
• Continues with the propagation at the speed of
light through space of energy in those waves
• Continues at the top of the atmosphere where
several different things happen to the short-wave
radiation
Figure CO: Chapter 2, The Energy Cycle--Sun and flares
Courtesy of SOHO/Extreme Ultraviolet Imaging Telescope (EIT)
consortium. SOHO is a project of international cooperation between ESA
and NASA
Radiation
• Is electromagnetic energy
• Has the form of electromagnetic waves in transit
– Wavelength is the distance between wave crests
• Solar radiation is short-wave radiation
• Long-wave radiation is emitted by the Earth and the
atmosphere
– Amplitude is half the height from crest to lowest point
• Takes the form of particles (photons) when
absorbed or emitted
Figure 06: Representation of a wave
Figure 07: Electromagnetic energy spectrum
Laws of Radiation
• All objects emit radiation
• Objects at higher temperatures emit much more
radiation than objects at lower T
– Stefan-Boltzmann Law
• Objects at higher T emit most of their energy at
shorter, more energetic wavelengths than objects at
lower T
– Wien’s Law
• A good absorber of radiation is also a good emitter
of radiation at that same wavelength
– Kirchhoff’s Law
Figure 08: Energy curves for three different temperatures.
Figure 09: Mad holding a match – infrared image
Courtesy of C. Brinkworth/NASA/JPL-Caltech
Amount of Radiation Absorbed
• Depends on several factors
– The radiative properties of the material
• Most objects are selective in which wavelengths they
absorb
– How long the object is exposed to emitted energy
• More time, more energy
– The amount of material
• Increasing thickness increases absorption
– How close the object is to the source of energy
– The angle at which the radiation is striking the object
The Greenhouse Effect
• Involves emission and absorption of infrared
(longwave) radiation by Earth and the
atmosphere.
• Involves selective absorption, capability of
greenhouse gases in the atmosphere to absorb
and emit longwave radiation, but only at
selected wavelengths. Without the greenhouse
gases, this radiation would be transmitted to
space
Radiation Interacting with Objects
• Absorption of radiation increases the energy of
the molecule
– If the energy is high enough (UV), chemical bonds
can be broken—called photodissociation
• Reflected energy is sent back
– Albedo is the percentage of reflected light
• Transmitted energy passes through an object
– It may change direction
Figure 10A: Venus
© 2010, Calvin J. Hamilton
Figure 10B: Earth
Courtesy of NSSDC/NASA
Figure 10C: Mars
Courtesy of NASA and The Hubble Heritage Team (STScI/AURA)
Figure B01_1: Production of O3 in the atmosphere
Figure B01_2: Destruction of O3 in the atmosphere
Figure B01_3: CFC contributes to the breakdown of O3 in the atmosphere
Figure B01_4: The hole in the ozone layer
Courtesy of NASA
The Sun and the Seasons
• The Sun is closer to the Earth in January than in
July
• Seasons are caused by the tilt or angle of
inclination of the Earth’s axis (22.5°)
• The axis always points in the same direction—
towards the North Star
• The Sun’s rays strike the Earth most directly at
noon, in the tropics, and in the summer season
• Daylight lasts longer in the summer season
Figure 11: The Earth’s tilt, or angle of inclination, determines the amount of
solar energy a given region of Earth receives as it orbits the Sun.
Figure 12: Sunlight hitting the Earth at three times of year
Figure 13: Solar zenith angle
Figure 14: Intensity of light on a surface when the solar zenith angle is
greater than zero
Figure 15: Path of the sun
Figure 16: Daylight hours versus month for different latitudes
Variations in Solar Energy
• The variation of solar energy at the surface by
latitude is cause by
– Changes in the angle that the Sun’s rays hit the Earth
• Called the solar zenith angle (0° with Sun overhead)
• Depends on time of day, time of year, and latitude
– The number of daylight hours
• Depends on latitude and time of year
– The amount of atmosphere the Sun’s rays have to
pass through
• More near sunrise and sunset, at higher latitudes, in
winter season, when the sun is closest to the horizon
The Solar Constant
• Isn’t really a constant
• Is the average amount of solar energy that
reaches the outer limits of our atmosphere on a
surface that is perpendicular to the solar rays
• Is about 1,368 watts per square meter
Figure T02: The solar zenith angle at local noon, the number of daylight
hours, and the average incoming solar energy for four different latitudes on
different days of the year
Figure 17: Solar energy at the top of the atmosphere
Figure 18: Sunlight incident at a higher solar zenith angle
The Greenhouse Effect
• Involves emission and absorption of infrared
(longwave) radiation by Earth and the
atmosphere.
• Involves selective absorption, capability of
greenhouse gases in the atmosphere to absorb
and emit longwave radiation, but only at
selected wavelengths. Without the greenhouse
gases, this radiation would be transmitted to
space
Radiative Properties of the Atmosphere
• Atmospheric gases are selective absorbers of
solar radiation
– Ozone absorbs UV radiation and a small amount of
visible radiation
– Water vapor, carbon dioxide, and the other
greenhouse gases absorb longwave radiation
– Gases absorb weakly or not at all in the atmospheric
window of longwave radiation
– Clouds are good reflectors of solar radiation and good
emitters and absorbers of longwave energy
Figure 19: Absorption of shortwave (left) and longwave (right) radiation by
the atmosphere
Figure UN01: N2O absorption
Figure B02: In IR radiometric images, cold objects (such as high clouds)
are white, and hot surfaces (such as deserts) appear black.
Courtesy of SSEC, University of Wisconsin-Madison.
More on the Greenhouse Effect
• It gets it name from a greenhouse, that uses
glass to receive maximum solar radiation. The
comparison is not exact.
• The atmospheric greenhouse effect is a series of
absorptions and emissions of infrared radiation
from/to different layers of the atmosphere and
Earth.
• Overall, the temperature is higher than it would
be without the greenhouse effect.
The Greenhouse Effect and the Energy
Budget
• Longwave radiation is absorbed and emitted by
Earth, the atmospheric greenhouse gases, and
cloud particles.
• The greenhouse effect recycles energy and acts
like a blanket to warm Earth and atmosphere.
• Radiation alone cannot balance the Earth’s or
the atmosphere’s energy budget.
Figure 20: Annual average energy budget of Earth
Other contributions to the energy
budgets of Earth and air
• Radiation overall cools the atmosphere.
• Radiation overall warms the Earth.
• Without some other process, the Earth would
warm and the atmosphere would cool more
every year.
• Two other processes transfer energy—
conduction and convection. Both transfer
energy from Earth to the atmosphere.
The other energy transfer processes
• Conduction transfers energy by direct
molecular contact
– In the energy budget, it overall transfers a small
amount of energy from Earth to atmosphere—Why?
Because on average the surface is warmer than the
air in contact with the surface
• Convection transfers energy by the movement
of molecules in large groups
– On average, convection transfers energy from the
Earth to the atmosphere
• Conduction and convection are sensible heating
Another important energy transfer
process—Latent heating
• When water changes phase/form between solid,
liquid, and/or gas, energy is transferred between
the water and its surroundings.
• Evaporation and condensation involve great
quantities of energy.
• This energy is called Latent heating
• On average, latent heating transfers energy from
the Earth to the atmosphere
Why advection is important
• It transfers energy in both the atmosphere and
the oceans.
• Advection transfers energy from the surplus in
the tropics to the deficit at the poles.
• Why is there a surplus and a deficit?
• The tropics receive more energy from the sun
than higher latitudes and the poles.
Figure 21: The radiation budget of the planet as a function of latitude
Radiative Forcing
• Is the change in the net radiation at the
tropopause
• Is defined by the IPCC (Intergovernmental Panel
on Climate Change)
• Is with respect to some reference state of the
Earth
• Has units of watts per square meter
• A positive value tends to warm the system
• A negative value tends to cool the system