Transcript AMS Weather Studies

AMS Weather Studies
Introduction to Atmospheric Science, 5th Edition
Chapter 3
Solar & Terrestrial
Radiation
© AMS
Driving Question
 How does energy flow into and out of the Earthatmosphere system maintain Earth as a habitable planet?
 This chapter covers:
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Electromagnetic radiation and the laws that govern it
How this reacts with the Earth-atmosphere system
Conversion of solar radiation to heat
Earth emission of infrared radiation
The greenhouse effect
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Case-in-Point
Ancient Astronomical Calendars
 Recurring patterns of seasons and seasonal
change have been important to humans since
the beginning of their existence.
 Stonehenge
 Earliest portions date to 2950 BC
 Aligned to summer solstice and mid-winter sunset
 Predicts solar and lunar eclipses
 Native Americans near present-day St. Louis
 Wooden posts arranged in circles (Woodhenge
calendars)
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Case-in-Point
Ancient Astronomical Calendars
 Nubian Desert of southern Egypt
 Predates Stonehenge by 2000 years
 Chankillo near Lima, Peru
 2300 year-old Peruvian ruin
 These devices predict important events
Model of Nabta calendar in
the Aswan Nubia museum
Sunrise on the
June solstice at
Chankillo, Peru
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The Electromagnetic Spectrum
 Electromagnetic radiation
 Energy transmitted through space or
materials as waves (solar radiation),
 Both electric and magnetic properties
 Wavelength
 Distance between successive wave
crests or troughs
 Wave frequency
 Number of wave crests that pass a given
point per second (hertz, Hz)
 Inversely proportional to wavelength
 Speed of electromagnetic radiation
 300,000 km/sec (186,000 mi/sec)
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 Commonly called the “speed of light”
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The Electromagnetic Spectrum
 Electromagnetic Spectrum:
 Radio waves
 Wavelength: from a fraction of a centimeter to hundreds of kilometers
 Frequency up to a billion Hz
 FM: 88 million to 108 million Hz
 Microwave radiation
 Wavelength: 0.1 to 1000 mm
 Microwave ovens
 Some used for radio communication (weather radio)
 Ultraviolet
 Beyond violet, short-wave radiation
 Visible radiation
 Portion of the spectrum perceptible to the human eye
 Violet end: 0.40 micrometer (one millionth of a meter)
 Red end: 0.70 micrometer
 Infrared
 Below red, long-wave radiation
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The Electromagnetic Spectrum
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Radiation Laws
 Blackbody
 At a constant temperature, it absorbs all radiation it receives and emits
all the energy it absorbs.
 Perfect absorber and a perfect emitter.
 Surfaces of real objects approximate blackbodies for certain
wavelengths of radiation
 Mathematical laws made simple
 Wavelength of most intense radiation emitted by a blackbody is inversely
proportional to its absolute temperature (Wien’s displacement law)
 Both Sun and Earth nearly blackbodies
 Sun is much hotter than the Earth, therefore, its most intense radiation is at a
much shorter wavelength than Earth’s
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Radiation Laws
 Wien’s Displacement Law
λmax = C/T
 Where λmax is the
wavelength of most
intense radiation
 C is a constant of
proportionality
 T is absolute temperature
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Radiation Laws
 Total energy flux (E) emitted by a blackbody across all wavelengths is
proportional to the 4th power of its absolute temperature (T)
E ~ T4
Flux of solar radiation at the top of the
atmosphere
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Earth
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Inverse Square Law
Doubling the distance
from the Sun reduces
solar radiation by ¼.
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Input of Solar Radiation
 Sun
 Composed of hydrogen (H) and helium (He)
 Source of solar energy is nuclear fusion reaction
 4 H protons fuse to form 1 He nucleus
 Excess mass in this fusion is converted to energy, E = mc2
 Some of energy is used to bond He nucleus
 Rest is radiated off to the Sun’s surface, then space
 Photosphere (visible surface of the Sun) cooler than interior
 Convective cells called granules
 Sunspots = cool areas on the Sun’s surface
 Accompanying bright areas called faculae
 Changes in numbers of sunspots/faculae affect Earth’s climate
 Chromosphere
 Sun’s atmosphere of superheated gases, mostly H and He
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 Solar corona – the outermost portion
of the Sun’s atmosphere
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Input of Solar Radiation
 Solar Altitude
 Solar radiation directly
overhead concentrates solar
energy in a small area
 Flashlight A
 Solar radiation at an angle
spreads the solar energy over
larger area
 Flashlight B
 Concentrated energy provides
for more heat per unit surface
area, hotter ground
temperatures
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Input of Solar Radiation
 Solar Altitude
 The noon solar altitude always
varies with latitude
 Earth presents a curved surface to
the incoming solar beam
 Ex: at equinox, the solar altitude
is 90 degrees at the equator
 decreases with latitude (poleward)
 Noon solar radiation striking
horizontal
 most intense at equator
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Input of Solar Radiation
 Solar Altitude
 Atmosphere not completely
transparent to solar radiation
 Incoming solar radiation has
more atmosphere to pass
through at low angles of
incidence.
 Low angles of incidence
allow for more atmospheric
scattering, reflection, and
absorption
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Input of Solar Radiation
 Solar Altitude
 Intensity of solar radiation striking local surfaces varies over the
year
 Inclination of Earth’s axis
 When the North Pole is tilted toward the Sun, the Northern
Hemisphere receives more solar radiation.
 This is spring or summer in the Northern Hemisphere.
 When the North Pole is tilted away from the Sun, the Northern
Hemisphere receives less solar radiation.
 This is fall or winter in the Northern Hemisphere.
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Input of Solar Radiation
 Earth’s motion in space and the seasons
 June 21 solstice, Sun is directly overhead the Tropic of Cancer
 23.5° N latitude
 Beginning of Northern Hemisphere summer
 September 23 equinox, the Sun is directly overhead the equator
 0° latitude
 Beginning of Northern Hemisphere fall
 December 21 solstice the Sun is directly overhead the Tropic of
Capricorn
 23.5° S latitude
 Beginning of Northern Hemisphere winter
 March 21 equinox, the Sun is directly overhead the equator
 0° latitude
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 Beginning of Northern Hemisphere spring
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Input of Solar Radiation
Perihelion and Aphelion
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Input of Solar Radiation
Northern
Hemisphere
tilted
away from
the Sun
Northern
Hemisphere
tilted
toward
the Sun
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Circle of
Illumination
Equinox
N. Hemisphere summer solstice
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N. Hemisphere winter solstice
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Path of the Sun
at the equator
At N. Hemisphere midlatitudes
At the North Pole
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North Pole webcam
 http://www.arctic.noaa.gov/gallery_np.html
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North Pole webcam
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Variation in the length of daylight increases with increasing latitude.
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Input of Solar Radiation
 Solar constant
 Rate at which solar radiation falls on a surface located at the outer edge of
the atmosphere, oriented perpendicular to the incoming solar beam, when
Earth is a mean distance from the Sun
 Averages about 1.97 calories per square cm per min
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Distribution of
solar radiation
received at the
top of the
atmosphere by
latitude and
day of year.
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Solar Radiation and the Atmosphere
 Solar radiation
 Passing through the Earth’s atmosphere interacts with
gases and aerosols via scattering, reflection, and absorption
 Law of energy conservation
 Within the atmosphere,
 + % solar radiation absorbed (absorptivity)
+ % scattered or reflected (albedo)
+ % transmitted to Earth’s surface (transmissivity)
= 100%
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Solar Radiation and the Atmosphere
 Scattering
 Particles disperse solar radiation
 Wavelength dependent
 Preferential scattering of blue-violet light by O2
and N2 molecules
 This is why daytime sky is blue
 Reflection
 Special case of scattering
 Takes place at the interface between two media
when the radiation striking that interface is
redirected (backscattered)
 Fraction of incident radiation backscattered by
airborne particles or reflected by a surface is the
albedo of that surface
 Albedo = (reflected radiation)/(incident
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Atmosphere viewed from space
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Stratospheric Ozone Shield
 Absorption
 Converts radiation to heat
energy
 UV absorbed in stratosphere
 Chemical reactions involved
in formation and dissociation
of ozone
 Significantly reduces the
intensity of UV that reaches
Earth’s surface
 Causes marked warming of
upper stratosphere
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The
Stratospheric
Ozone Shield
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Stratospheric Ozone Shield
 Chemicals in the ozone layer
 From natural and industrial sources.
 Enter the stratosphere through deep tropical convective currents
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Stratospheric Ozone Shield
 Antarctic Ozone Hole
 Circumpolar vortex cuts off Antarctic atmosphere
 Loses ozone through absorption of UV radiation
 Circumpolar vortex weakens in spring
 Warmer ozone rich air invades, replenishes ozone
 Cold Antarctic stratosphere, with stratospheric ice, accelerates the
reaction with CFCs as a catalyst
 No comparable ozone hole in Arctic due to warmer temperatures and
weaker circumpolar vortex
 The Montreal Protocol was an international agreement to limit CFC
production
 Violators receive economic sanctions
from other signing countries
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The Antarctic Ozone Hole
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The Antarctic ozone
hole on 24 Sept
2006, when it tied
with 9 Sept 2000 as
the largest area, at
29.5 million square
km (11.4 million
square mi).
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Solar Radiation and the Earth’s Surface
 Albedo
 Lighter the surface, higher
the albedo
 Varies with solar altitude
 Water has highest albedo at
lowest solar altitude
 Near 100% at sunrise and
sunset
 Decreases rapidly as solar
altitude increases
 Global average oceanic
albedo = 8%
 92% of solar energy reaching
oceans is absorbed
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Solar Radiation and the Earth’s Surface
Lighter the surface, higher the albedo.
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Solar Radiation and the Earth’s Surface
Water absorbs
red light more
efficiently,
while greener
and bluer light
scatter to our
eyes,
explaining the
color of the
open ocean.
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Global Solar Radiation Budget
 Earth’s surface principal recipient of solar heating and main source
of heat for the atmosphere
 Evident in the vertical profile of the troposphere
 Global radiative equilibrium
 Solar radiational heating of the Earth-atmosphere
system is balanced by
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emission of heat to space in the form of infrared radiation
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Outgoing Infrared Radiation
 Greenhouse effect
 Heating of Earth’s surface and lower atmosphere by strong
absorption and emission of IR by greenhouse gases.
 Earth emits IR (long wave) while the Sun emits UV and visible
radiation (short wave)
 Greenhouse gases transparent to short-wave radiation, absorb
long-wave radiation.
 Same net effect as a greenhouse
 Allows shortwave radiation through
glass while the glass strongly absorbs
and emits infrared radiation
 Warms the greenhouse
 Earth is kept warm by greenhouse
gases
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 Without, life as we know it would not exist
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Outgoing Infrared Radiation
Absorption of
radiation by
greenhouse
gases within
the
atmosphere.
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Outgoing Infrared Radiation
 Callendar effect
 Theory that global climate change can be brought about by
enhancement of Earth’s natural greenhouse effect by
increased levels of atmospheric CO2 from anthropogenic
sources
 Systematic monitoring of carbon dioxide began in 1957
 Keeling curve (Mauna Loa record)
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Outgoing Infrared Radiation
A. Mauna Loa record to date
B. Monthly mean values
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Globally averaged
atmospheric methane
concentration, from
1983 to 2011.
Globally averaged growth
rate of atmospheric
methane, from 1983 to
2011.
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Average
atmospheric
concentration of
nitrous oxide
beginning in 1978.
Average
atmospheric
concentration of
CFC-11 and CFC-12
beginning in 1978.
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Outgoing Infrared Radiation
 Possible impacts of global warming
 Climate zones may shift poleward by as much as 550 km (350 mi)
 Heat and moisture stress would cut crop production in certain areas
 Possible farming at higher latitudes.
 Rising sea levels of 20-60 cm (8-24 in.) during the 21st century
 Inundation of low islands and coastal plains
 Many are heavily populated
 Decreased snow cover and sea-ice extent
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Average Annual
Temperature
Departures from the
Long-Term Average
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Outgoing Infrared Radiation
 Agreement that action should be taken to head off possible
enhanced greenhouse warming
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Sharply reduce oil and coal consumption
Have greater reliance on non-fossil fuel energy sources
Have higher energy efficiencies
Halt to deforestation, massive reforestation
 Even if it were not for enhanced greenhouse warming, doing
this would help other problem areas
 Example: cutting air pollution
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Monitoring Radiation
 A pyranometer measures
the intensity of solar
radiation that strikes a
surface
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