CHAPTER #2
Radiation
Radiation: a form of energy that travels as waves without exchanging
mass in the presence or absence of a medium. Electromagnetic radiation
is the form of radiative energy that exhibits both electrical and magnetic
properties. It travels through space as well as through gases, liquids,
and solids. In a vacuum, electromagnetic radiation travel at its maximum
speed, 300,000 km (186,000 mi.) per second. As electromagnetic radiation
passes from one medium to another, it may be reflected or refracted at
the interface. It can also be absorbed and converted to heat.
Electromagnetic radiation travels as waves, which is usually described
in terms of wavelength and frequency. Wavelength is the distance
between successive crests or successive troughs of a wave. Wave frequency
is the number of crests or troughs that pass a given point in one second.
Passage of one complete wave is called a cycle, and a frequency of one
cycle per second equals one hertz (Hz). Frequency is inversely proportional
to wavelength; that is, higher the frequency, the shorter the wavelength.
Figure 2.2, page #34 (Moran and Morgan)
Electromagnetic spectrum: the range of different forms of electromagnetic
radiation arranged by wavelength and frequency. It ranges from wavelengths
of about 10-15 meter (or frequencies of 1024 Hz)
to wavelengths of about 104 meter (or frequencies of 104
Hz) and includes gamma radiation, X-rays, ultraviolet radiation, visible
light, infrared radiation, microwaves, and radio waves.
Gamma radiation ranges from about 10-11 meter to about
10-15 meter, while X-rays range from about 10-8 meter
to about 10-13 meter, having a substantial overlap with the
gamma radiation. Both are used to treat cancer patients.
Ultraviolet radiation (UV) ranges from 0.20 to 0.39 micrometer
(µm); 0.20-0.29 µm (UVC), 0.29-0.32 µm (UVB), and 0.32-0.39
µm (UVA). The sun emits 7% of its radiation in this zone. UVC is
absorbed by the ozone in the stratosphere, while UVB and UVA can be responsible
for sunburn and skin redness, respectively.
Visible light ranges from 0.39 µm at the violet end to
0.76 µm at the red end. The colors within the visible light are violet,
blue, green, yellow, orange, and red. Visible light stimulates the sensation
of color, and regulates the timing of animal activities such as migration,
and photosynthesis of plants. The sun emits nearly 44% of its radiation
in this zone.
Infrared radiation (IR) ranges from 0.76 µm to about 1
mm. It is responsible for all the terrestrial radiation emitted by the
Earth's surface and its atmosphere. The sun emits 49% of its radiation
in this zone; nearly 37% is radiated between 0.76 and 1.5 µm (near
IR), with only 12% radiated at wavelengths longer than 1.5 µm (far
IR).
Microwave radiation ranges from about 1 mm to about 1 meter.
It is used for the remote sensing of the atmosphere and also for microwave
ovens. Radio waves range from a fraction of centimeter up to hundreds
of kilometers, having a substantial overlap with the microwave radiation.
It is used for radio communication including FM (frequency of modulation)
waves which span from 88 to 108 million Hz.
Figure 2.1, page #34 (Moran and Morgan)
Blackbody: a hypothetical object that absorbs all the incident
radiation; that is, a perfect radiator neither reflects nor transmits any
radiation. In reality, no perfect radiators exist, but the sun and the
Earth are approximate blackbodies, therefore, blackbody radiation laws
may apply to the solar and terrestrial radiation. There are four blackbody
laws:
1) Planck's law states that the rate at which radiation is emitted
by a blackbody depends on the absolute temperature of the blackbody and
the specific wavelength (or frequency) of the radiation. It provides the
amount of emitted radiation at some wavelength at a specific absolute temperature.
2) Stefan-Boltzmann law states that the total energy radiated
by an object across all wavelength is proportional to the fourth power
of its absolute temperature. Since the sun radiates at a much higher temperature
than does the Earth-atmosphere system, the Stefan-Boltzmann law predicts
that the sun's energy output per square meter is about 160,000 times that
of the Earth-atmosphere system.
The total solar energy absorbed by planet Earth is equal to the total
terrestrial energy emitted by the Earth-atmosphere system back to space.
The balance between energy input and energy output is called global
radiative equilibrium.
3) Kirchhoff's law holds that a perfect absorber of radiation
of a given wavelength is also a perfect emitter of radiation at the same
wavelength (or frequency). In general, the efficiency of absorption, absorptivity,
equals the efficiency of emission, emissivity. The absorptivity
and emissivity of a black body are both 100%. The sum of emissivity, transmissivity,
and albedo is 100% following energy conservation law. For opaque substances,
such as ground, transmissivity is zero and the sum of emissivity and albedo
equals 100%.
4) Wien's displacement law holds that the wavelength at which
a blackbody emits the maximum intensity of radiation, lambdamax,
is inversely proportional to the absolute temperature, T, of the blackbody:
lambdamax = 2897 µm ûK / T
For the sun, with a surface temperature of 6000 ûK,
lambdamax = 2897 µm kK / 6000 kK = 0.5 µm
For the Earth, with a surface temperature of 288 ûK,
lambdamax = 2897 µm ûK / 288 kK = 10 µm
The sun emits radiation at wavelengths between 0.25 and 2.5 µm
peaking at a wavelength of about 0.5 µm, while the Earth emits radiation
at wavelengths between 4 and 24 µm peaking at a wavelength of about
10 µm. Therefore, the earth's radiation is often called longwave
radiation, whereas the sun's energy is refereed to as shortwave
radiation.
Figure 2.3, page #36 (Moran and Morgan)
Figure 2.4, page #36 (Moran and Morgan)
Sun: a gaseous body composed of hydrogen (about 80% by mass)
and helium. It is our closest star, about 150 million km (93 million mi)
from the earth. Based on the temperature of the region, the sun is divided
into four layers: core, photosphere, cromosphere, and the corona.
The core is extremely hot, with temperatures exceeding 20 million
ûC. The hydrogen nuclei (protons) collide at very high speeds that they
fuse together to form helium nucleus (alpha particle). In this reaction,
four hydrogen nuclei produce one helium nucleus, however, the mass of four
hydrogen nuclei is 0.7% greater than the mass of one helium nucleus. The
excess mass is converted to energy following the energy conservation equation
of E = mc2, where c is the speed of the light (300,000 km/second).
The photosphere is visible surface of the sun. It is much cooler
than the sun's interior, generally near 6000 ûC. The photosphere has a
honeycomb appearance that is due to a network of huge, irregularly shaped
convective cells, called granules. They have relatively cold spots,
sunspots, and relatively hot spots, faculae.
The cromosphere consists of ions of hydrogen and helium at temperatures
between 4000 and 40,000 ûC. It acts as a boundary between the photosphere
and the corona.
The corona is region of hot (1 to 4 million ûC) and highly rarefied
ionized gases that extends millions of kilometers into the space, the outer
limits of the solar system. It is visible during a solar eclipse. Because
of its low density, the corona radiates less energy than the photosphere.
The solar wind originates in the corona and the solar flares that erupt
from the photosphere into the corona intensify the solar wind.
Solar altitude: the angle of the sun above the horizon. It varies
with the time of the day and the latitude. When the sun is direcly overhead,
the solar altitude is 90û. The intensity of solar radiation decreases
with decreasing solar altitude since the solar radiation becomes less intense,
spreading over larger area at lower solar altitude. The path of solar radiation
also increases with decreasing solar altitude.
Figure 2.6, page #38 (Moran and Morgan)>
The earth makes one complete rotation on its axis approximately once
every 24 hours. As a result, at any point in time, half the planet is in
darkness (night) and the other half is illuminated by solar radiation (day).
The earth makes one complete rotation around the sun in one year, 365.25
days. The earth's orbit departures from a circular orbit, eccentricity,
therefore, the earth-to-sun distance varies by about 3.3% through the year.
Earth is closest to sun, 147 million km (91 million mi). on about January
3, perihelion, and farthest from the sun, 152 million km (94 million
mi), on about July 4, aphelion.
The earth is tilted by an angle of 23û27' with respect to normal to
the sun, obliquity. This tilt causes the earth's orientation to
change continually as the planet revolves about the sun and explains the
seasons. The Northern Hemisphere leans away from the sun during winter
and leans toward the sun during summer. When the Northern Hemisphere leans
away from the sun, the Southern Hemisphere leans toward the sun and vice
versa.
Figure 2.10, page #40 (Moran and Morgan)
The sun is directly over the equator at noon on March 21, Vernal
equinox, and on September 23, Autumnal equinox. Day and night
are equal length (12 hours) everywhere.
Figure 2.11, page #40 (Moran and Morgan)
The sun is directly over 23û27'N (Tropic of Cancer), its northern position,
summer solstice, at noon on June 21. The daylight is continuous
north of 66û33'N (Arctic Circle), while no daylight is present south of
66û33'S (Antarctic Circle). Elsewhere days are longer than nights in the
Northern Hemisphere, where it is summer, days are shorter than nights in
the Southern Hemisphere, where it is winter.
Figure 2.12, page #41 (Moran and Morgan)
The sun is directly over 23û27'S (Tropic of Capricorn), its southern
position, winter solstice, at noon on December 21. The daylight
is continuous south of 66û33'S, while no daylight is present north
of 66û33'N. Elsewhere days are shorter than nights in the Northern Hemisphere,
where it is winter, days are longer than nights in the Southern Hemisphere,
where it is summer.
Figure 2.13, page #41 (Moran and Morgan)
The total amount of solar radiation received at the earth's surface
varies seasonally. For the same intensity of solar radiation, any given
location on Earth accumulates more solar energy during long days of summer
than during short days of winter. The intensity of solar radiation, itself
also varies seasonally as the sun moves from its northern position (23û27'N)
on June 21 to its southern position (23û27'S) on December 21 and back again.
The solar radiation reaches its maximum where the sun is directly overhead
at local noon between 23û27'N and 23û27'S. The path of the sun through
the sky over the course of a year may be shown by the following diagram.
The seasonal (winter-to-summer) contrast in length of day increases with
increasing latitude.
Figure 2.14, page #42 (Moran and Morgan)
Figure 2.15, page #43 (Moran and Morgan)
Solar constant: the rate at which the solar radiation is received
at the top of the atmosphere perpendicular to the sun's rays when the earth
is at a mean distance from the sun. It is approximated as 1.97 calories
per cm2 in energy units or 1372 watts per m2 in power
units. The intensity of solar radiation is inversely proportional to the
square of the earth-to-sun distance such that the planet earth receives
about 6.7% more radiation at perihelion than at aphelion. Therefore, the
solar constant ranges from 2.04 cal/cm2 at perihelion and 1.91
cal/cm2 at aphelion.
As a result of perihelion/aphelion contrast in solar energy, the Southern
Hemisphere receives more radiation in summer and less radiation in winter
than the Northern Hemisphere. This may lead a greater winter-to-summer
temperature contrast in the Southern Hemisphere than in the Northern Hemisphere.
However, the relatively larger percentage of ocean surface area in the
Southern Hemisphere exhibits greater thermal stability, which modifies
the seasonal temperature and largely offsets the greater seasonal contrast
in insolation.
Reflection: a process whereby light bounces of a surface at an
angle equal to the angle at which it initially strikes the surface.
Scattering: a process whereby light is actually absorbed by a
particle and then quickly emitted in another direction. It is responsible
for the color of the daytime sky. Since air molecules are smaller than
wavelengths of visible light, they are more effective at scattering shorter
wavelengths (blue and violet) of visible light than the longer wavelengths
(red light). This property of being more effective at scattering particular
wavelengths of light is named as Rayleigh scattering, which is responsible
for the blue appearance of sky.
Another form of scattering called Mie Scattering, is responsible
for the white appearance of clouds. Mie scattering occurs when the wavelengths
of visible light are approximately, equally scattered. Water droplets and
ice crystals, in even small clouds, as well as dust and haze particles
effectively scatter all wavelengths of visible light in all directions,
thus making clouds appear white, and the sky is a hazy white in the presence
of high concentrations of aerosols.
Albedo: the ratio of reflected radiation to the incident radiation.
It is generally expressed as a percentage. The incident radiation is the
sum of direct and diffuse insolation. The light-colored surfaces have higher
albedo than the dark-colored surfaces. The albedo of fresh-falling snow
varies between 75% and 95%, while the albedo of alsfalt or dense forest
may be as low as 5%.
Cloud tops are the most important reflectors of insolation. The albedo
of cloud tops ranges from under 40% for thin clouds (less than 50 meter
thick) to 80% or more for thick clouds (more than 5000 meter thick). The
average albedo for all cloud types and thickness is about 50%, and clouds
cover about 60% of the planet at any given time.
Table 2.2, page #56 (Moran and Morgan)
The albedo of some but not all surfaces also varies with solar altitude.
The variation of albedo with solar altitude is especially pronounced for
ocean and lakes surfaces. The albedo of a water surface increases with
decreasing solar altitude under clear skies. The increase in albedo is
particularly sharp for solar altitudes less than 30û, approaching 100%
near sunrise and sunset.
On the other hand, when the sky is completely cloud covered, the variation
of the albedo with solar altitude is uniformly very low (less than 10%).
The average albedo of the ocean surface is only about 8% on a global basis;
that is, the ocean is a strong absorber of solar radiation.
Figure 2.21, page #56 (Moran and Morgan)
The surface albedo undergoes significant seasonal changes over land.
It increases as a result of snow cover and the formation of sea ice over
frozen lakes in winter and of loss of leaves in forested areas in autumn.
Planetary albedo: the fraction of solar radiation that is scattered
and reflected back to space by the earth-atmosphere system. It is about
31% as indicated by satellite measurements, while the moon's albedo is
only about 7% primarily because of the absence of clouds in the highly
rarefied lunar atmosphere.
Absorption: the process whereby a portion of the radiation incident
on an object is converted to heat. The amount of absorbed is generally
expressed as a percentage (one minus albedo). The earth-atmosphere system
absorbs 69% of solar radiation; 23% by the atmosphere and 46% by the earth'
surface, mainly due to low average albedo of oceans, covering 71% of the
globe.
Table 2.3, page #57 (Moran and Morgan)
Water vapor, oxygen, ozone, and various aerosols are the principal absorbers
of the solar radiation. Absorption by an atmospheric gases is wavelength
dependent, that is, these gases absorbs the solar radiation at particular
wavelengths.
The clear sky is essentially transparent to solar radiation at the wavelengths
of the visible light . Ozone and oxygen are the strong absorbers of solar
ultraviolet radiation in the stratosphere. Oxygen absorbs very short UV
(less than 0.2 µm), while ozone absorbs UVC radiation. As a result
of this absorption, a significant reduction in UV radiation at the earth's
surface and a warming of upper stratosphere are both observed.
Water vapor absorbs solar IR radiation at wavelengths greater than 0.8
mm. Clouds are poor absorbers of solar radiation. Typically, clouds absorb
less than 10% of the radiation that strikes the cloud top.
Absorption of solar radiation by ocean and lakes is also wavelength
dependent: red light is totally absorbed within about 15 m (49 ft) of the
surface, while blue-violet light may penetrates to depths of 250 m (820
ft) within clear, clean water. However, suspended sediments significantly
increase the rate of absorption, in fact, sunlight rarely reaches below
10 m (33 ft.).
Absorption of infrared radiation by the atmospheric gases is wavelength
dependent as well. Absorptivity is very low or close to zero near 8 µm
and 10 µm, called atmospheric window. The gases that absorb
the IR radiaiton are water vapor (principical), carbon dioxide, ozone,
methane, and nitrous oxide, called greenhouse gases. They are responsible
for rising the average surface temperature of lower atmosphere by 33 ûC.
A greenhouse effect also operates on Mars and Venus where the principal
greenhouse gas is carbon dioxide. The Martian atmosphere is thinner than
the Earth's atmosphere; the average surface temperature of Mars raises
by about 10 ûC (18 ûF). Venusian atmosphere, on the other hand,
is denser than the Earth's atmosphere; the average surface temperature
raise is estimated at 523 ûC (941 ûF).
Figure 2.24, page #59 (Moran and Morgan)
Clouds, which are composed of water droplets and/or ice crystals, also
produce a greenhouse effect. They reflect the solar radiation as they cool
the earth's surface; they absorb and re-radiate the IR radiation as they
warm the earth's surface. However, on a global scale, a greater cloud cover
would tend to cool the planet.
Carbon dioxide, on the other hand, could rise the average surface temperature
between 1.5 to 4.5 ûC (2.7 to 8.1 ûF) if it doubles its present value.
The numerical models based on the rise in carbon dioxide concentration
observed in Mauna Loa Observatory, Hawaii predicts that CO2 will double
by the middle of next century. However, many uncertainties in numerical
models particularly with regard to timing, magnitude, and regional patters
of climate change exist.
Fossil fuel combustion accounts for 80% increase in CO2 concentration,
while deforestation is likely responsible for the balance. The burning
of coal, oil, and natural gas produces carbon dioxide as a byproduct.
Figure 2.25, page #60 (Moran and Morgan)
The increase of the other greenhouse gases, methane, nitrous oxide,
and CFCs, is also evident. Although their concentration is considerably
less than the carbon dioxide, they are more efficient absorbers of IR radiation
since they strongly absorb within the atmospheric window.
Ozone in the troposphere help to global warming, while the carbon dioxide
in the stratosphere cools the atmosphere. The aerosols resulting from sulfur
emissions may have offset a significant part of the greenhouse warming
in the Northern Hemisphere during the past several decades.
Global mean surface temperature has increased by 0.3û to 0.6û
over the last 100 years, is consistent with predictions of climate models,
but it is also the same magnitude as natural climate variability. Natural
climate variability and other human factors could have offset a still larger
human-induced greenhouse warming.
Table 2.4, page #60 (Moran and Morgan)
Ozone, which is produced mainly by the subsequent combination of oxygen
atoms and oxygen molecules at altitudes between 10 to 50 km in the atmosphere,
absorbs the ultraviolet radiation at wavelengths less than 0.30 µm.
It exists at peak concentrations of 10 ppm at altitudes of 20 to 25 km.
Ozone is destroyed by colliding with other molecules and atoms. Chlorofluorocarbons
(CFCs), which is used as refrigerants, propellants in aerosol sprays, and
blowing agents for foam insulation, is one of the major source for ozone
destruction. CFCs in the stratosphere breaks down into chlorine (Cl) atoms
by UV radiation. Chlorine which acts as a catalyst in chemical reactions,
converts ozone to oxygen. A single chlorine atom destroys perhaps tens of thousands of
ozone molecules before it undergoes chemical reaction with another substance.
Fortunately, Cl atoms do not exist in the stratosphere, forever. They
are removed as chlorine monoxide (ClO) combines with nitrogen dioxide (NO2)
to form chlorine nitrate (ClONO2). Free Cl atoms combine with methane (CH4)
to form hydrogen chloride (HCl) and a new substance, CH3.
Table 2.1, page #47 (Moran and Morgan)
The depletion of ozone layer was first reported over Antarctica by British
Antarctic Survey team in 1985. It is attributed to the exceptionally high
concentrations of ClO. The Antarctic ozone hole exists during the Southern
Hemisphere spring (mainly September and October) and disappears by November.
It is about the size of the continental United States.
Satellite measurements show a negative trend in ozone concentration
in both hemispheres except near the equator where no significant change
was indicated. The ozone depletion in midlatitudes is observed in late
winter and early spring.
The concentration of ozone is expressed in Dobson units. It is
the depth of ozone produced if all the ozone in a column of the atmosphere
is brought down to sea-level temperature and pressure. One Dobson unit
is a hundredth of millimeter. A typical value of stratospheric ozone concentration
measured by the total ozone mapping spectrometer (TOMS) abroad the Nimbus-7
satellite is about 400 to 500 Dobson units.
Figure 2.20, page #55 (Moran and Morgan)
The reduction of ozone concentration in the stratosphere allows more
UV radiation to reach to the earth's surface, following a greater risk
of skin cancer, eye damage etc. In general, every 1% decline in stratospheric
ozone translates into a 2% increase in UV radiation. At the same time,
2.5% decrease in ozone layer could increase the incidence of human skin
cancer by 10%.
The most dangerous UV radiation for the human health is its UVB band.
Sand reflects up to 50% of the incident UVB, therefore one would still
expose to dangerous radiation even in the shade of beach umbrella. Water
transmits UVB to a depth of a meter or so, and a wet T-shirt allow 20%
to 30% of incident UVB to reach skin. One is also exposed to high levels
of UVB radiation at high mountain elevations during skiing. Snow reflects
UVB radiation more than the beach sand.
In general, if your shadow is shorter than your height, you should apply
a sunscreen, otherwise it is not a problem. Sunscreens have ingredients
that selectively block UV radiation. The sun protection factor (SPF)
is a measure of the time that the skin can safely exposed to the sun. The
higher the SPF value, the longer the protection lasts. It should be noted
that suntan lotions help to keep the skin moist, but they do not provide
any protection from UV radiation.
Pyranometer: the instrument that measures the intensity of solar
radiation striking a horizontal surface. It consists of a sensor enclosed
in a glass bulb that transmits total short-wave insolation. The sensor
is a disk consisting of alternating black and white segments. The temperature
difference of the black and white segments for the same intensity of solar
radiation is calibrated as a flux in units of cal/cm2 or W/m2.
The pyranometer should not affected by shadows, by any highly reflective
surfaces nearby, or by other sources of radiation. The glass bulb must
also be kept clean and dry.
Figure 2.27, page #63 (Moran and Morgan)
Figure 2.28, page #63 (Moran and Morgan)
