1) Radiation transfers heat through electromagnetic waves without a medium and travels at the speed of light. The amount radiated and wavelengths depend on an object's temperature according to Stefan-Boltzmann and Planck's laws. 2) Solar radiation reaches Earth's surface through reflection, scattering, absorption and transmission in the atmosphere. Gases like ozone and oxygen absorb most harmful UV rays while scattering by air molecules causes blue skies. 3) On Earth's surface, radiation is mostly absorbed and re-emitted, with some reflected depending on surface albedo. The atmosphere emits terrestrial radiation both upwards, lost to space, and downwards to the surface.

1. Radiation and Atmospheric Temperature
Dr. Akepati S. Reddy
School of Energy & Environment
Thapar University, Patiala

2. Radiation
• Electromagnetic waves (exhibit the characteristics of both electric
field and magnetic field)
– No medium is required for this mode of heat transfer (best in vacuum)
– Moves at the speed of light - spreads in all directions in straight lines
• Described by wavelength, frequency and velocity (relationship
among these: λ . V = C)
– ‘C’ speed of light: 299 792 458 m/s (3.0 x 108 m/s)
• Amount radiated and wavelength composition of the radiation
depends on the emission temperature of the object
– Stefen – Boltzmann law (radiation emitted is proportional to T4)
– Planck’s law (ideal emission spectrum or curve)
– Wein’s displacement law (λmax is object’s temperature dependent)
• Radiation spectrum: spectral distribution of radiative energy (over
different wavelengths)
• Black body (absorbs all the incident radiation) is an ideal emitter
(emissivity factor is 1.0) – grey bodies have <1.0 emissivity

3. Wein’s displacement law:
The wavelength (λmax) at which spectral radiance of a black body peaks
T
b
max
b is a constant of proportionality ( Wien's
displacement constant): 2.8977729×10−3 m K
T is absolute temperature
Stefen – Boltzman law
Total radiation emitted by a black body is proportional to the fourth
power of its absolute temperature.
I = irradiance per unit time per unit are (W/m2)
ε = Emissivity (for black body ε = 1)
σ (Stefan-Boltzmann Constant) = 5.6703 10-8 (W/m2K4)
T = absolute temperature in Kelvin (K)
I = εσ T4 A
4232
45
m
W
15
2
KhC
k
 
Planck’s law
Describes spectral density of radiation emitted by a black body
Bλ(λ,K) is spectral radiance at λ wavelength and K
temperature (W/m2- (λ,K))
kB (Stefan-Boltzmann Constant) = 5.6703 10-8 (W/m2K4)
C (speed of light) = 3.0 x 108 m/s
‘h’ (Planck’s constant) = 6.626196 10-34 W

5. Radiation
• Radiation is divisible into shortwave and longwave radiations
– Shortwave radiation (<1500 nm) includes UV, visible and near IR
– Longwave radiation (>1.5 μm) includes far IR and microwave
• Fate of radiation includes reflection, scattering, absorption and
transmission
• Absorption of radiation increases molecular motion, heating and
increases the temperature
– Earth is almost a black body, but gases are not – gases are selective
absorbers and emitters
– A substance, which is an efficient emitter in a given wavelength range,
will also be efficient absorber at the same wavelength range
• Radiations types:
– Solar radiation (Extra-terrestrial solar radiation and solar radiation
reaching the earth surface)
– Terrestrial radiation from the earth surface
– Terrestrial radiation from the atmosphere

7. Extra Terrestrial Solar Radiation
• Spectral characteristics
– Max. spectral density (λmax.) is at 480 nm (green of visible spectrum)
– 6.4% is UV radiation (<380 nm), 45.6% is Infrared radiation (>780 nm),
and the rest 48% is visible radiation (380 to 780 nm)
• Energy content
– Solar constant, GSC (extraterrestrial solar radiation received at the
mean earth-sun distance of 1.495 x 1011 m): 1367 w/m2
– The Earth – the Sun geometry is responsible for spatial and temporal
variations in the extra terrestrial solar radiation
– Varies with the time of the day (earth’s rotation), season and latitude
(tilt of the earth and Earth’s revolution around sun in the orbit)
– Earth receives 6.7% more radiation during Perihilion (3rd Jan – earth is
closest to sun) than during Aphilion (4th July – earth is farthest to sun)
– On equinox (vernal/spring & autumnal/fall) days (23rd March & 22nd
Sep.) everywhere on earth, day length and night length are equal
– Day length is maximum during the summer solstice (June 21st) and
minimum during the winter solstice (December 21st)
– On a summer solstice day, the arctic cicle (>66.5 N) experiences 24
hours day light and on winter solctice day no day light at all

11. Fate of Solar Radiation
Solar radiation while passing through the atmosphere is affected by
reflection/scattering and absorption
Reflection and scattering
• Reflection (albedo) is redirection of radiation by surfaces
• Radiation reflection varies with cloud cover, particulate matter in
air, angle of incidence of sun rays, and types of surfaces
• Gas molecules and small particles/droplets of the atmosphere
cause scattering
• Air molcules (nitrogen and oxygen) scatter shortwave radiation in
the blue-violet region in all the directions
– Diffused radiation from scattering is responsible for the blue sky
• Particles/aerosols (water and ice crystalls of clouds) scatter all
wavelengths equally
– scattering is more forward than backward and hence clouds, fog, haze,
etc. Appear while, grey or milky

12. Fate of the Solar Radiation
Absorption and transmission
• Clouds atmospheric gases and aerosols absorb solar radiation
• Ozone and oxygen are UV absorbers
– Ozone screens all the UV-C radiation and most of the UV-B radiation
and about half of the UV-A radiation
• None of the atmospheric constituents are visible range absorbers
(atmospheric visible range window: 0.3 to 0.9 μm)
Atmosphere is considered as transparent for solar radiation
• Under clear sky conditions as much as 55% of the solar radiation is
transmitted through the atmosphere and reach earth surface
– Overcast (cloud) conditions and aerosols through reflecting reduce the
direct solar radiation reaching the earth surface to as low as 4%
• Direct solar radiation reaching the earth surface is attenuated, and
have altered spectral characteristics
– The direct radiation has mostly the red light (sun appears red during
the sunset and the sunrise)
Atmosphere emits terrestrial radiation –radiation emitted upwards is
lost to space – radiation emitted downward reaches earth surface

13. Spectral characteristics and energy content of the solar radiation

14. Fate of the Solar Radiation
• Earth behaves like a black body and mostly absorbs the radiation
received (some fraction is reflected – albedo)
– Direct solar radiation
– Diffused solar radiation
– Terrestrial radiation from the atmosphere above
• Surface albedoes are different for different surfaces
– Fresh snow: 75-95%; sand: 15-45%; forests: 15-55%; grass land: 10-
30%; dry plowed field: 5-20%
• Earth looses energy as
– Terrestrial radiation from the earth surface – 100% longwave radiation
(1.5 to 100 μm)
– Latent heat through evapo-transpiration
– Sensible heat (winds and rising air parcels)
• Atmosphere is opaque for terrestrial radiation (green house effect)
– Some fraction of the radiation however escapes out through an
Atmospheric longwave range window (7-13 μm wavelength range)
– Gases with absorption ranges matching with the window cause global
warming

16. Name Formul a Pre-indust rial
concentration
Current
Concentration
Life-
span
Greenhous e
Potenti al
Water Vapor H2Ov Variable Variable weeks Most
impo rtant
Carbondioxid e CO2 280 ppm 375 ppm 100 yr 1
Methane CH4 700 ppb 1800 ppb 15 yr 21
Nitrous oxides NOx
N2O
275 ppb 315 ppb 100 yr 200
CFCs
Chlo ro-fluoro-carbons
CCF -12 CCl 2F2
0 0.5 ppb 100 yr 15,000
Sul fur hexafluoride SF6 0 0.03 ppb 3000 yr 24,000

17. Terrestrial Radiation and the Atmosphere
• Atmosphere is nearly opaque to terrestrial (longwave) radiation
– Atmosphere absorbs terrestrial radiation from the Earth surface and
radiates back to the earth surface – Earth’s green house effect
– Water vapour strongly absorbs terrestrial radiation at <7 μm
wavelength, carbon dioxide at >12 μm wavelength, and methane near
7 μm wavelength
– Liquid water droplets and ice crystals of clouds are excellent absorbers
of terrestrial radiation
– Earth’s actual temperature is +15C, while expected temperature in the
absence of green house effect is -18C
• Atmospheric longwave range window (7-13 μm) allows vent off of
some energy into space
– While 86% of the terrestrial radiation is absorbed, 14% of the radiation
escapes into space by atmosphere
– Upward terrestrial radiation emitted by the atmospheric gases and
clouds is also ultimately lost to space
• Clouds have net cooling effect during day time and warming effect
at nights in winter months

21. 100
19.6
49.1
8.8
22.5
22.5
31.3
7.0
22.8
114
102.3 11.7
94.7
8.8
48.2
68.7
11.7