1. NATS 101 Lecture 5 Radiation
Today WE’LL LOOK AT THE THIRD METHOD OF HEAT TRANSPORT, RADIATION.

2. Radiation
Any object that has a temperature greater than 0 K, emits radiation.
This radiation is in the form of electromagnetic waves, produced by the acceleration of electric charges.
These waves don’t need matter in order to propagate; they move at the “speed of light” (3x105 km/sec) in a vacuum.
Radiation -
Any object that has a temperature greater than absolute zero (0 K), emits radiant energy. The radiation is in the form of waves that have both electric and magnetic properties. Thus, they are called electromagnetic wave.
The waves are produced by electric charges moving.
This means of energy transfer does not involve matter, but instead electro-magnetic waves that travel at 300,000 km/sec. This is the major method of transferring energy from the sun to the earth, and at this rate it takes about 8 minutes for radiation to travel from the sun to the earth.

3. Electromagnetic Waves
Two important aspects of waves are:
What kind: Wavelength or distance between peaks.
How much: Amplitude or distance between peaks and valleys.
The picture of electromagnetic waves passing through the atmosphere is much like that of the waves on the surface of the water. There are two important aspects about the waves:
1. What kind of wave is it?: Wavelength or distance between the peaks of the waves.
2. How big is it?: Amplitude or distance between peaks and valley. When more radiation is emitted, the amplitude increases.
Wavelength
Amplitude
Frequency

4. Why Electromagnetic Waves?
Radiation has an Electric Field Component and a Magnetic Field Component
Electric Field is Perpendicular to Magnetic Field
Radiation has an Electric Field Component and a Magnetic Field Component
1. What kind of wave is it?: Wavelength or distance between the peaks of the waves.
2. How big is it?: Amplitude or distance between peaks and valley. When more radiation is emitted, the amplitude increases.

5. Photons NOT TO CONFUSE YOU, but…
Can also think of radiation as individual packets of energy or PHOTONS.
In simplistic terms, radiation with shorter wavelengths corresponds to photons with more energy (or more BB’s per second) and with higher wave amplitude to bigger BB’s
Not to confuse you, but I just wanted to touch on the fact that sometimes it is convenient to think of radiation in terms of particles or discrete packets of energy or “photons.” When doing this remember the relation, photons with shorter wavelengths have more energy. That is why, in spite of the fact that the sun emits more visible photons than UV photons, the UV photons have much more energy than visible photons and are thus more dangerous to humans than visible photons. Although visible photons can harm a person in large doses, i.e. looking directly at the bright light.

6. Electromagnetic Spectrum
Wavelengths of Meteorology Significance
The electromagnetic waves can occurs with any size of wavelength. In meteorology, however, the unit of measure commonly for the EM waves is the micrometer.
Danielson, Fig. 3.18
WAVELENGTH

7. Emitted Spectrum Emitted radiation has many wavelengths.
White Light from Flash Light
Purple
Green
Red
Prism
(Danielson, Fig. 3.14)

8. Emitted Spectrum
Energy from Sun is spread unevenly over all wavelengths.
Emission spectrum of Sun
Energy Emitted
Ahrens, Fig. 2.7
Wavelength

9. The hotter the object, the shorter the brightest wavelength.
Wien’s Law
Danielson, Fig. 3.19
The hotter the object, the shorter the brightest wavelength.

10. Warmer Objects => Shorter Wavelengths
Wien’s Law
Relates the wavelength of maximum emission to the temperature of mass
MAX= (0.29104 m K)  T-1
Warmer Objects => Shorter Wavelengths
Sun-visible light
MAX= (0.29104 m K)(5800 K)-1  0.5 m
Earth-infrared radiation
MAX= (0.29104 m K)(290 K)-1  10 m

11. Wien’s Law
What is the radiative temperature of an incandescent bulb whose wavelength of maximum emission is near 1.0 m ?
Apply Wien’s Law:
MAX= (0.29104 m K)  T-1
Temperature of glowing tungsten filament
T= (0.29104 m K)(MAX)-1
T= (0.29104 m K)(1.0 m)-1  2900K

12. Stefan-Boltzmann’s (SB) Law
The hotter the object, the more radiation emitted.
When the temperature is doubled, the emitted energy increases by a factor of 16!
Stefan-Boltzmann’s Law
E= (5.6710-8 Wm-2K-4 )T4
E=2222=16
4 times
Sun Temp: 6000K
Earth Temp: 300K
Aguado, Fig. 2-7

13. How Much More Energy is Emitted by the Sun per m2 Than the Earth?
Apply Stefan-Boltzman Law
The Sun is 160,000 Times More Energetic per m2 than the Earth, Plus Its Area is Mucho Bigger!

14. Radiative Equilibrium
Radiation absorbed by an object increases the energy of the object.
Increased energy causes temperature to increase (warming).
Radiation emitted by an object decreases the energy of the object.
Decreased energy causes temperature to decrease (cooling).

15. Radiative Equilibrium (cont.)
When the energy absorbed equals energy emitted, this is called Radiative Equilibrium.
The corresponding temperature is the Radiative Equilibrium Temperature.

16. Modes of Heat Transfer
Latent Heat
Williams, p. 19

17. Key Points MAX= (0.29104 m K)  T-1 E= (5.6710-8 W/m2 )  T4
Radiation is emitted from all objects that have temperatures warmer than absolute zero (0 K).
Wien’s Law: wavelength of maximum emission
MAX= (0.29104 m K)  T-1
Stefan-Boltzmann Law: total energy emission
E= (5.6710-8 W/m2 )  T4

18. Key Points Radiative equilibrium and temperature
Energy In = Energy Out (Eq. Temp.)
Three modes of heat transfer due to temperature differences.
Conduction: molecule-to-molecule
Convection: fluid motion
Radiation: electromagnetic waves

19. Reading Assignment
Ahrens
Pages 34-42
Problems 2.10, 2.11, 2.12