# ELECTROMAGNETIC RADIATION Electromagnetic spectrum Shortwave 3 m Longwave

• Slides: 21

Electromagnetic spectrum Shortwave < 3 µm Longwave > 3 µm

Solar (shortwave) : 8% UV, X-ray, Gamma 47% visible 45% infrared Terrestrial (longwave): 100% infrared

λ = wavelength

Atmospheric absorption Gases in atmosphere absorb at different wavelengths. Wavelengths not absorbed are TRANSMITTED.

absorption vs. transmission

Atmospheric absorption/transmittance graph White gaps are “atmospheric windows of transmission”; black areas are absorption bands

Longwave absorption

Shortwave absorbers: • Ozone: absorbs UV < 0. 3 μ • H 2 O vapor : absorbs > 0. 8 μ Atmospheric “window” of transmission: 0. 3 – 0. 8µ (shortwave) Longwave absorbers: • H 2 O vapor, CO 2, ozone, CH 4, N 2 O Another window: 8 – 11 µ (longwave)

Review SOLAR RADIATION: shortwave (visible, UV, IR); higher radiation intensity; absorbed by ozone and water vapor; atmospheric window of transmission : 0. 3 – 0. 8 µ TERRESTRIAL RADIATION longwave (IR); lower radiation intensity; absorbed by “greenhouse gases” (O 3, H 2 O, N 2 O, CO 2, CH 4) atmospheric window of transmission : 8 – 11 µ

Solar constant amount of solar radiation received at top of atmosphere on a plane surface perpendicular to sun’s rays. (not diminished by atmosphere) = 1372 W m-2 1 Watt = 1 J / sec

What is relationship between wavelength and temperature? What is relationship between energy and temperature?

Rules of Radiation 1. Wein’s Law all objects emit energy at wavelengths inversely proportional to their temperature Wein’s Law gives wavelength of peak intensity (λmax) λmax = 2897 / T (temp in Kelvins)

For earth: λmax = 2897 / 300 K = 9. 6 μ (infrared) For sun: λmax = 2897 / 6000 K = 0. 48 μ (visible) For molten lava? (temp = 1073 K) λmax = 2897 / 1073 = 2. 7 μ (shortwave infrared)

2. Stefan-Boltzmann Law E = σ (T 4) (temp in Kelvins) σ = Stefan-Boltzmann constant: 5. 67 x 10 -8 Wm-2 K-4

Humans maintain skin temp of 91°F (306 K). 1. In which portion of the spectrum do we radiate? Wein’s Law: λmax = 2897 / temperature 2. How much energy do we radiate? Stefan-Boltzman : E = σ (T 4) σ = 5. 67 x 10 -8 Wm-2 K-4