GEOL 1350 - 04

Selective Absorbers

Objects that selectively absorb and emit radiation.

Electromagnetic radiation.

- A form of transmitted energy. Electromagnetic radiation is so-named because it has electric and magnetic fields oscillate in planes mutually perpendicular to each other and to the direction of propagation through the space. - Electromagnetic radiation has dual nature: wave properties and particulate properties.

Radiation

- Energy transferred from the sun to the earth is called radiant energy, or radiation. - Radiation travels in the form of electromagnetic waves that release energy when they are absorbed by an object. - Electromagnetic waves do not require molecules to propagate. - All objects with temperature above absolute zero emit radiation.

Spectrum of Electromagnetic Radiation

- Higher (lower) frequencies have shorter (longer) wavelengths.

Wavelength & Frequency Specification

- Relation between wavelength and frequency. - λν = c - λ is wavelength, ν is frequency, and c is speed of light. - Since c is a constant, shorter (longer) wavelength corresponds to a high (low) frequency. - Most people in spectroscopy will use wavenumber instead of frequency. - Wavenumber is defined as a number of cycles in a given unit length. - ν' = ν/c = 1/ λ Where ν is frequency, c is speed of light, and λ is wavelength.

Warming the Earth & Atmosphere

- Sunlight warms the ground, and the air above is warmed by conduction (small impact), convection, and radiation. - Further warming occurs during condensation as latent heat is given up to the air inside the cloud.

Summary

- The Sun emits most of its radiation as short-wave radiation. The earth emits most of its radiation as longwave infrared energy. - The higher an object's temperature, the greater the amount of radiation emitted per unit surface area and the shorter are the wavelengths of emitted radiation. - Selective absorbers in the atmosphere, such as water vapour and carbon dioxide, absorb some infrared radiation from the earth and radiate an portion of it back to the surface, producing the atmospheric greenhouse effect. - Latent heat is an important source of atmospheric energy.

If the earth always radiates energy, why doesn't it cool?

- The earth is in a state of radiative equilibrium when incoming radiation is balanced by outgoing radiation. - Radiative equilibrium predicts surface temperature of ~ 255 K or ~ -18 degrees oC - But, the earth's observed average surface temperature is ~ 15 oC.

Incoming Solar Radiation

- When solar radiation enters the atmosphere, a number of interactions take place. - Some energy is absorbed by gases. - When sunlight strikes small objects (i.e: air molecules and dust particles), the light is deflected in all directions--called scattering. - Sunlight can be reflected from objects. Reflection differs from scattering in that more light is sent backwards.

Krichhoff's Law

Basic laws for radiation. Good absorbers are good emitters at a particular wavelength. - All gases in earth's atmosphere are selective absorbers. 1. The atmospheric gases are transparent to visible radiation. 2. Water vapour and carbon dioxide are strong absorbers (also emitters) of infrared radiation. These gases are also known as greenhouse gases, which helps to keep the lower atmosphere warm. 3. Ozone and molecular oxygen are good absorbers of ultraviolet radiation--keeping us from getting burned.

Stefan-Boltzman law

Basic laws for radiation. The amount of energy per square meter per second that is emitted by a blackbody is related to the 4th power of its Kelvin temperature. - If an object's temperature is high, it tends to release more energy. - E = σ T4 - where E is in J s-1 m-2 or Watts m-2 - σ = 5.67 x 10-8 W m-2 K-4 Stefan-Boltzman constant - As T increases, E increases by a power of 4. If T doubles, E increases by 16 times!

Wein's law

Basic laws for radiation. Wavelength of peak radiation emitted by an object is inversely related to temperature. - λmax = 2897 / T ~ 3000/T - (λmax is in μm and T is in Kelvin) - Solar radiation : λmax sun ~ 3000/6000 K ~ 0.5 μm, - Earth radiation: λmax earth ~ 3000/300 K ~ 10 μm, - Solar radiation is shortwave radiation Earth radiation is longwave radiation - Hotter the object the shorter the wavelengths of the maximum intensity emitted.

Radiation Characterized by Wavelength

Longer waves carry less energy than the shorter waves.

Atmospheric Windows

Portions of the electromagnetic spectrum where atmospheric gases absorb relatively little energy. - Visible band (0.25-0.8 micrometers) - vision has evolved to use these wavelengths. - Terrestrial band (wavelength range between 8-11 μm) where little absorption of infrared radiation takes place, allowing earth's radiation to escape to space. Clouds are good absorbers of infrared radiation, even in the wavelengths range of 8-11 micrometers.

Wave Nature of Radiation

Radiation can be thought of as a traveling wave. - Wave: wavelength, frequency, and speed. - Wavelength: distance between two consecutive peaks (troughs). - Frequency: number of cycles per second that pass a given point in space. - Speed: speed of light in a vacuum. c = 3 x 108 m/s

Electromagnetic Spectrum of the Solar Radiation

Solar radiation has peak intensities in the shorter wavelengths, dominant in the region we know as visible, but extends at low intensity into longwave regions.

The Fate of Incoming Solar Radiation

Solar radiation is scattered and reflected by the atmosphere, clouds, and earth's surface, creating an average albedo of 30%. Atmospheric gases and clouds absorb another 19 units, leaving 51 units of shortwave absorbed by the earth's surface.

the Greenhouse Effect

Solar radiation passes rather freely through earth's atmosphere, but earth's re-emitted longwave energy either passes through a narrow window or is absorbed by greenhouse gases and re-radiated toward earth. - Earth's atmosphere absorbs and emit infrared radiation that keeps atmosphere warm.

Spectrum of the Sun Compared to that of the Earth

The hot sun radiates at shorter wavelengths that carry more energy, and the fraction absorbed by the cooler earth is then re-radiated at longer wavelengths, as predicted by Wein's law.

Albedo

The percent of radiation returning from a given surface compared to the amount of radiation initially striking that surface. - Thick clouds have a higher albedo than thin clouds.

Wavelength & Frequency Units

Wavelength Units: - Angstrom (A): 1 A = 1x10-10 m - Nanometer (nm): 1 nm = 1x10-9 m - Micrometer (μm): 1 μm = 1x10-6 m - Frequency Units: 1/s or hertz (Hz) - Wavenumber Units: inverse length (cm-1)

Particulate Nature of Radiation

We can think of radiation as streams of particles, or photons, that are discrete of energy. - The energy of a photon is E = hv (high-frequency waves carry more energy and vice versa.) - Where h = Planck's constant = 6.6261×10-34Js-1, ν = frequency (hz). - Each photon's energy is related to the electromagnetic wave frequency (ν).