Radiation in the Atmosphere Radiation basics Solar radiation Radiation laws The Sun and Earth Optical phenomena in air

Different Forms of Energy Energy comes in different forms, and can be transformed between these forms: Kinetic energy: energy of motion. Energy/mass = (v is velocity, or speed). Potential energy: stored energy, as in gravitational potential energy. Energy/mass = (g is the gravity constant, and h is height) Thermal energy: heat internal to an object. Energy/mass = (T is temperature, and s is specific heat) Radioactive energy: nuclear energy released by radioactive decay. Energy/mass = (c is the speed of light; Einstein’s mass-energy equivalence) Light energy: energy of electromagnetic waves, or photons. Energy = (h is Planck’s constant*, c is the speed of light, and  is wavelength)

Waves Carry Energy The physical motion of a string, or water movement in an ocean wave, contains wave energy. In radiation, the oscillating electric and magnetic fields contain the energy. Radiant energy is also quantized into massless discrete “photons.”

The Electromagnetic Spectrum

The Spectral Ranges of Atmospheric Radiation Atmospheric radiation is usually broken down into specific spectral regions. “Solar” and “terrestrial” radiation usually refer to sunlight and Earth emission, respectively. These are also called “shortwave” and “longwave” radiation, respectively, which correspond to radiant emission temperatures of roughly 6000 kelvin and 255 kelvin, respectively. Visible radiation falls within the solar spectrum (blue, green, red), as does near ultraviolet radiation in the A, B and C bands. Infrared radiation falls above the visible spectrum in wavelength (which means it is below the red in energy). There is both “near” infrared (solar) and “far” infrared (terrestrial) radiant energy.

Solar Radiation and Light While we usually consider sunlight to be mainly visible radiation – the colors of the spectrum – in reality there is much more to the sun’s radiation. The ultraviolet component, which is a percent or so of the sun’s total energy output, has important consequences for life on Earth, while the near-infrared component at longer wavelengths carries nearly half the total energy of sunlight that we cannot see.

Sunlight After Filtering by Air The sunlight that reaches the ground is no longer “pure” solar radiation. It has been affected by absorption and scattering by atmospheric gases and particles. The light gray shading indicates the amount of sunlight that on average is scattered by air molecules (Rayleigh scattering) on clear days. The darker gray area shows the regions where the radiation is reduced by strong absorption bands, mainly by water vapor but also ozone and carbon dioxide.

Blackbody Radiation: Planck’s Function Radiant energy and heat are related, and readily converted from one form to the other. For example, the radiation emitted by an object at a specific temperature is defined by the “blackbody” radiation law, originally derived by physicist Max Planck. The figure shows the blackbody spectra for several temperatures. A blackbody at a temperature of about 6000 kelvin (K) has roughly the same emission spectrum and apparent surface intensity as the sun. That is, the solar radiation spectrum closely corresponds to the 6000 K curve above.

The Radiation Laws: Stephan-Boltzmann Here, FB is the radiant energy flux per unit area (joules/m2-sec), B is a known physical constant, and T is the temperature in degrees kelvin. Blackbody radiation obeys a few simple laws. One, the Stephan-Boltzmann law, states that the total radiant energy flux emitted by a blackbody – per unit surface area of the object – is proportional to the object’s temperature raised to the fourth power. This law means that the energy emission is very sensitive to temperature. If an object’s temperature doubles, its energy output increases by a factor of 16. In the figure, the energy emission rate, or flux, is equal to the integrated area under the blackbody curve – that is, the shaded area.

The Radiation Laws: Wien Blackbody radiation also obeys Wien’s law, which is illustrated in the diagram. It states the the peak intensity in the blackbody spectrum lies at a wavelength that is inversely proportional to temperature. The equation in the figure gives the peak wavelength in units of micrometers (microns, or m). At normal temperatures on Earth of ~290 K, the peak thermal radiation intensity is at ~10 microns.