Physics of the Infra-Red

For a molecular vibration to absorb (or emit) infrared energy a change in dipole moment must occur.

For a molecular vibration to absorb (or emit) infrared energy a change in dipole moment must occur.  CO2 has two modes of vibration which are IR active, H2O has three.

Note: Diatomic molecules such as N2 and O2 are IR inactive: they can not absorb or emit in the IR region.

Due to the limited vibration modes for CO2 – only specific wavelengths are absorbed (or emitted). H2O has many more vibration modes and absorbs over a wide range of wavelengths.

The GH gases radiate energy to space, cooling the Earth.

Schematic showing Heat Energy from the Surface – By Convection and Radiation. The left arrow shows radiation emitted to space.

The GH gases radiate energy to space, cooling the Earth

If these gases did not exist the Earth would be much hotter as shown below.

•If the concentration of GH gases increase then more heat energy would be radiated to space from the atmosphere
•Note: it is the GH gases that enable the atmosphere to radiate heat away from the Earth.
•The energy which is conveyed by the atmosphere includes the convective energy from the surface of the Earth that would otherwise have been trapped in the atmosphere.

It is the GH gases that COOL the Earth

•To maintain energy balance with incoming solar energy, the Earth cools by means of IR radiation. The total heat loss is 235 W/m2
•66 W/m2  is radiated from the surface of the Earth. 40 W/m2 is radiated directly into ‘space’ and 26 W/m2 is absorbed by the GH gases in the atmosphere
•If the concentration of GH gases increase more heat will be ‘trapped’ or ‘absorbed’ in the atmosphere.
•If more heat energy is be absorbed by the atmosphere then there will less radiated directly to ‘space’

To maintain energy balance more energy will be radiated into space from the atmosphere


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One Response to Physics of the Infra-Red

  1. Tryagain says:

    Oh good grief. Please read a basic undergrad texts on this topic. You are making some very embarrassing mistakes.

    Goody and Yung -Atmospheric Radiation: Theoretical Basis
    Thomas and Stamnes -Radiative Transfer in the Atmosphere and Ocean
    Liou -An Introduction to Atmospheric Radiation

    Or try asking Steve over at Science of Doom to explain it to you.

    http://scienceofdoom.com/roadmap/atmospheric-radiation-and-the-greenhouse-effect/

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