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What property of greenhouse gases cause them to conserve heat?

What property of greenhouse gases cause them to conserve heat?
Daniel from Hertfordshire (Aged 5-14)


3 Responses

  1. Hi Daniel,

    Greenhouse gases all have particular structures and components which enables them to absorb energy in a part of the electromagnetic spectrum known as the infra-red (IR). It’s energy in the IR range that we feel as heat.

    The greenhouse gases vibrate or rotate when they absorb energy in the IR and this contributes to their ability to store heat.

    For information on the electromagnetic spectrum have a look at http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html

  2. The answer to this question concerns the ability of molecules to absorb heat radiation.
    Heat radiation (just like light) has a spectrum of wavelengths which depends on the temperature of the surface emitting the heat.
    Molecules absorb radiation at particular wavelengths depending on how they are constructed. Water vapour and carbon dioxide, the two main greenhouse gases, are each composed of three atoms joined together by atomic bonds. The bonds vibrate at particular frequencies (you can imagine them being like springs) and the molecules preferentially absorb the wavelengths of radiation which excite the vibrations.
    It just so happens that carbon dioxide likes to vibrate at frequencies which have energy similar to that of the wavelengths of radiation emitted by the Earth’s surface. Furthemore, at these wavelengths water vapour is not very active so carbon dioxide has greater effect.

  3. Good question. The first thing is to lok at the properties of the molecules of greenhouse gases themselves. They have more than one type of atom, and are made of two or more atoms. For example, the most famous, carbon dioxide, is made of two oxygen atoms with a carbon atom in the middle. The join between the atoms act like a spring, which can vibrate at a particular frequency (different molecules have different frequencies, which is how we can tell them apart, even if we can’t take a sample). Now let’s look at the Earth. Light (which is a wave, remember) from the Sun hits the Earth and warms it. This heating makes the Earth give off infra red (heat) radiation, which is just the same as light, but has a lower frequency.
    Now, here’s the punch-line. The frequency of the heat radiation the Earth gives off matches the frequency the carbon dioxide molecules vibrate at, so they can absorb and trap the heat.
    So, the property of greenhouse gases that cause them to conserve heat is that the frequency they can vibrate at matches the frequency of heat radiation given off by the Earth.

    *Information about Paul Hatherly*
    Dr Paul Hatherly is a member of the Department of Physics and Astronomy at the Open University in Milton Keynes. His research interests inclue molecular physics, heritage science and the development of science education (through the OU’s piCETL programme).
    He lives in the Heart of Englad with his family, a dense dog, a killer cat and a number of miniature steam engines.

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