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    Written by: LEE Shuk-ming, Olivia     September 2010

  1. Has it ever occurred to you that your body is emitting radiation?  Yes, all objects, including human bodies, emit electromagnetic radiation.  The wavelength of radiation emitted depends on the temperature of the objects.  Such radiation is sometimes called thermal radiation.  Most of the radiation emitted by human body is in the infrared region, mainly at the wavelength of 12 micron.  The wavelength of infrared radiation is between 0.75 to 1000 micron (1 micron = 10-6 metres).  This wavelength is longer than that of red visible light so this explains the name 'infrared', meaning 'beyond the red'.

    The amount of thermal radiation emitted by an object depends on its surface temperature, area and characteristics.  Warmer object emits more thermal radiation than cooler one.  Handheld infrared ear or forehead thermometers are used to probe body temperatures by detecting the infrared radiation emitted by human bodies.  Infrared cameras are also used for fast screening of travelers with a fever at airports, ports and border crossings. 

    Infrared radiation detection also finds application in meteorology - images captured by satellite showing intensities of infrared radiation emitted by cloud tops and ocean/land surfaces.  Higher cloud tops are of lower temperature and emit infrared radiation of smaller intensity.  Hence we can observe temperature distribution of cloud tops on an infrared satellite picture.  The advantage of infrared satellite images over visible images is that the 'night vision' capability of the former makes them useful round the clock.  An interesting point to note is that weather forecasters usually prefer the use of different colours (red, blue, green, etc) to highlight different cloud top temperatures so as to facilitate his/her analysis of cloud structure.  Members of the general public, however, seem to prefer white and greyscale cloud pictures.  Hence on the Observatory's website, you will find higher cloud tops being depicted in brighter white colour in infrared satellite images.  Perhaps white clouds and blue sky are just so natural to us that any fancy-coloured cloud pictures are not consistent with our intuition!

     
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    Figure 1:  Infrared satellite picture in which cloud top temperatures are represented by different colours of red, green, violet, etc.

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    Figure 2:  Infrared satellite picture in which cloud top temperatures are represented by greyscale tones.

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Last revision date: <22 Jan 2013>