Friday, 21 June 2013

Physics of Haze

I couldn't help it but to apply what I know in Physics to explain the white/gray haze as a result of the smoke particles from the Sumatran fires.

To begin, let's try to understand how we see the colours of the buildings and the trees. We "see" the colour of the light that is scattered off the surface of the building, leaves, etc, as shown.
Scattering of light from surface (e.g. green wavelength em waves from the leaves) causes us to "see" the objects' colour

Light is also scattered by the air molecules (~ nanometer in diameter) along the path from the sun to the object by a process known as Rayleigh Scattering (refer to my previous post) towards the observer. However intensity from this scattering is low compare to the intensity of light reaching the objects and scattering to the observer. Generally our eyes cannot sense this low intensity in the presence of the light from the objects.
Effect of Rayleigh Scattering is small and hence don't produce the white haze 

With the dust particles, diameter in the region of microns (0.000001 m), suspended in the air, Mie scattering dominates. The scattering intensity by this process is larger than Rayleigh Scattering. The larger intensity of white light scattered from these particles (consisting of all visible wavelengths) is now more obvious to us.
Mie Scattering off the dust particles produces the white haze seen the above photo
The additional scattering also explain why the sky and the sun appears red around 6 pm, something we usually observe around 7pm when the sun is close to the horizon.
Red sky and sun around 6pm

For more information on the different type of scattering, you may want to refer to this.

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