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Light & colour in the atmosphere (Colour of the sky (Blue light…
Light & colour in the atmosphere
Effect of visible light
Clouds
Haloes
Caused by ice cirrus clouds- crystals in the sky
Can be 22 degrees (hexagonal) or 46 degrees (cubic)
Light hits crystal at slightly different angles- many different
refracted
angles 'bunch' mainly at 22
Rainbows are caused by clouds
Rainbows are lighter on the inside compared to the outside
For a double rainbow: the outer rainbow is a reflection of the colour and
less intense
Fogbow
Caused by ice fog
Droplets are much smaller than in rain
Colours of rainbow are lost- small droplets have complex scattering
Corona
Caused by water droplets in the sky- defraction
Light defracted around the outside of the droplet- causes interference pattern
Air
Aerosols involved in red colour cause visibility degradation
Aerosol=
Suspension of small particles in the air
Causes blue skies and red sunsets- blue has a shorter wavelength than other colours which is
scattered more effectively
than red (by light)
Why are clouds opaque?
Adsorption
Complete removal of light (conversion of heat)
Scattering
Channeling of light into different colours-
reflecting light in lots of different directions
Both processes that reduce light passing through an object lead to extinction of light from direct beam
Light in a cloud
A typical cloud is one-millionth liquid water by volume
A 1km depth of cloud is equivalent to 1mm of water
Therefore: clouds do not absorb much light
How much is scattered?
Amount of scattered light is proportional to surface area
A 1km3 of cloud contains 1000m3 of water
surface area when squashed to a droplet= 480m2**
SA when distributed across 100 droplets/ cm3= 2x10^8m2 (factor of 400,000 higher)
Efficient scatterers
Weakly absorbing substances distributed into tiny pieces
Guinness froth
Sugar (small crystals)
Soap foam
Snow (small ice crystals)
Polar bear fur- fur is transparent NOT WHITE
Why are clouds white?
Each droplet scatters small amount of light in all directions- all colours scattered with equal efficiency
When coloured substances appear white it implies extinction by scattering dominates absorption
Light transmitted through water is blue
Blue comes from preferential absorption of other colours in spectrum
Clouds dont absorb much light so they aren't blue
Net effect of many scatterers is white light
Black clouds
Underside of the cloud looks dark
Scattered white light is lost from the direct beam
Visibility
Backscattering of light by aerosols is much weaker than forward scattering
Visibility looking into the sun is less than visibility looking away from the sun
- whitening of direct beam reduces contrast
Caused by scattering of light by aerosols
Visibility=
How far away you can see a black object against the sky
Colour of the sky
Blue light scatters 7x more efficiently than red
Direct beam looks red
Air molecules scatter light
Scattering efficiency depends on 1/wavelength^4
Blue light wavelength= 400nm
Red light wavelength= 650nm
White light is composed of
seven colours
Dimensional analysis= Lord Rayleigh- Rayleigh scattering
Without an atm, there is no scattering: sky is black and sun is white
A moonlit sky on Earth is black because scattered blue light is very weak
Lower light intensity
at night makes our eyes see the sky as black when it is the
same colour as during the day