Skyglow: the Effect of Poor Lighting

Skyglow is the orange "smog" that hangs over town and cities at night. It is caused by inefficient lights shinning needlessly into the sky, and is predominantly caused by street-lighting (hence the colour).

Skyglow in the Countryside

This photograph (right; click to enlarge) shows light pollution from towns and cities as seen from a rural location in South West England, where towns can be easily seen from over 35km away.

What causes Skyglow?

In the diagram (below right) there is a globe-light which is shining in all directions. To the left is an observer.

Light scatters (reflects) off particles in the atmosphere, and in the diagram below arrows indicate in which direction light preferentially scatters. The length of the arrows indicate the relative strengths of the scattering in the various directions.

When light scatters through large particles in the atmosphere (e.g. aerosols, dust, water), only a small fraction of the light is scattered backwards or to the sides, and the majority of the light continues to travel in the same forward direction. This is known as Mie Scattering, and it predominantly occurs at low altitudes.

However, when light scatters through small atmospheric particles (e.g. in clearer, cleaner skies), a larger fraction is reflected backwards, and to the sides. This Rayleigh scattering predominantly occurs at high altitudes, and is the reason why the sky appears blue on a clear day.

The diagram shows both these effects - the Mie scattering dominating at low altitudes, and Rayleigh scattering at high altitudes.

From the extensive research by Dr Chris Baddiley into the cause of skyglow, we know that
sky-glow is mainly caused by light emitted close to horizontally,
and it is for this reason that neither non cut-off nor shallow bowl semi cut-off lighting should be used for street-lighting.

In his research, Dr Chris Baddiley reached the following conclusions:

• Light has a preference to scattering forwards along its path (like stones skipping across water) rather than at right angles, due to aerosols (large pollution particles) in the lower atmosphere.
  
  
• Shallow bowl luminaires cause more sky glow than full cut-off street-lights, even when fewer are used per given road length. This is because they have more side emission and the higher reflectivity of surfaces at these angles cause more skyglow.
  
  
• Shallow bowl lights are also visually more obtrusive (i.e. they shine into peoples' and car drivers' eyes). They should not be used in open areas and should be restricted to town centres.
  
  
• To minimise skyglow in the countryside, all lighting should be shielded from the horizontal view (e.g. use full cut-off lighting).
  
  
• The reduction in skyglow by universally adopting full horizontal cutoff lighting in all areas outside of town centres can be a factor of 3 to 5 according to elevation and distance of view.
  
  
• Measures such as dimming and switching off when not necessary, should be used wherever possible.
  
  
• In city centre's, skyglow is predominantly caused by ground reflection, and not direct illumination, since light emitted close to the horizontal is usually blocked and reflected by buildings.
  
  
• Outside the high-rise city centre, skyglow is predominatly caused by street-lights shinning at low to mid angles above the horizontal.
  
  
• The reflected light component is mainly reflected off the surfaces surrounding the roads (e.g. grass), and not the road itself. However, reflection along a road at shallow angles can be significant.