This fogbow is unusual for its many well defined and coloured supernumerary arcs – a sign of generation by droplets having a very narrow range of diameters.

Fogbows form like rainbows – a single reflection inside water drops is responsible.   The difference is that, in tiny cloud or fog droplets, diffraction effects become much more influential compared to those in large raindrops.   The result is a broad and only weakly coloured bow.   

Supernumeraries – when present – are a good gauge of droplet sizes.     Smaller droplets give more widely spaced supernumeraries and a broader primary fogbow.

At left IRIS has simulated two fogbows using Mie theory.  The supernumeraries pull together, the bow narrows and colours intensify as the droplet size increases.