Mountain Weather
Information Service

Fogbows, glories and spectres...

It's been a superb few days of inversion conditions once again! (See previous MWIS blogs for more on how these situations arise). 

As high pressure built to the north during the past week, the charts were looking good for inversion conditions, and wow did they deliver...  You've been sending us truly mesmerising scenes on the Highland tops, with many of you spotting some classic optical phenomena. 

Tag us into your pictures on social media and we'll share! @the_mwis #mwis Check out a gallery of your best pics below the text!

Here's a little science behind the magic...


These ghostly white features are similar but different to your standard rainbow. They appear opposite to you from the sun, and require the interaction of light and water droplets - refraction, and reflection within a droplet. 

However, in the fogbow, the water droplets are much smaller than raindrops, causing diffraction of the light as it leaves the droplet, which interferes with the light, meaning all we see is a white light, rather than distinct colours. 

Usually, fairly thin fog is required, so that you get the essential combination of bright sun and fog in the same place!


Look into the centre of a fogbow, and often a glory can be seen - appearing directly opposite to the sun from your viewpoint. A similar light process occurs to fogbows and rainbows, but here we're dealing with very subtle 'surface waves' of light. 

Some fairly complex physics is going on here, but essentially it is light which 'clings' to the tiny water droplet briefly as it refracts. Diffraction produces the ring of light, and varied angles of light wavelength generate the colours much like in a standard rainbow.


The classic 'Brocken Spectre', so named after the highest point in the Harz Mountains of Germany, is the spooky shadowy figure seen on the fog banks. 

It is your own shadow, and can appear giant sized - but this is an optical illusion, because you are seeing the shadow on nearby fog, whilst referencing it against objects further away. The peculiar triangular form is all about perspective.