Unusual Double Fogbow Imaged by Marko Krusel (atmospheric optics site) on the Tallinn-Tartu highway, Estonia, 06:35 on 24th August '09. ©Marko Krusel, shown with permission.

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The rarity is the faint outer secondary fogbow from light waves reflected twice within the fog droplets. Secondaries are rarely seen let alone photographed in naturally lit fogbows although they are obvious in night fogbows produced by bright lamps.

The bright inner primary fogbow has little colour, just a slight red outer edge. That tells us that the fog droplets were small.

There is a characteristically dark band inside the primary but no clear supernumeraries - the droplets therefore had a range of sizes.

Morning fog that has formed in the night from radiative cooling of the earth and lower air layers tends to be 'old'. 'Old fog' as opposed to the freshly formed variety has a range of drop sizes, the result of collisions and preferential evaporation of small drops to feed condensation on larger ones. Fresh fog droplets have not had time to evolve.