"Old Fog" Fogbow

Imaged by Les Cowley off the coast of Western Greenland 70 - 71° N June '11.

A thick sea fog had formed in the early hours and during the morning the ship was cautiously making way SE through waters strewn with icebergs that would loom suddenly out of the fog.

Towards 0900 local time (solar elevation 34°) weak sunlight filtered through. Would there be a fogbow?

The antisolar point (a fogbow's centre) was down over the stern rail and sure enough arcing over ship’s wake was the first inkling of a fogbow which steadily became more definite as the fog thinned and the sunlight strengthened.

A red outer rim and a delicate blue inner one were evident particularly in the lower section where the bow was viewed against the dark sea. In that area a weak single supernumerary was occasionally glimpsed.

Around the shadow of my head on the sea was a weak single ringed glory which strengthened later on (lower image). The glory's small diameter, the fogbow colours, narrow width and indistinct supernumeraries together indicated that the fog droplets had a large median size with a substantial spread of actual diameters.

Old fog! Freshly formed fog has a narrower droplet size distribution and smaller droplets. Fresh fog gives the best fogbows and glories.

Subsequent comparison with IRIS Mie-Lorentz simulations indicated a median droplet diameter in excess of 40 micron with a very wide size distribution – old fog indeed.

Atmospheric
Optics

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Fresh & Old Fogs

Freshly formed fog consists of small droplets of fairly uniform size because all the droplets are of similar history. The droplets are well separated and do not often collide. However, after time the combined effects of occasional collisions, evaporation and condensation broaden the droplet size distribution to 1- 100 micron diameter and the average size gets larger.

The left hand IRIS simulation is for a fogbow and glory formed by young fog with almost (3% std dev) monodisperse droplets of 16 micron diameter. The bow is broad and there are several supernumeraries. The glory is large with several rings.

The right hand simulation is for an evolved fog with droplets of average diameter 50 micron but with a wide size range (35% std dev). The fogbow is much narrower and the glory miniscule. A supernumerary is visible more by the characteristic dark space between it and the primary than by the intensity of the supernumerary itself.

Fogbows form like rainbows by a single reflection of sunlight within spheres of water. Raindrops spheres are 1mm or more in diameter but those of fog are sufficiently small that diffraction effects become significant and the bow is considerably broadened.

The diminutive glory accompanying the 'old fog' fogbow.