Rare ghost rainbow phenomenon seen over Watten

After a spectacular rainbow over Watten on Saturday afternoon, a ghost rainbow, or fogbow, was seen in the same position today (Monday, January 8).

The dry and cold conditions in Caithness today led to a blanket of freezing fog across the county along with strong sunshine which helped create the phenomenon, seen around 1pm today.

A fogbow is similar in some respects to a traditional rainbow forming from sunlight interacting with water droplets contained in fog, mist or cloud rather than interacting with raindrops as it does in a classical rainbow.

Wick weather watcher Keith Banks said: "Fogbows are an optical water droplet phenomenon.

"They occur when the diameters of the droplets suspended in the atmosphere are extremely small. The optimal conditions for fogbow development arise when the diameters of the water droplets that are suspended in the atmosphere are all the same size – typically just 0.05mm.

"Rainbows are formed when light enters a water droplet and is then refracted twice and internally reflected once. However a fogbow scatters light of all wavelengths, and this manifests as an essentially white bow. The principle physical process involved in fogbow genesis is called diffraction."

Fogbows also go by the names white rainbow, cloudbow or ghost rainbow.

The Met Office says that conditions in Caithness today are dry but cold. Its bulletin says: "Early freezing fog patches for some, only gradually clearing. Temperatures slow to lift despite the prolonged periods of weak winter sunshine, with colder spots remaining sub-zero. Winds light mainly from the east. Maximum temperature 2 °C."

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