White Rainbow at Malvern

    Abstract

    AN account of a white rainbow has been received from Mr. R. H. Stevens of "Rockland", Cowleigh Road, Malvern. The bow was seen at about 10.30 a.m. on November 8 after a short snowstorm, but no snow could be seen to be falling at the time of the observation. The white rainbow, also known as the 'fog-bow' or 'Ulloas Ring', is a rare phenomenon due, as in the case of the ordinary coloured rainbow, to refraction and reflexion of sunlight in falling drops of rain, but the raindrops composing the cloud must be very small—0·1 mm. or less in diameter. When this is the case, the first maxima of intensity for the different colours of the spectrum are spread out over a wider angle than with the larger drops, and are nearly coincident, the result being approximately to restore the original colour of the sunlight. The bow is only bright enough to be visible in exceptionally favourable circumstances and when the observer is near the cloud which contains the small drops. In this instance the previous occurrence of snow must be assumed to have been fortuitous, the snow probably having originated from clouds at a higher level than that of the cloud which carried the water drops.

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