IT CAN be a moving experience suddenly to see a rainbow. In D. H. Lawrence's novel, for example, the rainbow of the title becomes a symbol of hope for Ursula Brangwen as she looks out disconsolately over the bleak dank collieries of Cossethay.
"Steadily the colour gathered mysteriously from nowhere, and the are bended and strengthened itself till it arched indomitable, making a great architecture of light and colour in the space of haven. She saw in the rainbow the earth's new architecture - the old, brittle corruption of houses and factories swept away, and the world built up in a living fabric of Truth".
Others, however, are able to describe their feelings without resort to such hyperbole - as did a reader who noticed the recent Weather Eye on rainbows and was kind enough to write to describe her own experience many years ago in Co Wexford: "I saw a rainbow in the moonlight, white of course, but magical. It did not last for very long, but I have never quite forgotten it."
Whenever it may occur, a rainbow is always formed by the reflection of light by little drops of water in the air. Usually when we see one, the source of the light is the sun, but it does not have to be provided there are water drops in the right place, the are can be seen at night as well - originating from the light of the moon.
Sufficient light for a "moonbow", as it is sometimes called, is normally available only at full moon, and even then, moon rainbows are much weaker than their day time equivalent. They are seldom coloured just as all feebly illuminated objects usually appear colourless at night. And like the daytime rainbow vis a vis the sun, a moon rainbow is always seen on the opposite side of sky to the moon. Moonbows are less common than solar rainbows, because the showery type of rain needed for their formation is less frequent at night than during the day.
But white rainbows can be seen by day as well. They occur when the water droplets that reflect the light are provided by a bank of fog, rather than a shower of rain. The tiny droplets that produce a "fog bow" are only a fraction of the size of raindrops, and do not separate the spectral colours of the white sunlight with anything like the same effectiveness as raindrops do. Fogbows, therefore, are generally white, with just the very slightest tinge of colour a case perhaps when Shakespeare's dictum
To add another hue unto the rainbow.
Is wasteful and ridiculous excess does not apply.