Weather Watch: Colourful science at the end of every rainbow


A more benign consequence of the lively thundery showers recently has been the opportunity to see some spectacular cloud formations, as well as colourful rainbow displays.

Rainbows are optical and meteorological phenomena caused by reflection and refraction of sunlight in water droplets, nearly always appearing in the part of the sky opposite the sun, so for example an evening rainbow will be seen in the eastern sky.As sunlight passes through rain droplets, multiple rainbows are actually produced.

The most clearly visible is the ‘primary bow’, involving refraction of light as it passes into the raindrop, total internal reflection within the raindrop and then further refraction as it moves back out.

The arc of light will be at an angle approximately 42 degrees to the line between sunlight and observer, with the light dispersing so that red shows on the outside and violet on the inside of the bow.. This means that when the sun is higher than 42 degrees above the horizon - the case in Bucks during the middle of the day in summer - it’s impossible to witness the primary rainbow when on flat ground.

The ‘secondary bow’ involves an additional reflection within the raindrop. As more light is ‘lost’ before it finally passes out of the drop, the secondary arc is harder to see, usually only visible when there is a particularly dark cloud in the sky opposite the sun.

The colours of the second arc are reversed, with red on the inside, and it appears higher in the sky than the primary bow.

Even weaker ‘tertiary’ and ‘quaternary’ bows are formed from sunlight reflected multiple times within the droplet. These actually appear in the direction of the sun, but are rarely seen with the naked eye.

Rainbows can also be caused by mist and spray from waterfalls and fountains, whilst sunlight shining through a glass object can lead to a similar dispersion of light and colour spectrum.

Spells of warm sunshine across Aylesbury Vale over the coming days, but an increasing chance of scattered showers too, perhaps with more prolonged rain on Friday night and early Saturday. As a result, there could well be the opportunity to see further colourful rainbows.



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