<P> Rainbows span a continuous spectrum of colours . Any distinct bands perceived are an artefact of human colour vision, and no banding of any type is seen in a black - and - white photo of a rainbow, only a smooth gradation of intensity to a maximum, then fading towards the other side . For colours seen by the human eye, the most commonly cited and remembered sequence is Newton's sevenfold red, orange, yellow, green, blue, indigo and violet, remembered by the mnemonic, Richard Of York Gave Battle In Vain (ROYGBIV). </P> <P> Rainbows can be caused by many forms of airborne water . These include not only rain, but also mist, spray, and airborne dew . </P> <P> Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind the observer at a low altitude angle . Because of this, rainbows are usually seen in the western sky during the morning and in the eastern sky during the early evening . The most spectacular rainbow displays happen when half the sky is still dark with raining clouds and the observer is at a spot with clear sky in the direction of the sun . The result is a luminous rainbow that contrasts with the darkened background . During such good visibility conditions, the larger but fainter secondary rainbow is often visible . It appears about 10 ° outside of the primary rainbow, with inverse order of colours . </P> <P> The rainbow effect is also commonly seen near waterfalls or fountains . In addition, the effect can be artificially created by dispersing water droplets into the air during a sunny day . Rarely, a moonbow, lunar rainbow or nighttime rainbow, can be seen on strongly moonlit nights . As human visual perception for colour is poor in low light, moonbows are often perceived to be white . </P>

When does a rainbow appear in the sky