Asked by: John, Southampton
A rainbow is created when sunlight splits into various colours as it enters raindrops. These rays then bounce off the interior and re-emerge at slightly different angles, depending on their colour. Seeing a rainbow thus depends on where you're standing relative to the Sun and the raindrops. So while forecasters can predict the conditions of patchy cloud and showery conditions that make a rainbow likely, they can't tell if you'll be in just the right place to see one.
Robert is a science writer and visiting professor of science at Aston University.