How do rainbows form?

Rainbows are formed when sunlight is scattered from raindrops into the eyes of an observer.

Most raindrops are spherical rather than the often depicted 'teardrop' shape and it is this spherical shape that provides the conditions for a rainbow to be seen.

The position of the sun and the raindrops in relation to the observer need to be just right for a rainbow to form:

- The sun needs to be behind the viewer

- The sun needs to be low in the sky, at an angle of less than 42° above the horizon. The lower the sun in the sky the more of an arc of a rainbow the viewer will see

- Rain, fog or some other source of water droplets must be in front of the viewer

The size of the raindrops does not directly affect the geometry of a rainbow, but mist or fog tends to disperse the effect more (see fogbows).

Rainbows only appear semi-circular over level ground at sunrise or sunset, when the sun is exactly on the horizon, the majority of the time a smaller segment of an arc is seen.