A rainbow is caused by the sun hitting water droplets in the atmosphere and reflecting the light so that people see a spectrum of colors. Rainbows are bowed in the center due to the fact that rainbows result from a group of circular arcs that have one common center.
Rainbows appear when there is a lot of water in the atmosphere. Most often they occur during rainstorms when the sun comes out from behind the clouds, illuminating the portion of raindrops and water droplets that the sun points at once it appears form a spectrum of light. For this reason, rainbows always are in the direction of the rainfall and the sun is always in front of the rainbow.