Rainbows require two things: sunlight, and water in the air in the form of mist or rain. The best place to consistently find both of those things is a tropical island with a tall mountain or ridge on it. read more
So if you're on the right side of the mountain, it's easy to have sun and rain at the same time, and get a rainbow. The American state of Hawaii, for example, gets so many rainbows it calls itself "The Rainbow State" - you can usually see rainbows multiple times a day from Waikiki. read more