A rainbow "forms" the moment mist (tiny water droplets in the air) is present between the observer and the sun at such a position that the water droplets diffract the white sunlight, splitting it into its constituent colors. read more
The moment the mist is there, you can see the rainbow, so the time required to "form" a rainbow is the time required to form the mist, or for the mist to arrive. The rainbow doesn't really "form", in the sense that a cloud forms. read more
Quick Answer. Rainbows form when water in the air reflects and refracts sunlight and disperses the normally white sunlight into colored light. White light is composed of colored light that bends at different angles when reflected and refracted, separating the white light into the distinct colors of a rainbow. read more
The potential for these arcs to form in Los Angeles is 5-10 times higher than in London. Aside from the position of the sun, the other ingredient to forming circumhorizontal arcs is cirrus clouds. Cirrus clouds are the thin, wispy clouds that occur at higher altitudes. read more