Rainbows form around the antisolar point, the point exactly opposite the sun (look at the ground on a sunny day and that is where the shadow of your head is!) If the sun is moderately high in the then rainbows that form can appear very close to the ground, as is the case here. Statistically you are less likely to see a rainbow like this because the cloud producing the rain is more likely to block the sun!
I took perhaps 20 shots of this rainbow but it was the first hand held shot that showed the best colour. This was the second rainbow of the day. The first is here: [link]