|Rocky Mountain Iridescence - Scott Carlin saw these iridescent clouds from Denver, Colorado facing west towards the mountains. ©Scott Carlin, shown with permission.
Uniform sized droplets are the key to vivid iridescence like this. They are best sought in thin freshly forming cloud at the edges of larger opaque masses.
The iridescent patches are often small, here they occupied the area marked by the rectangle for around 15 minutes. Look fairly close to the sun taking care to shield it first behind a tree or building. Sunglasses help to reduce the glare, another option is to look at the sky reflection in water or a black glass.