Panayoti Kelaidis writes:

 

I and my family have observed iridules on several occasions in Colorado: they have usually been in the winter time, although my daughter pointed one out to me recently in early June. I am not sure they are a reflection of a rainbow at all, but in fact a distant, high altitude rainbow that occurs when light hits a cloud at just the right angle at a certain time of day (just as rainbows usually occur only in the afternoon or morning depending on direction of sun and rain). I think they may be a phenomenon more widespread in continental regions, or especially mountainous terrain.