A circumhorizontal arc is an optical phenomenon - an ice-halo formed by the refraction of sun- or moonlight in plate-shaped ice crystals suspended in the atmosphere, typically in cirrus or cirrostratus clouds. In its full form, the arc has the appearance of a large, brightly spectrum-coloured band running parallel to the horizon, located below the Sun or Moon. The distance below the Sun or Moon is twice as far as the common 22-degree halo. Red is the uppermost colour. Often, when the halo-forming cloud is small or patchy, only fragments of the arc are seen. As with all halos, it can be caused by the Sun as well as (but much more rarely) by the Moon.
So, although the rainbow looks curved, it was straight across the sky and there is no sign of rain anywhere nearby (in fact, we are on the verge of having fire dangers in the area). It surely is a great sign today of "living blessed on the prairie!"
Please let me know if you have any better explanations for this, because it seems to be such a weird occurrence!