Today I saw the phenomenon in picture below. It was not raining (at least nearby me). What can that be? What is the technical explanation?
Edit: Just seen today in Southern Brazil another circumhorizon arc, this time together with a halo. Really beautiful.
This is not a "rainbow". Quoting from the linked site, "not all colored patches in the sky are rainbows". It instead is a circumhorizon arc. Rainbows are caused by internal by refraction and reflection in water droplets. Halos such as the circumhorizon arc portrayed in the question are caused by internal refraction and reflection in ice crystals.
that type of rainbow is created by light refraction within an extended cloud or layer of ice crystals, in which the crystals are similarly-shaped and have been aligned by wind shear so their principal axies are all pointing in more or less the same direction. they then behave like a cloud of microscopic prisms suspended in the air and if you are observing them from the right angle on the ground, you'll see the rainbow.
