The evening of May 17th stood out by being very hazy. Sunset colors were an unusual ochre (see photographs by Dutch KNMI meteorologist Jacob Kuiper here). But more excitedly, both Jacob and I managed to observe (and in my case, photograph) the rare Bishop's Ring.
Below image was taken by me on the late afternoon of May 18th, when remnants of the ash cloud passage still lingered in the atmosphere.
Visible is a diffuse disc of light around the sun (the sun itself is just behind the roof tip). The outer edges are reddish, the inner part is more bluish, as can be seen from this version where I depicted the RGB color values for two parts of the ring in the color spectrum:
With "normal" halo's, due to ice particles where refraction is the dispersal mechanism, the blue is on the outside and the red on the inside. Here however, it is the other way around, which confirms it is due to diffraction by dust.