Eyjafjallajokull Subtleties 
Volcanic ash continues to hang in European skies. As outlined in the previous OPOD – and contrary to colourful media images – its optical effects are subtle.  Compared to the very violent volcanic eruptions that inject ash and aerosol forming SO2 high into the stratosphere, Eyjafjallajokull's dust is lower in the troposphere and the particles are larger.

Top image (1) could be titled “Large and Small”. Peter Paul Hattinga Verschure captured it on Sunday April 18 at Deventer in Holland. A huge Bishop’s Ring aureole spreads out from the shielded sun. Diffraction by volcanic dust produced it. The exact same effect is present on a much smaller scale. Close to the masked sun are the comparatively tiny rings of a corona made by much larger pollen grains. “Today another Bishops’ring event [earlier one], but now simultaneously seen with double pollen corona. Two diffraction phenomena in an absolutely cloudless sky! I succeeded in making them visible by processing the image with an unsharp mask. In the unprocessed image the pollen corona is too weak to reproduce. Data: 18 April, 1225 UT, Nikon D80, 20 mm focal length.

Sunset image (2) taken in Northern Ireland by Martin Mc Kenna (site) shows an almost cloudless sky with a very faint sun pillar. Over the last few days there have been many faint 22 degree halos, tangent arcs, sundogs and pillars. Connected with volcanic ash? The absence of contrails and contrail induced cirrostratus might allow these faint halos to be seen? The dust might be nucleating halo forming ice crystals?

Martins second image
(3) shows clearly the faint and high horizontal clouds – possibly ash originated – that show up at sunset and dawn.

(4) John McConnell’s 16th April sunset over Northern Ireland also shows the narrow horizontal clouds plus yet another faint sun pillar. His picture sums up the sunrises and sunsets over Europe – no blazing colours but a sullen sun in deep haze.

John’s lower image
(5) shows the early dawn twilight on the 16th. “There were still plenty of stars visible, but the colour is quite striking. No alteration was made to the image. However, I have taken plenty of images at or close to dawn but have never seen this colour. The camera was also on default settings so this is what the camera saw.” We might expect enhanced purple twilights from any sulfate aerosol that has formed in the lower stratosphere. The purple will be largely masked by scattering from the lower tropospheric dust until the latter dissipates – presently there is little sign of that happening.

Images ©as described and shown with permission.
Atmospheric
Optics

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