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Lunatic Cirrus Corona

My last night's evening walk in Boulder was worth facing the stormy wind, as on my way home I happened to observe the most stunning and colourful corona display that I've ever seen. With naked eye I was able to distinguish not just the typical first maxima rings but even the second maxima. Of course I'd left my camera at the office but with my video camera and some photo editing I was able to get a reasonable image of the corona display showing at least the first maxima.

Another reason why the corona made me so lunatic was the fact that it seemed to be displaying through a high lying cirrus cloud. During the day no low level clouds were to be seen and the persistent cloud mass indicated a high level cirrus cloud.

Of course as an atmospheric scientist I checked the radio sounding data from that evening provided by the University of Wyoming from a sounding from Denver. The sounding show cold temperatures up to -60°C under the tropopause. If indeed a high level cloud, this cloud had to be completely glaciated.

Normally corona is formed when moon or sun light shines through thin liquid clouds of droplet sizes around 10 µm. The uniform size of the cloud particles is the key for the phenomena and so it was long thought that ice clouds have a particle size distribution that is too large and too wide for producing the corona effect. Yet, in the 90's series of field observations reported a corona observed through a cirrus cloud. An explanation of how ice crystals in cirrus clouds can produce a corona, similar to water droplets of uniform size, was later given in my first paper as a PhD student (Järvinen et al., Appl. Opt., 2014). It was a laboratory study, where we showed that a rapid homogeneous freezing can lead to a narrow ice crystal size distribution with small (~17 µm) and compact crystals (see microscopic images from the paper). Now we are not talking about those pretty and pristine crystals that are responsible for the halo-phenomena.

For a scientist it's rewarding to be able to use your own findings to explain the world, even if its something simple as a colourful night sky.

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