Saturday, 28 May 2011

Parhelion revisited

From rainbows to the aurora, our atmosphere is capable of generating some spectacular light effects. A few days ago, we posted a photograph of a parhelion. This is a light effect, caused by the scattering of sunlight from ice crystals in the atmosphere. Now, thanks to a recommendation from Jenny Shipway at the InTech Planetarium, we've found some interesting software that allows anyone to experiment with this phenomenon.

The recommended site was http://www.atoptics.co.uk/ — a site dedicated to atmospheric phenomena. The relevent section for what we saw from the KAIRA site is under 'Ice Halos'. Additionally, there is a link to some Windows software that anyone can download and use. And, that's what we've done here to re-create the effect that was seen at Kilpisjärvi.

The software does need some data. There was the time and date of the photograph (11-May-2011 16:48 UTC), from which we can use another online tool to get the solar elevation (as we didn't measure it at the time!). This gives a solar elevation of 16.9 degrees.

Below is the simulation (top image) and actual photograph (below it). As you can see from the comparison, it does a pretty good job of modelling what we actually saw.



Obviously there are a lot of lens flares within the photograph due to the limitations of the camera (both the lens and the CCD sensor therein) and I could have spent more time tweaking the camera angles to get the alignment correct. Still, it is a good match and indicates that the physics is well understood within the software.

The programme is pretty sophisticated and one can tweak parameters such as the ice crystal shapes and orientations to explore different effects.

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