ice halos and other optics

Moon halos from Mustavaara snowguns 23/24 November 2010

When it is windy you don’t get diamond dust halos in Finland. For some reason (could it be that in still weather radiative cooling is stronger and thus relative humidity can rise to reach ice saturation?) there is not enought moisture in the air for ice crystal growth in windy weather. This night is was windy but I nevertheless drove 25 km to Mustavaara to check the situation. As expected there was no diamond dust and I was thinking of going back to Joensuu. Still it looked like it was on the treshold so I decided to wait. This paid off as eventually the ice fog from the snow guns starting spreading and 22° halo was seen in it. Further away the display was a bit better, with also parhelia visible (there was also high cloud in the background, which may have partly contributed to the display, in the second photo the 22° halo on the left is in high clouds). I tracked the cloud extent to 10 km from the source but at that time it was already receding, so probably it was wider at its best.

So this display broke the “no wind” curse for me in Finland. At polar areas where it is hardly ever calm, diamond dust halo displays seem to occur at any wind speeds.

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