Saturday, 4 March 2017

Odd radius halos with Wegener and subhelic arcs in the UK

A short-lived display of relatively high quality occurred in Berkshire, UK, in the morning of 4th March, 2017. A routine check at 9:35 local time suggested the presence of faint 18° and 23° parhelia, in addition to the usual 22° stuff, so I collected my camera, tripod and blocker and set up the system in a nearby park area. After just 25 minutes the display was all over, but there had been enough time to capture three 30-frame series for stacking.

In addition to the 18° and 23° parhelia, both visible to the eye during the display, the processed stacks suggest parts of 20° and 35° (and possibly 24°) circular halos. Unfortunately, the region of 9° halo is over-exposed in my photos. However, as far as my visually-based observations are to be trusted, there simply were no halos 9° from the Sun.

Given that we are dealing with a cirrus display here, I find the presence of both supralateral and infralateral arcs indicative of fairly ideal column crystals. This is confirmed by the side-view stack, as faint Wegener and subhelic arcs can be identified. The latter crosses the parhelic circle near the tall tree at the bottom. Just slightly further away from the Sun, there is a 120° parhelion too.


  1. Reima, is that the first time you have photographed the 20° and 35° halos in particular here in the UK? I'm located to the north of you in Lancashire but only managed to get a meagre, weak 22° on the same day. I'm slightly envious to say the least!

  2. I had one case of 20° and 35° halos in the Summer of 2015. But that was just on the threshold of detection even after stacking and colour subtraction. This case is more clear (although still far from being visible to the eye on the field).

    In general I don't recall seeing odd radius halos as often in the UK as I used to when living in Finland. Could be just sampling noise, though.

  3. In the lower row br I tend to see a similar segment as the 35° halo but closer to the sun. Around perhaps the 28 degrees...

  4. I've just had a look at the area Marko mentions and there definitely appears to be something there. I've really cooked the png and it shows quite well.