Tag Archive | strange weather days

Fallstreak holes

Oct10

I was in Salisbury this morning for a dental appointment, and was very excited to notice the unusual skies: the high-altitude small cobbler-like clouds (I know they have a name but I don’t know it) had four or five oval ‘gashes’ in them, each of which was filled with a fluffier, whiter cloud. These are called fallstreak holes.

Not one of my fallstreak holes from this morning - this one was over Oklahoma City in the US in January 2010. It is very similar to what mine looked like though.

Not one of my fallstreak holes from this morning – this one was over Oklahoma City in the US in January 2010. It is very similar to what mine looked like though. Photo by Paul Franson.

Wikipedia explains their formation thus:

‘A fallstreak hole (also known as a hole punch cloud, punch hole cloud, skypunch, cloud canal or cloud hole) is a large gap, usually circular or elliptical, that can appear in cirrocumulus or altocumulus clouds. Such holes are formed when the water temperature in the clouds is below freezing but the water, in a supercooled state, has not frozen yet due to the lack of ice nucleation. When ice crystals do form, a domino effect is set off due to the Bergeron process, causing the water droplets around the crystals to evaporate: this leaves a large, often circular, hole in the cloud.

It is believed that the introduction of large numbers of tiny ice crystals into the cloud layer sets off this domino effect of evaporation which creates the hole. The ice crystals can be formed by passing aircraft which often have a large reduction in pressure behind the wing- or propeller-tips. This cools the air very quickly, and can produce a ribbon of ice crystals trailing in the aircraft’s wake. These ice crystals find themselves surrounded by droplets, grow quickly by the Bergeron process, causing the droplets to evaporate and creating a hole with brush-like streaks of ice crystals below it. Such clouds are not unique to any one geographic area and have been photographed from many places.’

But sadly not in Salisbury this morning by me, because I didn’t have my camera with me. Buggeration. At one point I could see five holes. When I came out of my dental appointment 45 minutes later they had gone, and in their place were fluffy cumulus clouds.

I love Strange Weather Days. I still remember the excitement when Chap and I saw our first (and still only) ever mammatus clouds, in New Zealand in 2008. Well, we have to get our jollies somehow, don’t we?

Sun dog days

Jul15

Today has been a beautiful sunny summer’s day, with fluffy white clouds bumbling by all day. This evening we decided to enjoy the last of the sun’s heat in the garden with a bottle of Chenin blanc and some olives. The sky was clearer, bluer, with the occasional mares’ tails were high in the sky. The house martins and occasional swallows (and two swifts—a rare sighting round here) were zooming overhead.

And that’s when we saw a beautiful arc of a rainbow, high in the sky and directly over the sun, but curved back against it.  I thought it was a sun dog, but a quick google has revealed that it was a circumzenithal arc, as it was inverted, rather than bending around the sun.  I was so excited I snapped lots of photos and ran over to our neighbours’ cottage to tell them, but they were out. But looking at the sun from their cottage, the rainbow looked different—shorter and stubbier. It wasn’t until I got back to our garden that I realised that the shorter stubbier one was something different, on the same level as and to the right of the sun: a right side sun dog.  We hadn’t noticed it initially from our garden because of all the foliage of the trees and shrubs.

west of Salisbury, Wiltshire, 15 July 2014, 6.35 pm.

Circumzenithal arc west of Salisbury, Wiltshire, 15 July 2014, 6.35 pm. The sun is just below the bottom edge of the photo.

So exciting!  We love weird or unusual weather phenomena. The last I can remember seeing were our first (and so far only) mammatus clouds when we were in New Zealand.

We watched the arc and sun dog for about a half hour, from 6.30ish onwards, and then they gradually disappeared. There were lots of cirrus clouds (mares’ tails) around at the time, and it is the refraction of light through the ice crystals in these very high clouds (typically 5,000 m (16,000 feet) in temperate zones) that cause this atmospheric effect. A strange weather days first for us!

Cirrus clouds (mares' tails) above our village, evening of 15 July 2014, at the same time the circumzenithal arc and sun dog were visible elsewhere in the sky.

Cirrus clouds (mares’ tails) above our village, evening of 15 July 2014, at the same time the circumzenithal arc and sun dog were visible elsewhere in the sky.

I took a couple of short videos. The second shows the sun dog as well as the circumzenithal arc:

This post has reminded me that I must get round to joining the Cloud Appreciation Society.