Storm has blown over as quickly as it came and I can see at least two parhelia. Parheliae? Lovely. But it isn't cold enough for them to be icecrystal ones, I don't think; perhaps the London smog has similar refractive properties?
Wikipedia say: Because it occurs due to internal reflection in ice crystals, it is produced only by clouds cold enough to consist entirely of ice particles rather than liquid water droplets. Such clouds are usually found only at temperatures below about -20°C.
This is at least partly bullshit, though, because I've seen them on days when there is no visible cloud at all. It was -30C at the time so any moisture in the air would definitely have been frozen, but it clearly doesn't have to be visible as a cloud.
I'm going to stop looking at the sun now, though, SAD aside it isn't doing my eyes any good.
Just because you can't actually see the cloud, doesn't mean there isn't one there - and cirrus clouds are comprised entirely of ice crystals, rendering them wispy and often transparent (http://en.wikipedia.org/wiki/Clouds#High_clouds_.28Family_A.29). So no, it's not bullshit.
And the air temperature at ground level is entirely irrelevant to the air temperature at 3-5 Km up, which is where cirrus clouds form. It could be a warm balmy day of 20°C at ground level yet still be -20°C 5 Km up.
(Meteorology is a pet subject of mine, and not just in the extreme weather sense.)
I'm possibly getting these confused with parhelia caused by diamond dust (http://www.sundog.clara.co.uk/halo/diamonds.htm), which are more the sort of thing I'm used to.
That does sound likely. You won't see diamond dust here except possibly in the mountains in Scotland and Snowdonia, and even then only extremely rarely. Just about all the parhelia you will see in the UK will be due to cirrus. I've seen a few parhelia in London but far more in Wales.
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timeasmymeasure
Isle of Sundogs
Date: 2006-03-09 04:59 pm (UTC)Re: Isle of Sundogs
Date: 2006-03-09 05:06 pm (UTC)Because it occurs due to internal reflection in ice crystals, it is produced only by clouds cold enough to consist entirely of ice particles rather than liquid water droplets. Such clouds are usually found only at temperatures below about -20°C.
This is at least partly bullshit, though, because I've seen them on days when there is no visible cloud at all. It was -30C at the time so any moisture in the air would definitely have been frozen, but it clearly doesn't have to be visible as a cloud.
I'm going to stop looking at the sun now, though, SAD aside it isn't doing my eyes any good.
Re: Isle of Sundogs
Date: 2006-03-09 05:29 pm (UTC)And the air temperature at ground level is entirely irrelevant to the air temperature at 3-5 Km up, which is where cirrus clouds form. It could be a warm balmy day of 20°C at ground level yet still be -20°C 5 Km up.
(Meteorology is a pet subject of mine, and not just in the extreme weather sense.)
Re: Isle of Sundogs
Date: 2006-03-09 05:46 pm (UTC)Re: Isle of Sundogs
Date: 2006-03-09 06:00 pm (UTC)