Yes, having done some more research (or rather, having read a bit more carefully ), these clouds do appear to have been nitric acid arihydrate “Type 1” PSCs. They only need about -78C to form.

Type 2s, the pure water-ice ones, are the ones that are formed by stratospheric mountain waves (as explained in the first “afterglow” article).

Type 1s, as explained in Ed’s link, tend to be less colourful than Type 2s, but these sunsets were clearly spectacular in their range and depth of colour. When I initially wrote the “more afterglow” article I was thinking that the PSCs were responsible for the colouration, which didn’t quite make sense not least as I’d observed them myself to be white on Tuesday evening.

Where I was wrong was thinking that it was the PSCs themselves that were producing the colour. On further thought I reaslised that couldn’t be the case, so came up with volcanic ash as the reason for the colour and amended the “more afterglow” article to reflect that.

I was still thinking that somehow the ash had crossed the tropopause and entered the stratosphere, even though that’s quite unlikely.

I now realise that the PSCs were being lit by the sunset occuring below, and it was material in the lower troposphere that was absorbing the blues leaving the reds to light up the underneath of the PSC. What was the material? Over the weekend and into Monday there was a Sahara dust event—dust had been brought up from Africa and on Monday evening there was a large concentration in the Atlantic southwest of England. I’d initially dismissed it thinking the cause of the colours must have been at the same level as the cloud—not so!

I’ll amend the “more afterglow” article in a minute…

http://www.spaceweather.com/archive.php?view=1&day=21&month=02&year=2008

They think volcanic effects are not involved and say “Type1 PSCs are composed of nitric acid compounds and are less colourful and bright than the better known Type2 or nacreous clouds made of ice crystals…”. They’ve also got some nice pictures if you follow the links.

Not sure by any means. The proof of the pudding will be over the next few days. Volcanic effects would continue regardless of stratospheric temperatures. So when we next get clear skies, if the volcanic theory is correct, we should still see some “Mount Pinatubo type” sunsets. I recall them well from the early 1990s and indeed, the appearance was similar to the recent spectacle.

I somehow doubt that anything unusual will be noticed the next clear evening and if that is indeed the case, the volcanic theory is a dead duck.

Jack Harrison

But one other observation is interesting. During the day I saw an aircraft contrailing (short and non-persistent)at a remarkably low altitude, probably around 15 to 18,000 feet. I believe in the Antarctic, contrails can form at ground level (but that might be 10,000 feet or more above sea level)

Jack