More on the “afterglow”
Back | HomeI went out to observe it myself yesterday evening, and it was quite eerie to be looking up at the glowing halo above the horizon and know it was over 20 km up. Low-level mist obscured the view slightly and robbed it of any colour, but it was still quite a sight. However people who saw the glow both yesterday and on Monday report that last night’s was only 1/3rd the brightness.
If, like me, you missed Monday’s display a photographer on the south coast did a very good job of capturing the remarkable colours and glow.
The favoured explanation is Type 1 Polar Stratospheric Clouds (PCSs)—-nitric acid bound in ice crystals forming at -78C and lower at levels far into the stratosphere. The following image shows a “bubble” of air at or below -85C high over Britain (bear in mind that the IDV shows the vertical scale an order of magnitude larger than the horizontal—-the top of the box is 25 km up) at 6pm on Monday evening:
These low temperatures have been caused by the large high that was over us (now declining rapidly) pushing the tropopause (the boundary between the troposphere—-where our weather is—-and the stratosphere) and the stratosphere above upwards, like a dome. The following shows the 200 hPa level, roughly the tropopause (the top of the high):
Now, the tropopause is always higher towards the equator and lower at the poles, simply because the warmer air around the equator is “thicker”—-as air warms it expands. So in that image you can see the usual down-slope from the equator to the pole. What’s significant is the arrowed feature: this where the high pressure system has pushed itself under the tropopause from the southeast, lifting it upwards into a ridge-like structure. As the air above is lifted, it cools and if conditions are right hey presto, cloud forms. Cloud in this case being made up of very tiny ice crystals (only a millionth of a metre across).
For a look at the temperatures and humidities that were actually observed, here’s the very top of the Nottingham sounding that evening, at some 22 km (76,800 ft):
While the relative humidity is only 39%, at -89C and a pressure of just 1/30th of that at the surface water vapour can be deposited directly as ice crystals. In the very tiny space around any existing ice crystals, the relative humidity can reach 100% or more, allowing the crystal to spontaneously grow.
However ice alone can’t account for the incredible colours that were observed. That was, undoubtedly, the result of volcanic ash. Together the ice crystals and ash combined to produce the extraordinary sunsets and sunrises.
In fact, there were two processes at work. We’ve already established how the PSCs formed at some 20-25 km height, but these clouds reflect white light. What was causing the colours was a mass of Saharan dust that had been transported up from north Africa over the weekend, far below down in the troposphere. This dust alone would produce spectacular sunsets, but it was also causing only red light to pass through, resulting in the underside of the PSC cloud sheet to be lit up such spectacular colours.
The odds on these two events coinciding are… long! So if you saw it, count yourselves lucky. What isn’t so good is that the process that forms the nitric acid also starts a cascade of chemical reactions that ends with the destruction of ozone. The “ozone layer” is also at around 25 km, so that beautiful sunset you saw was also eating away the ozone layer…
I saw it from the passenger seat of the car at sunset and can add nothing to the excellent explanations. Thanks for putting up the graphics.
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
You say:
“However ice alone can’t account for the incredible colours that were observed—-that was, undoubtedly, the result of volcanic ash. Together the ice crystals and ash combined to produce the extraordinary sunsets and sunrises.”
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
SpaceWeather.com discussion:
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.
Think I’ve finally figured it out… thanks for keeping me on my toes!
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…