I’m not a fan of airports – particularly those big, busy ones of the shopping-mall variety. But once in the air I’m in my element. As long as I get a window seat – that’s the crucial part.
I do like to travel to unfamiliar places by train. But there’s still a special kick to be had from looking down on the land from high above. And if it’s cloudy – well, clouds are pretty cool too.
Cataloguing my photograph collection has made me realise just how drawn I am to clouds. Among which you have to count the contrails left by high-flying aircraft, which can often be extremely beautiful.
It’s strange to think how baffling and bizarre those straight, or sometimes not so straight, lines across the blue would have seemed to a skywatcher only a century ago. They seem so natural to us. Especially in this part of the world where the drone of flying engines is so much part of the background noise we scarcely really notice it.
And in a way they are natural. The atmosphere’s natural reaction to engines flying through it. Gavin Pretor-Pinney, in The Cloudspotter’s Guide, calls them “the new bastard son of the cloud family”.
I’ve seen them referred to a few times lately as “chemtrails” – which isn’t entirely wrong, since water is a chemical. And that’s what they are, like any other cloud. Condensed water vapour or ice crystals.
As Pretor-Pinney is at pains to point, though: “Aircraft exhaust contains a lot more than just water vapour. The other ingredients include carbon dioxide, oxides of sulphur and nitrogen, hydrocarbons, carbon monoxide, unburned fuel and tiny particles of soot and metal. The particles have an important role in the formation of the contrail, by acting as nuclei on to which the water vapour can begin forming into droplets and crystals.”
What he doesn’t say – but which you can sometimes observe if you have a window seat near or behind a wing – is that the air pressure change which gives a plane lift can also cause condensation of the water vapour already in the atmosphere. Contrails sometimes stream from wings themselves, not just the engines on them.
But however they are formed, depending on atmospheric conditions they can puff away in a moment or hang around for hours, gradually widening and spreading into “real” clouds.
It’s not that one aero-engine produces that much vapour. Rather, it can start a chain reaction, causing the vapour in the surrounding air to condense.
These clouds look gorgeous, especially in the low sun of early morning or late evening. But they’re not quite as benign as they look.
Cloud cover has two contradictory effects on global warming. By day, it reflects the sun’s light away, keeping the earth relatively cool. By night, it acts like a blanket, keeping the earth warmer. You know this already. Cloudy nights are warmer than clear ones.
What you may not know – I didn’t until I read Pretor-Pinney’s excellent book – is the way these two effects balance out.
Thick, low clouds tend overall to have a cooling effect. While the thin, high cirrostratus such as that created – or rather triggered – by passing aircraft have a nett warming effect.
So much so that climate scientists suspect clouds that start as contrails may contribute even more to global warming than the greenhouse gases emitted directly in aeroplane exhaust.
All of which seems to have been ignored in the heated toing-and-froing over the now-approved expansion of Heathrow airport. But probably shouldn’t have been.