(Phil) The instrument does get a tiny bit shorter (about 2 mm at -50°C) but the overwhelming effect is the reduction in the speed of sound as the temperature goes down. Since there is a standing wave in the instrument the lower speed causes it to take longer to go down and up the instrument, thus lowering the frequency and therefore the pitch. In effect the instrument becomes longer as far as the standing wave sees it. Of course the player's breath warms it up quite a bit, about halfway beteen ambient and body temperature. Even allowing for this (which I didn't in my original posting), the adjustment is about one inch on the tuning slide for 20 degrees and this is just a bit more than is usually available. So if you're playing outside at 0°C all you could do would be to play "short", as they say. Fortunately I don't do marching bands or Sally Army stuff.

So presumably in old, draughty churches without heating it must be impossible to get accurate tuning for the organ for the same reason.

It's pretty difficult to get accurate tuning on any kind of church organ in any case...

True. Accurate was the wrong word... consistent?

(Darren) That's correct. The whole instrument would be flat and it wouldn't sound wrong but any other instrument accompanying it would have to tune down a bit. Rather difficult with a piano. One way round it would be to have a small bleed of a lighter gas into the air the organ uses. You could use hydrogen, helium, neon, methane, ammonia or hydrogen fluoride. Perhaps I should patent this lunacy.

Wouldn't some of those risk producing the Amazing Exploding Organ? That puts me in mind of the Large Hot Pipe Organ which produces sound by exploding a propane/air mix in its pipes.

(Darren) It's blown itself to pieces and taken the website with it. I drew a blank with your link, alas. Helium would be safe enough and it's much cheaper than it used to be. It would be amusing to use sulphur hexafluoride, totally inert and non-toxic. It would lower the pitch by over an octave. I think I'd better shut up now.

Large Hot Pipe Organ (thanks to google), with MP3s (although the site doesn't seem to have the bandwidth to supply them very well).

My curiosity is partially because I'm playing carols at the village Christmas tree tomorrow evening with the local brass band. Getting 40 or so of us in tune may be interesting if we get a cold evening.

What instrument do you play again, Phil?

(Phil) If all the brass is out of tune by the same amount it won't matter. I suspect there will be enough left on the tuning slide to accomodate an average cold evening.

[Darren] I'm currently reviving my interest in playing B-flat cornet, having not really played it for nearly 3 years.
[Rosie] That was what I thought. I might have to get a bit of practice in tonight, and give the old horn a good clean.

(Phil) I hope your lip won't have gone. I wouldn't dare go three days without blowing a few raspberrires down my nine-foot tube. Brass is a treadmill; constant practice just to stand still.