What's the science behind that then, Rosie? I would have expected extreme cold weather to reduce the size of the instrument, thus shortening the column of air one needs to vibrate. I would expect this effect to sharpen the instrument.
(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.