Cold, dry weather is much worse for fine wood orchestra instruments than warm, humid weather is. The contraction and lack of humidity can cause cracks in the wood. Plus, fingers don't work so well in colder temps, and the strings and bows don't respond very well.
However, I expect that if the concert were scheduled during the afternoon, they'd reconsider-- direct sunlight will heat an instrument up above the air temps, and the hide glue that holds those things together starts to get iffy. In the workshop the glue is heated to around 140 degrees for liquid application.
I'm still a little surprised they will still play tonight, as lots of sweat isn't great for the wood, strings, or lacquer on brass instruments either. Having played outside earlier this week for roughly the same amount of time they will tonight, I guarantee that both they and the instruments will be quite crabby.
But it's doable, and I'm sure scheduling considerations figure into it. Wonder who will show up. Die-hard Abba fans?