My guess would be altitude, not cold, is what caused the bubble to occur. My Brunton developes a bubble when I hike at altitude that dissappears when I get back home (lower elevation). This is Brunton's pricey 8099 Eclipse model.

No more expensive fluid filled compasses for me. My next purchase wil be one of the US military Cammenga's. I wasn't sure if I would be able to use this type of compass with my older eyes and bifocals, so I bought a pair of cheap trainer/imitations from SportsmansGuide. They don't sell these anymore, but it was something like "two for $9.99" back when they carried them. These trainers allowed me to test out how the lensatic compass design would work for me before spending the money on a Cammenga (it works just fine - no problem with old eyes and bifocals).

I see no need for the tritium model, so I plan to get the less expensive phosphorescent model:

http://store.cammenga.com/scripts/prodView.asp?idproduct=23