It turns out that the blue fog is actually the result of frozen carbon dioxide combining with icy clusters inside the bottle. That might sound odd, given that the champagne is rather warm. It all comes back to that adiabatic cooling principle. When you start with cold bubbly, the gas inside the bottle isn't at very high pressure, and therefore doesn't undergo much expansion or much adiabatic cooling. Because the lukewarm champagne starts out at high pressure, it gets to expand much more, and therefore gets cooled much more. In the end, the gas from the warm bottles actually ends up colder than the gas from the chilly ones. And it gets cold enough to temporarily freeze carbon dioxide and water.