I am planning to travel to places like Switzerland (Mt. Titlis) which has temperatures going up to -15 C I have the Nikon D3100 SLR camera. I am not sure about the performance of the camera in such conditions (and the precautions to take) I am planning to put 2 small pouches of silica gel in the camera case/bag.

How useful is the silica gel in protecting the camera for any kind of condensation issues that may arise due to change of temperatures?

Also in such cold weather, by how much does the battery output decrease ?