Keep Your Camera Safe in Cold Weather, article.
Over time, this water works it’s way into lenses, and onto sensors. Mold can grow inside lenses, ruining the image, and particles that may have been shaken free by the internal sensor cleaning now become adhered, which quickly become hundreds of spots in the skies of photographs.
An airtight box such as above may help, but it is not strictly necessary. The main matter is that the camera and lenses do not go suddenly from cold to warm environs, They should be wrapped for an hour or so, to warm up gradually and not get condensation. (The thing is that warm air will hold more moisture, so when it cools down, it dumps that moisture in the form of water.)
(Good idea to take out battery and memory card before going indoors, so you can charge/copy those without delay.)