Breathing through the nose is always best but as we move into colder weather it becomes even more important. The body has many built in regulators and filters and the nose does amazing things to the air we breathe as it passes through the nose and through the pharynx, larynx and trachea before reaching the lungs.
The nose has thirty different functions that include filtering dirt and other foreign matters with the cilia that line the nasal passages. It also works as a heater which comes in handy in cold climates.
If it is 32 degrees outside and we take in air through the mouth it arrives in the lungs at 32 degrees. It is a pretty straight shot without a lot of interference except for the tonsils (if you still have them) which act as a filter for the throat
When you are breathing through the nose the 32 degree air takes a serpentine journey in and around the nasal conchae (seashell like bugles that create nooks and crannies inside the nose) until it reaches the lungs at a perfect 98.6.
The body is a machine meant to work in a specific fashion that we are free to take advantage of, or not. I always like to point that when we get in touch with how the body works we have a much better chance of ageing gracefully through life.
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