It's inevitable. With the advent of rain, snow and cold weather, city streets and highways are filled with cars and trucks that have their windshields or other windows partially obscured by fog or frost – not on the outside but on the inside.
It needn't be so.
There are many ways to fog up the inside of the glass but they all come down to one thing – the air inside the vehicle is very moist and that moisture turns to fog or frost when it comes in contact with the colder inside surface of the glass.
Of course, the Defrost mode of the climate control system is designed specifically to remove such deposits. But its work is often made more difficult and its progress delayed by another button or switch found in most vehicles.
It's called the Recirculation, or "Recirc" control. Its purpose is to help accelerate warmup of the interior in winter by closing off the intake of fresh outside air to the heater so that it draws air only from inside.
In that way, as the inside air begins to warm up, the heater has warmer air going into it and even warmer air going out. In air-conditioning season, the effect is exactly the reverse – taking in only already cooled air from inside helps the car cool down even quicker.
The problem with that scenario is that, as the people inside breathe, the air they exhale is high in water vapour content – which is why we can see our breath when it's cold.
So, with no fresh air coming in to dilute it, the air inside becomes more and more saturated with water vapour – which turns to fog or frost when it reaches the colder window glass.
The more people there are in the vehicle, the more serious the issue becomes. And other forms of water in the vehicle, such as snow on clothing and boots, makes it worse.
It's a problem that's easily alleviated. Just turn off the "Recirc" switch as soon as the vehicle begins to warm. Or leave it off altogether and accept a slightly slower warmup as the trade-off for clearer windows.