Winter has arrived with snow, ice, and grey days. In the past, when it got cold, it used to produce clear blue skies but it seems in recent years, there is a lot more moisture in the air.
Along with the arrival of snow, there are the sounds of winter. The stillness of a forest glade, the Christmas muzak in the stores, and the neighbour's snow blower churning away at their driveway are all part of this time of year.
One of the more distinctive sounds, for me, is the very fine scraping noise that accompanies cleaning the car windows. There is no other sound like it.
Ever notice, though, that the front window of your car, truck, or whatever gets frosted up but not the side or back windows?
Or at least the side and back windows are never frosted up to the same extent?
At this time of the year, scraping the front window in order to be able to drive to work is almost a mandatory morning activity unless you have a garage.
The reason that only the front window gets frosted has to do with, as the physicist Richard Feynmann once put it, the "strange interaction of light and matter". Individual atoms and molecules scatter light in all directions. Indeed, this is why the sky is appears to be blue.
In fact, everything (except a black hole) scatters light to some extent. It is this scattered or reflected light that allows you to see objects around you.
For most of the light striking a surface, there is a wavelength shift when the energy gets re-emitted. This results from the absorption of light by molecules at a surface causing them to get excited. This makes the molecules appear to be "hot". In order to "cool off", the molecules must release the energy they have gained.
They do this by simply releasing photons of energy but not at the same wavelength. Rather, re-emitted light is in smaller packets or at longer wavelengths. While an object may absorb light in the visible region of the spectrum - say, for example, green light from the sun - it re-emits it in the infrared region of the spectrum where it appears as "heat".
This process of absorption and emission occurs year round and everywhere you can see. Indeed, part of the reason that you can see is that objects around you are reflecting visible light.
This is also why sitting in front of a window on a sunny winter day can make you quite warm even when the outside temperature is below zero. The molecules of your clothing and skin are picking up this solar energy and getting excited.
There really is no difference between heat generated from solar radiation and that obtained from a flame or a heating element - it is just a matter of intensity. Both sources of heat depend upon photons to excite molecules.
Anyway, you ask, what has this to do with car windows and frost?
Well, since everything is absorbing and re-radiating energy, how warm something is depends, in part, upon how much radiation it is receiving from its surroundings. Every part of your car receives sunlight, either directly in the form of sunshine or indirectly from clouds and the sky.
But in addition to this light, your car is also exposed to re-radiated light - in the form of heat - from its more immediate surroundings, such as the car in the next stall or the building at the edge of the parking lot or your house right next to the passenger door. The closer an object is to your vehicle, the better it is at keeping your car warm.
Now, consider the front windshield of the average car. It points upwards at the sky - which means that it has a large "Sky View Factor". The sky, relatively speaking, is very cold and very far away. It is not very effective at warming a car window.
But the side and back windows of a car are more vertical and point at the surrounding objects (cars, buildings, trees) which are much closer and warmer. They have much smaller "Sky View Factors", but a bigger "Surroundings View Factor", so they stay warmer and frost free.
The objects surrounding your car help to keep the side windows ice free even when the front windshield is totally iced over.
Of course, a good strong wind and freezing rain will coat any part of a car with frost or ice, but all things being equal, it is the "Sky View Factor" that causes your front window to frost up first and worst.
Which means that at this time of year, one thing is certain - we all get to do a lot of scraping before driving.
