Monday, December 13, 2010
The Physicist's Guide to Surviving a Montreal Winter
In the sixties, a band called “The Zombies” sang: “It’s the time of the season for loving.” Now that December has rolled around, it’s the time of the season for freezing if you live north of the equator, and reasonably far from it. Those of us who actually experience four distinct seasons usually agree that it is good for the soul to see our environment change. Still, as we dump cat litter under our snow tires before giving the car yet another push, many of us would like to skip over winter. We curse its arrival every year, and even vacation far from it when the opportunity presents itself.
Winter is very particular. You can examine a picture of downtown Montreal in mid-February, and know with absolute certainty that you are looking at the winter season. Snow is the symbol of winter, and after a quarter of a metre of the stuff got dumped on my walkway and driveway last week, I set out at 6:30 am to displace it.
The first thing I observe every winter as I dig my shovel into the fresh snow is how quiet it is. I live less than one kilometre from a highway, and yet I hear no car engines as I stand outside. The reason for this is that the ground surface is covered in snow, which absorbs more sound than pavement otherwise would. I really like the quiet. After five minutes of intense work, if I listen carefully, I can actually hear my heart beating. The next thing I remark is that the air is so fresh. Again, it is the snow that has covered the surface, normally teaming with dust particles.
Later, as the sun rises, the intensity of light striking my eye is nearly blinding. I am not staring at the sun, but bright light is attacking my eyes from all directions. While the snow absorbs the sound, it actually reflects the light. The colour white actually contains the entire colour spectrum; it is a linear combination of the three primary colours, red, blue, and yellow. The black pavement represents an absence of colour. It absorbs the photon energy from the sun and heats up, but does not reflect much.
The snow, with its white colour, has no need for the wavelength of the Sun’s light and so it reflects it. One can spend a long morning skiing on a clear day without sunglasses on to appreciate just how reflective a surface the snow is. If you were to then enter a chalet for lunch, you would think someone forgot to turn the lights on, as it would seem so dark by comparison, and the human eye takes some time to adjust.
Oh, and did I mention it gets cold? It gets very cold, particularly at night, when the Sun’s radiation does not reach us. The temperature of the air can be as low as -30 degrees Celsius, and even colder if you are somewhere that is essentially uninhabitable like Winnipeg. But even minus 30 C is beginning to approach absolute zero temperature, the minimum allowable value for matter to exist. Absolute zero corresponds to 0 degrees Kelvin (-273.15 degrees Celsius). I suppose it could be worse than Montreal. The temperature of satellite components in orbit can be as high as 300 C, and as low as -250 C, depending on whether the Earth is blocking the Sun or not, and the direction that the satellite is facing. The surface of the Earth avoids these giant temperature fluctuations thanks to the atmosphere, which acts as a spongy, protective barrier.
We must note however that the -30 C mentioned above may not include wind chill. What is the wind chill factor anyway? In the absence of wind, the warm skin exposed to the cold air loses its heat due to conduction, the most basic method of heat transfer. Conduction is what happens when your warm bare feet cool as they step along the colder kitchen floor. If the air is moving with respect to your exposed skin (i.e., wind), then conduction’s evil cousin, convection, will take effect. Depending on the wind speed, -30 C can quickly feel like -45 C. The precise decrease in apparent temperature depends on the surface area of exposed skin, but this can be approximated. Safety tip: mittens will prevent frostbite for a longer period of time than gloves, since gloves force each poor finger to fend for itself.
While we are on the subject of freezing, have you ever noticed that the sunniest days are also the coldest? This occurs because, in the absence of clouds, the snowy surface reflects the Sun’s radiation back to where it came from; we don’t keep as much of the heat. The beautiful blue sky is then a tease as you stare outside your window in the morning. Only your furnace, which is currently in overdrive, is aware of the cold recurring smacks in the face that await you as you head to your car on this morning, cursing that your garage is full of junk and that your automatic starter battery died last winter.
Speaking of driving, the winter roads require their own specific survival guide. If you can manage to see through your windshield, which is affected by changes in temperature and humidity, and once you regain feeling in your fingers, then the real challenge of winter driving begins. In the best of cases, where one can see and be seen (Clean the snow off of your car ... Yes, all of it!), all drivers must contend with surface irregularities. During the course of a one hour drive, the road conditions can change a fair bit.
Traction in a car is directly related to the friction coefficient between the car’s tires and the surface itself. A lot of friction is a good thing in a car, as it allows you to speed up or slow down much faster. When the friction coefficient reduces to near zero (as it does when driving over black ice), one’s speed and direction is left mostly in the hands of God. The force of friction is dependent on the contact force between the surface and the friction coefficient, or abrasiveness of contact. We can maximize the coefficient by installing winter tires, but it will only take us so far; reducing speed is a necessity for safe driving in winter on icy or snowy days.
The contact force between road and tire should be maximized for the wheels responsible for breaking and accelerating. The engine is directly above the front tires, making for a high contact force for front-wheel-drive cars. Though not good for gas consumption, rear-wheel-drive cars should have about 50 kg of bags of dry cement placed in the trunk above each rear wheel to increase traction during the winter.
With all of the above precautions taken, applying the break can still cause the entire car to rotate, which should surprise anyone new to driving. Why does this occur? If the friction coefficient is high on one side of the car, where the surface is pavement, but low on the other, where the surface is snow, then the force on one of the braking tires is greater. The difference in applied forces about the center of mass of the vehicle and its passengers causes a moment or torque in it, which then causes a corresponding angular acceleration. In lay terms, if your left wheel has better traction than the right, your car will spin about this tire (it will involuntarily turn counter-clockwise). If, in the same scenario, you tried to accelerate, a clockwise rotation would occur. Driving a four-wheel drive vehicle can ease winter driving, though not fully eliminate the "car = bull riding" phenomenon.
Considering the additional challenges that winter imposes on drivers, it is not surprising that half of all car accidents in Montreal occur during the three winter months of December, January, and February. I would not be surprised if these stats extended to other snow-covered cities. The stat implies that the frequency of car accidents increases by a factor of three during winter. To keep our safety constant, we should all drive three times more carefully in the winter. So, put down the cell phone, stop texting, and eat your sandwich later.
Why don’t we all just move away? It seems that some retirees smarten up and move to Florida, where snow is a figment of one’s imagination. The joke may be on the Floridians, as their climate will become too hot during certain months by the year 2050 if global warming trends continue. By that time, snowbirds that fly south will be replaced by sweathogs that stampede north. Who knows, Montreal’s climate may become agreeable year round. Consider this carefully before putting your northern property up for sale.
Hopefully these tips and insights into winter have been helpful for you. I will end this commentary with a look at why we have seasons at all. For this, as with most big, important questions concerning the Earth, the answer lies with our immediate master: the Sun. If the axis of Earth’s rotation were exactly perpendicular to the plane of our orbit around the Sun, we would have no seasons. January and July would be indiscernible for any given location on Earth. Thankfully, Earth’s axis of rotation is inclined by about 23.45 degrees; this is its angle of inclination. This angle changes ever so slightly over time, but the effects of this minor wobble on our seasonal oscillations are negligible.
Why is the Montreal winter so different from the Vancouver winter? After all, the two cities are roughly the same distance from the equator. The main reason for this is due to their proximity to two very different large bodies of water. Large bodies of water distribute heat around our planet, functioning like massive conveyer belts. An oceanographer or atmospheric scientist could offer a more detailed description on these dynamics than I. As much as I would like to research this topic right now, I cannot, because I need to start scraping the sheet of ice covering my car if I plan on arriving to work on time.