Last week it was warm and raining. In Prince George in November, no less.
Then the winds came through and the mercury plummeted. An Arctic high-pressure air mass has settled over the northern reaches of the province.
It is not unseasonably cold. Normally, we have one or two of these blasts before Christmas just to remind us that we are living in the North and winter is an actual season.
But in combination with the warm days last week, it seems a little bit colder. Certainly there is more ice on the ground where pooled water froze before flowing away. The roadway in front of my house is an ice rink.
Even my poor dog finds it hard to walk and she has four-paw drive.
There are some benefits to an Arctic blast, though. For example, windshields generally don't need scraping. It is not snowing. And many of the main thoroughfares are virtually ice and snow free.
But why is it that when it gets too cold like it is now, we don't see frost or snow?
The answer has to do with relative humidity, high pressure ridges, and the Arctic air mass. Collectively, these combine to ensure that it is too cold to snow. Yes, too cold for the white stuff to come down.
The Arctic air mass that occasionally wanders our way and envelopes the North is the result of a high-pressure ridge settling in. It results in beautiful, crisp, clear, blue skies of the sort that make winter in the North seem not so bad. (If you have ever lived in the Lower Mainland, then you know that their winter seems to be nothing but gray for days and days.)
The air inside an Arctic high-pressure ridge is very, very dry. This is why people get chapped lips and sore noses at this time of year. Why is it so dry? Well, the amount of water vapour that the atmosphere or air around us will support depends upon the temperature.
Think about water in a kettle on a stove. As the temperature in the kettle increases, the kinetic energy of the water molecules also increases. That is, the water molecules start to move through solution with an increased velocity. They start to bang into the walls of the kettle with more force.
More importantly, when they hit the
surface of the water, they can break free.
They have sufficient energy to bust out of the solution. It is like a molecular version of the game Red Rover where fast moving molecules can break through the line. The result is that above the water the amount of steam or water vapour increases.
The reverse is also true. As the water gets colder, it gives off less vapour. Molecules in the air have less energy and when they hit the surface of the water, they get stuck instead of rebounding away.
The amount of water vapour within a kettle is in equilibrium with the water and depends upon the temperature of the water itself. It is always in balance.
It never gets to zero, which is why ice cubes do shrink if left in the freezer for too long.
Even solid ice is in equilibrium with water vapour. However, the amount of water or the humidity in the air can get pretty low.
Molecules of water in the air simply don't have the energy to stay there.
As a consequence of this low humidity, both the Arctic and Antarctic are defined as deserts from a climatological point of view.
Not a lot of rainfall in either place.
The moisture that does get introduced to the air around town by such things as automobile exhausts and people breathing quickly gets frozen out. These are the little crystals that you can sometimes see floating in the air on a sunny day.
Of course, clear skies are another reason that it doesn't snow - no clouds.
To understand this, think of the atmosphere as a fluid. It is hard for a fluid to flow uphill. It naturally flows away.
A high-pressure region in the atmosphere is bit like a hill and clouds have a tendency to flow away to accumulate in low pressure basins.
Clouds also tend to form at the boundary between a hot air and a cold air mass. Think of the small clouds you breathe out when the hot moist air from your lungs hits the cold outside. Normally, a warm air mass rises resulting in the cooling of air and the formation of a cloud.
However, with a strong, stable, cold, air mass, there is no boundary. Without the moisture from below, clouds and snow do not form.
A stable air mass - such as an Arctic winter ridge - doesn't produce snow. Just cold and lots of it.
