Ok, maybe the title is a bit misleading as we’re not talking exactly about where all this wonderful air we breathe comes from. Rather, we’re talking about where some of our air masses come from that determine how it feels outside (and sometimes inside). We’ll be getting into the nuts and bolts of of these gigantic chunks of air we call an air mass.
A quick definition, and one that I use with kids in the Bill’s Weather 101 classes, of what an air mass is. It’s simply a gigantic slab of air (continental size) that takes on the characteristics of where it was born or where it’s been hanging out for a while. Air masses are born, but they can also modify as they move from their origin points.
For today’s purposes we’re focusing on 2 different types of air masses, Polar and Arctic. There are others, but these are the ones that will be influencing us the next week (and have been for the past month).
Our primary air mass type is Polar air. Now by its name you’d assume that its origins were up near the North Pole, but alas life is not that easy. Most Polar air we enjoy originates over Canada and primarily the central part east of the Rockies. There can be different ‘flavors’ of this type of air, some of it colder or drier than others depending on its origin area. Occasionally we’ll pull down some air from Alaska or areas northwest of Hudson Bay that can be colder than other batches that develop over Alberta or Saskatchawan. Polar air is our ‘normal’ cold air. When it arrives our temperatures will be below normal for a time with the degree varying by the character of the airmass. Polar airmasses can also modify fairly quickly on their southward journey via the lowering of latitude and increasing sun angle. Also, if it’s origin was over a snow field, the lack of snow will also modify these quickly. These aren’t especially dry air masses as dew points don’t head to very low numbers and most cold fronts we see have these tucked in behind them. When they move east of us, we can also warm quickly on their return flows.
Arctic air masses are a different story. They are far less common visitors to the mid latitudes, but they are generally more memorable. Arctic air originates in the very northern latitudes, up around Greenland, the North Pole, Alaska’s North Slope and our very coldest Arctic air masses can orginate in Siberia and come across on a cross polar flow (not to be confused with a Polar airmass). Characteristics of Arctic air usually stand out well. Obviously the extreme cold air, for the season, running 20 degrees and more below seaonal normals. That is an extreme in and of itself. Spectacularly cold air masses can run 30 and 40 degrees below normal and are usually associated with catchy names like ‘The Siberian Express’ or even the now cliche’d ‘Polar Vortex’…sounds scary doesn’t it…What also helps to identify Arctic air is how dry it is. Dew points will usually drop down below 15° indicating it is very dry air. We’re looking at forecasted dews Sunday morning down around 10.
The term we’ve been using to describe this weekend is ‘Modified Arctic Air’. Yes it’s going to be cold, but it’s not a January Arctic air mass. The sun angle is still relatively high and there is not a vast snow field yet (although most of Canada is above normal for early season snow) so the airmass’s temperature can moderate (it’s still going to be about 20 below normal) but this is not going to be bitterly cold. What doesn’t moderate is the lack of moisture…the aforementioned dew point around 10. The bottom line is its a cold and very dry air mass.
What that lack of moisture also does is raise the spectre of static shocks around your house. When you bring that very dry air inside and heat it to 70 degrees, you indoor relative humidity can sink below 10%. You get lots of shocks and chapped lips.
In a way we’re kind of wasting an Arctic shot this early in the season. 20 or 30 degrees below normal in November is a whole lot warmer than the same departures from normal in January. We could see some flakes fly very early Saturday morning (predawn) and then perhaps a changeover for a while on Tuesday with the next batch of Arctic air’s arrival. No excitement yet, but it’s early. By the way, after the midweek batch the pattern looks to relax some, and Polar air becomes our friend again, but that’s out beyond Day 8.
We’ll talk more about air masses as the cold season continues to unfold.
Until then, make the best of Friday’s cold rain (yick).
All the best
Bill
