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The problem with spring weather forecasts

Here’s when warm and cool air masses are their strongest, relative to each other

Springtime is one of my favourite, yet least favourite, times of the year. I love the fact that it’s warming up; as the older I get the more I find myself thinking about how I can end up spending most of my winters in the tropics. I love the melt season (usually), as the little kid in me is allowed to come out when I get to play in the water, making sure things are draining properly. The only difference is I get to use bigger tools than when I was a kid and I have a “reason” for playing in the water. Finally, there is something special about starting plants and getting the greenhouse going at this time of the year.

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On the downside, trying to figure out the weather in the spring can be a real pain in the butt! From the big-picture point of view, the atmosphere is heating up toward summer levels in the south, while in the north, winter cold is still in place. Weather across our region is mostly caused by these two air masses battling it out for dominance — and at this time of the year, they’re both at their strongest relative to each other. This is why we can see some of our biggest storm systems at this time of the year, and also why forecasting can be so difficult; the atmosphere can’t make up its mind on whether it should be summer or winter.

Nature’s A/C

Now, on top of this large-scale battle between warm and cold air, we have a couple of smaller-scale phenomena that can play havoc with spring weather and weather forecasting. The biggest player in this spring weather havoc is the cold ground. I have written in the past about the effects of snow cover on temperatures. Snow reflects sunlight which would, without snow cover, be absorbed by the ground and turned into heat. So, the reflectivity of snow helps to keep us cooler. Snow is also nature’s air conditioner. Snow is, well, cold — it’s below-zero frozen water. To melt snow it takes energy, lots of energy, and that energy comes in the form of heat.

Just how much energy does it take? To melt snow you first need to warm the snow to just below the freezing mark. It takes about 2,000 joules per kilogram of snow to warm it by 1 C. So, if we take an example of one kilogram of snow at -10 C, it would take 20,000 joules of energy to warm it up to the freezing or melting point. Now, to get the snow or ice to change phase into water takes a lot more energy. This seeming little push of temperature from just below freezing to just above freezing takes about 33,500 joules per kilogram. This is more energy than it took to warm the snow up by 10 C!

Now you can see why areas with no snow cover can be significantly warmer than areas with snow cover. This isn’t the only thing to play havoc with spring weather. The second havoc wreaker is the temperature inversion. This is when warm air gets trapped below a layer of colder air, can occur at any time of the year, and can result in temperature differences of 10 C or more over relatively short distances. Fortunately, or maybe unfortunately, temperature inversions don’t usually last for that long around here.

Elevated warmth

There is a second type of inversion, or maybe the word inversion isn’t the right term. This is when warm air moves into a region but it rides up and over a very thin layer of cold air that, in essence, gets stuck at the surface. This type of inversion, for lack of a better word, is very stable and can last for longer periods of time. We often see this happen in the spring, thanks largely to the cooling effect of all the snow on the ground. Sometimes it is a very large-scale event where ground temperatures are cool or cold — say, in the -10 C range — while, only a couple of hundred feet up, temperatures can be as warm as +10 or +15 C. This is why western regions that are higher in elevation can sometimes have very mild spring weather while the rest of the region is stuck in the cold.

Along with being a large-scale event, this pattern often develops due to local topography. The most commonly affected region is the Red River Valley. While the Red River Valley does not bring about images of a classic river valley, it is a very shallow and wide valley — but not so shallow that cold air can’t get trapped in it. This occurred just last weekend, as mild air flooded into the province from the south but wasn’t able to push the cold air out of the valley on Saturday. While this setup creates colder-than-expected temperatures across an affected region, it also has an impact on cloud cover. This warm air over a shallow layer of cold air will often result in overnight fog, which then transitions into low clouds that can sometimes stick around for days. Fortunately for those of you who live in this region, winds were strong enough last weekend to finally scrub out the cold air by the end of the day on Saturday.

Try to remember these different weather havoc wreakers when the spring weather forecast calls for +10 C high three days from now, then switches it to a +2 C forecast 12 hours later. That’s just the weather models trying to figure things out.

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.

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