Weather school: Understanding heat waves, Part 1

A blocking pattern left parts of Manitoba under the influence of a hot ridge of high pressure

The more water vapour there is in the air, the more potential energy exists to help a 
thunderstorm develop.

Most areas felt the effects of the early-summer heat wave that brought high daytime temperatures and even warmer overnight temperatures as well as some high humidities, light winds and, for some of you, torrential rains, so I thought we should re-examine a topic I seem to be revisiting every year or two: blocking patterns.

Early this year in weather school I talked about upper-level lows, and these blocking patterns can often be associated with these upper lows. We live in the zone that stretches right around the globe, where warm air moving northward battles it out with cold air surging southward. This is what gives us our constantly changing weather.

Our atmosphere is fluid — that is, it flows — but for most of us it is difficult to picture air flowing. So, picture this; the top of our planet is covered in a heavy substance that we call cold air. This substance wants to run down the planet toward the equator, but instead of simply sliding down in all directions, just parts of it start to sag in large undulations. Since there is not an infinite amount of this cold air, if some of it sags southward in one area, other areas will pull northward to replace what has sagged southward. Altogether, at any one time in the Northern Hemisphere, we have between three and six regions with cold air sagging southward. Due to the shape and size of these sags, they are referred to as long waves.

Now, making this more complicated is the fact that our planet is rotating. To put things very simply, this causes the air to rotate. Take the picture you have created in your mind of a pile of cold air sitting on the top of our planet, with three to six areas of it sagging southward, and have that whole thing start to slowly rotate around the Earth. Now you have a very basic picture of what causes our weather to change from periods of warm to periods of cold. When one of the sags of cold air moves across us, we have an outbreak of cold air. When it moves away we return to warmer conditions. The boundary between the cold and warm air is where the majority of our “weather” or storm systems occur, and this is the typical location of the jet stream (a ribbon of high-speed winds in the upper atmosphere). We end up having a pattern that repeats itself over and over, stormy weather as cold air moves in, followed by a period of cool or cold weather, then possibly some more stormy weather as the cold air moves off, and finally a period of warm weather.

Parked waves

We now have a fairly basic understanding of how the weather occurs in our region of the world. The next question is, why do we sometimes get the same type of weather for very long periods of time? The quick answer to that is blocking systems.

Blocking is a phenomenon in which the normal movement of weather systems in the atmosphere stops and can lead to prolonged periods of either fine or unsettled weather, depending on the precise location of the block. As the long waves develop and move across the globe, certain shapes and patterns of these long waves tend to become very stable and at times can cause the movement of these long waves to stop, preventing other long waves from moving across that region. This is what happened last year over Russia and created that region’s worst heat wave on record.

The most well-known blocking system is an omega block. It is called an omega block due to its resemblance to the Greek letter omega (Ω) and this is what brought the heat and humidity during the last week of June and the first week of July.

When an omega block occurs, there is usually a large strong area of high pressure, with two areas of low pressure on each side (our long waves). This particular setup causes the jet stream to flow down and around the bottom of the area of low pressure, then up over the high, then back down around the low, producing the omega symbol. Areas that are under the high pressure will see clear skies and warm, dry conditions, while areas under the lows will see unsettled weather with excessive moisture and cool conditions. Typically, these omega blocks will last anywhere from one week up to a month or longer, sometimes forming and then partially breaking down, then re-forming again.

This is what brought the heat, humidity and rain to some areas over the last few weeks. This blocking pattern placed most of southern and central Manitoba under the influence of the hot ridge of high pressure. Just to our west, the trough of low pressure brought western regions thunderstorms earlier this month. Now, the million-dollar question for the rest of summer is whether we will see this type of pattern continue — or will it break down? The next few weeks will tell the story. If we see the overall blocking pattern break down next week, there is a good chance a different weather pattern will move in. If we see the block redevelop, then all bets are off. It wouldn’t surprise me if this hot, humid pattern re-establishes itself a few times this summer.

Next issue we’ll look at just how certain conditions can conspire to bring about those sweltering summer heat waves.

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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