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Long Waves And Blocking Patterns

If you don’t like the weather, just wait a minute.” This is probably one of the most used and often true statements made about the weather in our part of the world. We live in the zone that stretches right around the globe, where warm air moving northward battles it out with cold air surging southward. Last weekend we experienced a classic example of this. After nearly a week of sunny, warm weather, everything came crashing down weather-wise as a strong, late-spring storm moved into our region.

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, other areas will pull northward to replace what has sagged southward. All together, at any one time in the Northern Hemisphere, we have between three and six regions seeing cold air sag southward. Due to the shape and size of these sags, they are referred to aslong 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. So now, 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 thejet 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.

GOODBYE WAVES

Now we have a fairly basic understanding of how our 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, or seem to have one type of weather dominate most of the time? Well, the answer to that isblocking systems.

Blockingis 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 that created that region’s worst heat wave on record.

The most well-known blocking system is anomega block, so called due to its resemblance to the Greek letter omega.

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 and then up over the high, and then back down around the low, producing the omega symbol. Areas 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 reforming again.

This is what brought us the nice sunny, warm weather last week. We were under the southern end of a blocking pattern. What was particularly unique about this blocking pattern was that all of our weather was coming from the east instead of the west. As we all know, that blocking pattern broke down last weekend and we saw clouds, rain and cooler temperatures move in. The $1-million question for the rest of this spring and summer is whether we will see this type of pattern redevelop. The next few weeks will tell the story. If we see another blocking pattern develop, there is a good chance this type of pattern will dominate the weather this summer, bringing hot and dry conditions. If we do not see a redevelopment of this pattern, there’s a good chance the current cool and wet pattern will continue.

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Ifweseeanotherblockingpatterndevelop, thereisagoodchancethistypeofpattern willdominatetheweatherthissummer, bringinghotanddryconditions.

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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