The boundary between the cold and warm air is where the majority of our ‘weather’ or storm systems occur
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 a zone that stretches right around the globe, where warm air moving northward battles it out with cold air surging southward.
Our atmosphere is fluid, that is, it flows. But for most of us, it is difficult to picture air flowing. One way to look at it is that the top of our planet is covered in a heavy, gooey substance that we call cold air. This substance wants to run down the planet towards 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 southwards, 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 that are seeing cold air sag southwards. Due to the shape and size of these sags, they are referred to as long waves.
Now, what makes this more complicated is the fact that our planet is rotating, and 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 goop sitting on the top of our planet with three to six areas of goop sagging southward, and now 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). So, 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.
Now that 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 like we’ve seen several times over the last year or so? Well, the 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.
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.
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 that are under the high pressure will see clear skies, and warm, dry conditions, while the 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 and then reforming again.
In the next issue we’ll look at other types of blocking systems such as the Rex Block, cut-off highs and lows, and split flows, so stay tuned.