At the end of our last weather school article I hoped that we would see an early strengthening of the subtropical high which would then hopefully bring a warm and early start to spring. Instead we saw what our part of the world is famous for, big intense areas of low pressure. This week in weather school we will start to take a closer look at just why we are subject to these chaotic storm systems that make forecasting the weather a real challenge.
To begin with, we’ll take a big-picture look at what causes the weather in our region and then we’ll begin to focus in on the various combinations and permutations of weather and land forms that help create the daily weather we all experience. Let’s just say that while we live in one of the most dynamic weather regions of the world, it also means that we live in one of the most difficult to understand regions as well.
I always feel that to best understand a subject or topic you need to have some kind of general overview to help put things into context.
So far, we have come to an understanding that over the poles of our planet the air is generally cold. Cold air is heavy and therefore sinks, and sinking air creates high pressure. So the regions around the poles of our planet are generally regions of high pressure. This sinking air needs to spread out when it hits the ground and the only direction it has to go is towards the equator. As this air at the North Pole travels south, the Coriolis force deflects it to the right. So, now our southwards winds are blowing easterly resulting in the polar regions having prevailing winds from the east.
To our south, we have our subtropical high where air is also moving downwards but this time because it is being forced to do so. This forcing of the air downward creates a region of high pressure that is warm and dry. When this air hits the ground it too, is forced to spread out, however, for this area the air can either flow southwards back towards the equator or northwards towards the poles. The air that flows northwards will also be deflected to the right due to the Coriolis force. This deflection results in these southerly winds becoming westerly given our region of the world our predominately westerly winds.
OK, we now have an understanding of why we have the general winds we have. Now let’s start to examine what happens with these winds that gives rise to our stormy and hard to predict weather.
The first thing we need to look at is the area around the globe where the regions of northeasterly winds meet up with the westerly winds. Our atmosphere behaves very much like a fluid so maybe I will try a water analogy here to try to create a picture of what happens. I think everyone over the course of their lives has watched water flowing in some form or another. Often water is flowing in just one direction, but every so often we see water flowing from one direction meet up with water from another direction (just watch spring run-off flowing from culverts to see this). If you have seen this then you have an idea of what happens.
You will often see the water from one direction flowing past the water coming from another direction – oh great, that is really helpful. If our flow is strong enough it will be easy to see what I am going to talk about but if the flow is weak then you have to look closely to see. Some of you already know what I am talking about – eddies. That’s right, those little (or sometimes big) swirls of water. To get eddies to form we either need water flowing in opposite directions or some kind of object in the way of the flowing water to induce spin.
Well guess what? The atmosphere behaves in pretty much the same way. Along the region where the northeasterly winds meet up with the westerly winds eddies will form. Sometimes they will be weak and not last long while other times they will be very big and strong and last for a long time. Also the atmospheric eddies are not all the same. Some will be cold and some will be warm, depending on how the air is mixing, but they will almost always end up creating stormy weather.
Well, we will have to end it there for this lesson. We now have the big picture on what helps to create the areas of low pressure that so influence our weather, but so far we have only just scratched the surface. Just like the eddies in a stream there a numerous regions why they form and just because one forms at one time doesn’t mean another one will form there again. But I digress – more on that next time.