Weekend Weather Word #3 – Of Highs and Lows

Good afternoon, everybody. I hope you were able to get through Friday and Saturday while staying reasonably dry. With the desert firmly out of my system and me firmly entrenched in everyday life here in Cork, it’s time for another Weekend Weather Word. In the last two installments, I wrote about a few resources that everyone can use, as well as about the tools that I use to come up with my sometimes adventurous forecasts. Today, I’m going to take things a bit further, and start talking about some of the terms of the “trade”, so that you finally understand what the Azores High is not some kind of new drug, and that a frontal occlusion does not happen during plastic surgery. For the start, we’re going to be talking about two absolutely basic elements of every weather pattern: Highs and Lows.

The interplay of high and low pressure areas as well as weather fronts on a weather chart published by the US National Weather Service.

As you all know, we are permanently surrounded by a colourless and theoretically odourless mixture of gases that are collectively known as air. This stuff is what keeps us alive, both by giving us something to breathe and, in tandem with the Earth’s Magnetic Field, by acting as a shield against high energy solar and cosmic radiation. While it may at first seem to be featureless and uniform, the air around us, and indeed the entire atmosphere, is a chaotic mix of different densities, or temperatures. This is caused by numerous factors, the rotation of the Earth, its orbit around the sun, the terawatts of heat we constantly receive from the sun, different surface temperature on the day and night sides, and so on. This is the engine that drives our weather, and two of the key elements of these are high pressure areas and low pressure areas.

High Pressure Areas (H on weather charts)

This satellite Image by NASA shows a high pressure area somewhere near Australia. It’s the clear area in the center of the image, showing the typical clear skies associated with a high.

As the name implies, these are areas in the atmosphere where the air pressure is higher than in the surrounding areas. Think of it as a huge pile of air molecules sitting over a certain spot on our planet. They are formed when an airmass begins to sink within the troposphere, that’s the lowest layer of the atmosphere, where pretty much all of our weather happens. The reasons for this sinking movement can be varied, and I won’t go into details here. High pressure areas, or highs, as they are known for short, are in my experience very stable, and can remain steady for weeks on end. The sinking movement I mentioned earlier causes the air to dry out through a process known as compressional heating. This drying out means that clouds cannot form, meaning that a high is usually associated with clear skies. This also leads to the atmosphere heating up more during the day, and cooling out quicker during the night. Also, there is often little wind associated with high pressure systems, apart from a gentle breeze caused by the sinking of the air mass, some temperature differentials, or the air flowing outwards from the center of the high. This outward flow is often twisted by the Coriolis effect, causing air to circulate clockwise around the high. For us here in Ireland, this means that being on the northern or eastern edge of a high usually means a cold airflow from Scandinavia or the Arctic, while being on the southwestern or western edge usually means a warm airflow.

A schematic view of the Azores High from NOAA. It shows hwo the airflow around it facilitates the creation of Hurricanes.

I mentioned above that high pressure areas tend to be very stable structures in the atmosphere. This stability is so pronounced that some highs are named after the locations of their centers, because they remain constant for months on end. One such permanently resident high pressure area is the Azores High, one of the KEY elements of a larger weather pattern known as the North Atlantic Oscillation. It’s center is a bit to the south of the Azores, a group of mid-atlantic island governed by Portugal. It is pretty much a permanent resident there, expanding and shrinking with the changing of the seasons. Due to the way the air circulates around a high pressure area (Shame on you if you don’t know that by now 😉 ), it’s northern edge acts like a conveyor, transporting low pressure areas towards Europe. The strength of the Azores High is usually one of the deciding factors when it comes to the paths of these lows, i.E. if Ireland or France get drenched first. Incidentially, the southern flank of the Azores High is a key factor in the spawning of “Tropical Waves”, the nuclei of future Hurricanes. But enough about highs, let’s get low!

Low Pressure Area (L on weather charts)

NASA Image of the Icelandic Low. The typical spiral structure and counterclockwise airflow can be clearly seen. This is almost a textbook image!

A low pressure area is in many ways the polar opposite of a high-pressure area. It is an area within the atmosphere where the density of the air is lower than in its surroundings. In other words, at the core of a low pressure area, there’s something missing that should be there. As nature traditionally abhors a vacuum, a fact that is apparently contradicted by the existence of walking vacuums like Kim Kardashian, Kanye West, or Justin Bieber, but lets not go there today. Anyway, back to the topic at hand. The lower pressure that is at the center of lows can be created by numerous different mechanisms, so I won’t go into detail on those. However, the laws of physics dictate that this void will be filled. In the case of a low pressure area, that means that air streams into the center of the area. Unlike high pressure areas, lows are generally chaotic, short lived weather systems, although there are some exemaples of permanent-, or semi-permanent lows. As there is often an updraft element associated with a low, air will cool down when caught in this updraft, causing any water vapour to condense and form into clouds. That’s why more often than not, you’ll find rain, or even thunderstorms, associated with such a low, as well as strong winds as the air spirals in to its center. Once again, the Coriolis effect twists the path of the air streaming into the low, causing air to circulate counterclockwise around the center. For us here in Ireland, this means that the southern and eastern edges of a low are usually associated with a warm airflow, while the northwestern edges generally mean that cold air is sucked in from the north Atlantic, Greenland, or Iceland. Oh, and if you’re loooking for an extreme example of a low pressure area, just look at a hurricane. ALL major storm systems on this planet, whether they’re called, hurricanes, typhoons, cyclones, or whatever, are nothing more than low pressure areas on sterioids.

Now, I mentioned earlier that there are some stable or semi-stable low pressure areas. Once again, we here in Ireland are “lucky” that we have one of those right on our metaphorical doorstep. This is the Icelandic Low, which, as the name suggests, has its center somewhere near or over Iceland for most of the time. Like the Azores high, it waxes and wanes, and changes position, with the seasons, often weakening in summer, and strengthening in the winter months. Due to its general “area of operations”, Ireland, and northwestern Europe in general, finds itself on the southern edge of this low. As the air along the southern edge of a low flows in the same direction as the air along the northern edge of a high, the Icelandic Low and the Azores High often forma kind of atmospheric jet, or conveyor, which facilitates the rapid formation and movement of weather systems like the North Atlantic lows, that bring rain, and more often than not storms to Ireland.

That’s going to be all for today, that was more than enough theory for one day. I hope that this, admittedly very general, overview helped you understand the key elements of the weather, and what the hell I talk about in my regular weather foreasts. Next weekend, I’ll tackle another key element of the weather in our latitudes, weather fronts. Until then, have a great week, stay dry, and watch the skies, as well as this blog. Once again, any feedback, criticism, or suggestions are highly welcome. Just leave a comment.


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