Niclas Eriksson
05.06.18

Coriolis, Isobars and PGF - Meteorology for dummies.

WRITTEN BY
Niclas Eriksson
PUBLISHED
05.06.2018

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What could be more appropriate than a post about weather now that we’re getting close to the midsummer festivities? For all you non-swedes, weather happens to be somewhat of an obsession during the days leading up to midsummers eve, everyone is keen to know whether it will rain or not (hint, it usually does)...

Our instructor Nicklas has prepared a post about wind – something that pilots inevitably must deal with on a daily basis. In order to discuss wind from a pilot’s viewpoint it’s important that you first understand the basics – so in this post Nicklas will give you a bit of a crash course on the subject, you’ll learn, among other things:

 

  • How high and low pressure systems work
  • Everything you need to know about Coriolis and Isobars
  • Why a sea breeze occurs

It’s quite easy to understand that wind essentially occurs when air move from one place to another. In order for air to move some sort of force is necessary, in this case, this force occurs due to pressure differences which forms naturally around our planet.

We have all heard the meteorologists on TV talking about high and low pressure systems sweeping in from the coast line, taking with it, different forms of weather. Between different pressure systems there are pressure differences. Both high and low pressure systems can take form as smaller and temporary systems, or as large and relatively permanent occurrences. The pressure systems can be caused by factors such as temperature or terrain, but also by the circulation of air around our planet.

Nature will try and even out these pressure differences, in order to do this, air from the high-pressure system will move towards an area with lower pressure, this force is known as Pressure Gradient Force, and it is essentially the cause of wind.

The Pressure Gradient Force (PGF) is not only the source of the wind, but will also play an important role in the velocity of it. A higher PGF equals a greater difference in pressure, which in turn will affect the force of the wind – the greater the difference, the more powerful the wind. The winds are typically stronger in a low-pressure system due to the impact of the fall in pressure.

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The Coriolis Effect

When it comes to wind direction, things start to become a bit more complicated, The Pressure Gradient Force strive to pull the high-pressured air into low-pressure systems, however, due to a phenomenon called Coriolis, it won’t succeed.

Coriolis happens since the earth is spherical (or very close to spherical at least…), the circumference grows greater closer to the equator, this also means that due to the rotation of the earth around its own axis, the speed of the rotation will increase the closer to the equator you get. In short: an object located on the earths pole will have a rotation speed close to 0, while on the equator, this speed will be equal to nearly 1700 km/h.

This relative change of speed causes objects to automatically steer to the right in the norther hemisphere, and to the left in the southern. This is a very simplified explanation of the phenomenon, in case you want to learn more, there are a ton of videos on YouTube on the subject.

So, now we have a basic understanding of the two main forces affecting the winds of our planet. The Pressure Gradient Force and Coriolis. To summarize: nature wants to even out the differences between high and low-pressured air (the Pressure Gradient Force) but the air will simultaneously steer of 90 degrees to the right because of the Coriolis effect. 

This wind is known as geostrophic wind and it is the foundation for all the winds in our atmosphere. Winds will be further affected by other factors such as friction from the ground, the centrifugal force and terrain, which in turn will affect the force and direction.

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Isobars

Have a look at the weather system above, the map shows a high-pressure (H) and a low-pressure system (L) over Europe and the north Atlantic. The numbers beneath the pressure systems indicate the air-pressure in hectopascal in the center of the system. The thicker lines dressed with triangles and half-circles specifies the front of the system. The thin lines are what’s interesting in this case – they are called isobars.

An isobar is a line that connects coordinates with the same air-pressure, they are often pictured as circles around different pressure systems. The isobar also has a specific number displaying the air-pressure in hectopascal. If you have a look at a low-pressure area you can see that the isobars decrease closer to the center.

Isobars are perfect indicators of how the air will move. Since the Pressure Gradient Force will move 90 degrees from the isobars, from high to low pressure, and Coriolis will cause another 90 degree turn, the wind will blow parallel to the isobars, as long as it is not affected by other factors such as friction or terrain.

The distance between the isobars will also decide the velocity of the winds, the narrower the distance between them, the stronger the wind.

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A Sea Breeze

So, let’s take all this new knowledge and put it into a context. Let’s discuss how a typical sea breeze forms.

You know now that for a wind to blow we need a difference in pressure.

When the sun rise and starts heating up the surface of the earth, the ground will warm up quicker than the water, this means that the air over land will be warmer than that over water. A change of temperature will also cause a change in density, the warmer the air gets, the lighter. As with a hot air balloon the warmer air will rise.

As the air rises further up in the atmosphere, the pressure further down will decrease, a low-pressure area is created.

Above water on the other hand, air will not heat up as quickly, this will in turn create a high pressure. The colder air above the sea will move in towards land – a sea breeze.

A breeze like this often increases in strength throughout the day and reaches its maximal force in the afternoon. Coriolis will also have its effect on the sea breeze steering it off from the coast.

I hope that you have gotten a better idea of how winds work, and maybe you’ve even become a bit interested in the wonderful world of meteorology. Wind is one of the foundations in this complicated subject, including everything from tropical storms to the construction of our atmosphere – hopefully I’ll be back to tell you more another time!

 

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