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If I were to check the weather report on a typical day where I live I would probably see that the air pressure is something like 101kPa. I understand that this has been normalized to sea-level.

In fact, the city I live in, being situated on the Canadian shield, is roughly at 1000m altitude. In reality, I am subjected to a value that is much closer to about 90kPa. Why doesn't it make sense to report that number in the weather report? What is the logic behind normalizing it to 101?

How does this make sense to you and I as regular citizens who want to know what I will experience if I step outside?

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  • $\begingroup$ So that you can look at a larger scale weather map and make sense of the isobars shown on it. $\endgroup$ – Jon Custer Sep 8 '17 at 21:29
  • $\begingroup$ @Jon, I suspect that has a lot to do with it, but a weather report is for the citizens, not for the scientific community. I have clarified by adding an extra sentence to my Q. I'm hoping I'll get a better explanation of that in an answer. $\endgroup$ – Octopus Sep 8 '17 at 21:32
  • $\begingroup$ But I want to look at the weather map and have a clear picture of the fronts without worrying about keeping all the geography in the western US in mind also - is that weird feature weather, or the Colorado Plateau? I live at a higher altitude then you, but it isn't that far to being 1000m lower (or higher) - that would all really complicate things in my head. $\endgroup$ – Jon Custer Sep 8 '17 at 22:05
  • $\begingroup$ @Jon, Drawing normalized isobars on a map doesn't mean they have to report that same number on the weather channel though. $\endgroup$ – Octopus Sep 8 '17 at 22:22
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If you are comparing pressure across a large region it makes sense to compare it all at the same altitude. This enables isobars to be drawn.

Along the coast, but at an altitude of 1000m (high up in the air) is the pressure that can be compared to that you experience at home.

It is an arbitrary choice to normalise pressure to sea level, rather than 1km up. (At least it means costal measurements don't need to be normalised.)

This comparison dictates air flow direction at that altitude.

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