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Old 03-31-2012, 07:38 AM   #1  
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Default How are isobars drawn?

Hello,

I was just curious as to how isobars are drawn on prog charts. I know what they mean and what you can tell by the spacing, etc. But my question is how do they know that the pressure is equal along those lines? What do they use to know where there will be a large pressure gradient in one area and a small gradient in another. My guess is that they use the altimeter settings in various metars to get a big picture, but that would only be good for a current anaylsis, not a forecast.

Thanks guys!

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Old 03-31-2012, 10:30 AM   #2  
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Meteorologists use observed pressure conditions to plot isobars. When isobars are plotted, the map starts out with lots and lots of pressure readings from lots and lots of weather stations across an area. When plotting isobars, a meteorologist will start at the lowest (or highest) pressure value and trace a track where that pressure exists on the map. The since isobars never just "end" in the middle of a map, the isobar will continue until either hitting the edge of the map or connecting with itself again (circular isobars that appear around pressure centers). Once one isobar is completed, they move on to the next value up (or down). The amount of isobars showing up on a given map depends on the pressure difference between the isobars being plotted. If an area is plotted using an isobar every 4 mb apart vs ever 8 mb apart, there will be twice as many isobars being drawn on the 4mb plotting vs the 8 mb plotting. Once the meteorologist goes through the entire range of reported pressure readings, the map is complete.

Obviously, nowadays meteorologist don't do this by hand very much anymore. Computers can calculate EXACTLY where each isobar needs to be located by differentiating the pressure readings across every single square inch of an area but, most meteorologists do have to learn how to plot by hand in their schooling.

As for forecast plots, they are generated by various computer models that use basic fluid movement (momentum) equations to predict where and how the weather will change over time. Once these models predict the pressure readings across an area... the whole process repeats itself.

Plotting isopleths (lines of equal quantity... pressure, temp, humidity, height, etc.) is not an exact science because no one can REALLY know what the EXACT pressure is over an entire area however, isopleths are used by meteorologists to see how a certain quantity is distributed over a given area. Ask 3 meteorologists to plot isobars over the same area and you will get 3 different renditions of the same map but overall, they will show the same major trends (a trough here, a high pressure there, etc.).

Here is a cool site that allows you to practice plotting your own isobars in an area. You can trace your own lines and then reveal what the actual conditions are.
https://www.e-education.psu.edu/file...tool_p0207.swf

Enjoy
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Old 03-31-2012, 02:13 PM   #3  
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Picking up bits here and there over the years, I believe the weather products we get from NOAA & AWS are derived from

• radiosonde data gathered at various locations across the country each day. See wiki on radiosondes. They have a radiosonde that is released at Atlantic Aero here in Greensboro daily, it is a small balloon with a radio unit attached to the rope. They try and recover the radios, but I guess they lose quite a few.

• METARs and weather stations at airports across the world.

• ACARS data from airliners. See wiki on ACARS weather data.

This probably leaves out other pieces of the puzzle. But the basic idea is a huge supercomputer constantly running weather modeling equations with scads of variables indexed to real weather sensors. This is called "indexing to boundary conditions" -see wiki on boundary conditions. It works fairly well, weather modeling is pretty accurate these days, although they still have trouble nailing it because the earth's atmosphere is so complex.
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Old 03-31-2012, 02:23 PM   #4  
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I attended a weather seminar this morning hosted by the FAA Safety Team and it had a lot of good information on using the various weather tools available to a pilot on the ADDS website. For instance - I had never used the Java Tools feature and I had never given much thought to the Infrared (color) Satellite loops before. Somewhat a handy tool if you have time to digest all of the weather information before a flight. It is certainly an example of information overload!

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Old 03-31-2012, 03:19 PM   #5  
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With ink.......
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Old 04-01-2012, 05:06 AM   #6  
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Default Fly Boy Knight, is 100 % correct.

Speaking as a Meteorologist, Fly Boy Knight, is 100 % correct.
I did need to learn all that stuff in college, and actually needed to do it on the job, back then the NWS only came out with the surface analysis once every 6 hours.
There is an 'art' to it, isobars generally run parallel to the winds with lower pressure to ones left with the winds coming to your back, in the Northern hemisphere. Sharp kinks indicate frontal boundaries, squall lines, and troughs, this is where the 'art' comes into play, the analysis, as to where the fronts are.
When I taught aviation weather at Dowling College, I made sure all my students did it, just so then would have an understanding as to what the charts meant.
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Old 04-01-2012, 11:37 AM   #7  
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By a cranky man in a poorly lit cubicle to the faded tunes of the 60s when being a G-Man was still cool.
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Old 04-01-2012, 02:46 PM   #8  
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Quote:
Originally Posted by jedinein View Post
By a cranky man in a poorly lit cubicle to the faded tunes of the 60s when being a G-Man was still cool.
He died, and they gave his job to a supercomputer some years ago. The isobars he drew had more natural style and shortly after his demise some of the code hacks noticed this and attempted to incorporate routines aimed purely at duplicating his lines. So far none have proved accurate, although a few have come mighty close over the years.
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Old 04-01-2012, 04:32 PM   #9  
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Quote:
Originally Posted by Cubdriver View Post
He died, and they gave his job to a supercomputer some years ago. The isobars he drew had more natural style and shortly after his demise some of the code hacks noticed this and attempted to incorporate routines aimed purely at duplicating his lines. So far none have proved accurate, although a few have come mighty close over the years.
Peaked my interest, and google found this:

Weather chart from 1980

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