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Thread: Big Wind Contest ...next 48 hours

  1. #51
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    Neil, excellent post - and even I understood it.
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    Quote Originally Posted by Neil
    Thought I'd add my two cents since I know a thing or two about the Hays chart...

    1.) The hays chart is calibrated for an assumed standard pressure and temperature. Namely the average barometric pressure and temperature for MWO. Since the device works off a differential pressure transducer this means that large temperature or pressure variations away from the normal will have a noticeable impact on the recorded windspeed. In order to come up with the actual speed a relatively straightforward calculation needs to be performed using the displacement measured on the chart, the air temperature, and the actual pressure. Technically moisture content of the air could be considered as well but especially at low temperatures it has a rather neglible impact.

    2.) The above discussion is why there is a discrepency between what is written on the hays chart and what is officially recorded on the obseravtory F6 and thus with the NCDC as well. After correcting for temp/pressure, the true gust is 178 for Dec 4 1980.

    3.) As far as the Hays chart being able to record winds up to 180ish range... First of all it couldn't, the Dec 4th chart has the peaks well off the page. There was a second hays chart, calibrated for 15 inches of water instead of 10". This was used similarly to the way the Barton is currently used. Furtheremore the Hays chart that currently reads to around 145 mph is no longer a 0-10" chart. It was damaged in the late 1990s and then recalibrated to compensate for the damage. It is now only good up to around 8 or 9 inchs of displaced water column. This explains why the charts that are from the big wind summer wind of 1996 could be record peaks above 150 mph on the Hays while yesterdays 148.5 gust topped the chart.

    4.) Unless things have changed since I left, the 148.5 was not measured with the Hays at all. It was measured with a second device, the setra pressure transducer which is a digital device that ranges from 0-30 inches of water displacement. It is an excellent device with high precision, accuracy and response time. It was calibrated at the factory in the summer of 2006 and should be considered the gold standard. The charts are excellent back up however. Also unless things have changed with automation in the data base the setra wind speed would still need to be corrected for pressure and temperature. Could make a difference of a few mph.

    Other than that, glad to see the summit getting some good winds.
    And Jim, glad to see you've still got a knack for nailing the wind speeds.

    Brian, thats probably my fault for never explaining that the Hays chart for 1980 was actually a bit off. It was just easier to tell tours the number on the chart than to launch into the above discussion.

    -Neil
    Very helpful info

    I was wondering if you have any pics of these measuring devices?

    Thanks

    Eric

  3. #53
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    Here is the Hays Chart for March 21, 2008

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    Quote Originally Posted by edog2007
    Wow,thats just crazy wind.Time to fly a kite.
    So are you saying that this would have worked better if the wind was 180+ ?



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    Quote Originally Posted by mtruman
    So are you saying that this would have worked better if the wind was 180+ ?



    This is Jack who we met at Lakes hut last summer. This is how he spent his 70th birthday!
    It would be a BLAST!!!!

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    Quote Originally Posted by Neil
    Thought I'd add my two cents since I know a thing or two about the Hays chart...
    Thanks for all that info Neil. That actually answered several questions I have had for a while and always forget to ask Ken...assuming he knows the answers.

    Quote Originally Posted by Neil
    Also unless things have changed with automation in the data base the setra wind speed would still need to be corrected for pressure and temperature. Could make a difference of a few mph.
    Nothing has changed in the sense that there still isn't an automated process in place to determine the "real wind". This wind event has certainly opened up a can of worms regarding that subject.

    Quote Originally Posted by Neil
    Brian, thats probably my fault for never explaining that the Hays chart for 1980 was actually a bit off. It was just easier to tell tours the number on the chart than to launch into the above discussion.
    No worries. You're right, it is much easier just to tell them the gust was 182. The majority of the people on our tours in the summer probably wouldn't care to hear that entire explanation anyways. Afterall, it is only a 2% difference.
    Brian Clark

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    Default Corrected wind gust

    As Neil pointed out, wind speeds/gusts measured by the Setra pressure transducer are not corrected for temperature and pressure. Therefore, the 148.5 mph gust is not entirely accurate considering the actual temperature and pressure outside at the time the wind gust was measured.

    I was unable to use the Setra to calculate the corrected wind gust because we only save the high resolution differential pressure data used by our computer database to calculate that gust for 20 minutes. So, the Barton high speed wind chart had to be used to calculate it.

    So after running through the proper calculations, the official corrected wind gust is.....(drum roll please).....

    136.5 mph

    That also means that my buddy Mr. Jim is not the winner of this contest. It is actually WSR88D with a guess of 138 mph. And no Jim, you can't argue that your guess was for the uncorrected wind gust. You should have known from that start that 148.5 was uncorrected Perhaps in the future it will be best to specify that guesses are for CORRECTED gusts.

    Look for a more detailed writeup about this in the observer comments at some point later today.
    Last edited by bclark; 03-27-2008 at 01:36 PM.
    Brian Clark

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    Quote Originally Posted by bclark
    As Neil pointed out, wind speeds/gusts measured by the Setra pressure transducer are not corrected for temperature and pressure. Therefore, the 148.5 mph gust is not entirely accurate considering the actual temperature and pressure outside at the time the wind gust was measured.

    I was unable to use the Setra to calculate the corrected wind gust because we only save the high resolution differential pressure data used by our computer database to calculate that gust for 20 minutes. So, the Barton high speed wind chart had to be used to calculate it.

    So after running through the proper calculations, the official corrected wind gust is.....(drum roll please).....

    136.5 mph

    That also means that by buddy Mr. Jim is not the winner of this contest. It is actually WSR88D with a guess of 138 mph. And no Jim, you can't argue that your guess was for the uncorrected wind gust. You should have known from that start that 148.5 was uncorrected Perhaps in the future it will be best to specify that guesses are for CORRECTED gusts.

    Look for a more detailed writeup about this in the observer comments at some point later today.

    Oh you guys and your new math...I'm only calabrated to compute data the uncorrected way...

    You win...

    Congrats to WSR88D
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    137 versus 149? That's not a trivial difference.

    I'm going to ponder the ramifications of this. In general should I look at the MWO wind data and think that speed is within 10% of the actual number?
    Bill
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    Quote Originally Posted by Bill O
    137 versus 149? That's not a trivial difference.
    I agree that it is not a trivial difference, however I also wouldn't call it a significant difference. Afterall, it is "only' an 8% correction.

    Quote Originally Posted by Bill O
    In general should I look at the MWO wind data and think that speed is within 10% of the actual number?
    Absolutely not.

    First of all, the correction curve is not linear by any means. A lower wind speed at the exact same temperature and pressure as this most current big gust would not have as big of a correction. For instance, a gust of 101 mph under the same conditions as this gust, recorded by the Barton chart(4.25 inches of displaced water column) would only need a trivial 2% correction to 99 mph.

    Additionally, different temperatures and pressures will cause big changes in the correction. I will get into this more in the observer comment I am working on, but I will explain briefly here. The Barton high speed wind chart was calibrated for a temperature of 27 degrees F, according to my boss Ken. I would assume it was calibrated to a "standard pressure" of 800 millibars. The closer the outdoor temperature and pressure is to these values at the time the wind is measured, the more accurate the uncorrected value will be. For this instance, the Barton read 8.125 inches of water, which roughly equates to 140 mph. That means that 137 mph is only a 2% correction. Again, this is small enough to consider trivial in my book.

    The Setra pressure transducer, the device used to measure the 148.5 mph gust, works slightly differently. That will be explained in the observer comment.
    Brian Clark

    MWO Observer and Meteorologist

    http://mountwashington.accuweather.com<---- My blog on AccuWeather.com

    We are....PENN STATE!!

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