Pressure - Windfinder vs W underground

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MILO
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Pressure - Windfinder vs W underground

Postby MILO » Tue Aug 05, 2014 8:35 am

I use both these sites to see whats happening weatherwise.

At the same time this morning:
SITE / PRESSURE / ALT / TEMP
Weather ug 1021 4900 9
Windfinder 866 4900 11


The pressure per windfinder is probably corrected for MSL at 15c etc.
- Does someone have the calc?
- I am used to 1021 -1024 being a good range to fly in provided the number is increasing. What is the equivalent range adjusted per windfinder?

I accept there will be differences between the two sites
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Wargames
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Re: Pressure - Windfinder vs W underground

Postby Wargames » Tue Aug 05, 2014 9:31 pm

Hi Milo,

What is the altitude of the place you are searching for.

The 866 is the qfe or actual pressure on site given.

To get QNH from there you do the following math:

Altitude / 30 + QFE = QNH

Eg. if you altitude is 4500ft.

Your calc will be 4500 / 30 = 150 + 866 = 1016.

That should give you the closest idea. According to the reverse calculation your field is 4650ft above sea level.

Hope this help.

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nicow
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Re: Pressure - Windfinder vs W underground

Postby nicow » Wed Aug 06, 2014 8:08 am

Wargames wrote:Hi Milo,

What is the altitude of the place you are searching for.

The 866 is the qfe or actual pressure on site given.

To get QNH from there you do the following math:

Altitude / 30 + QFE = QNH

Eg. if you altitude is 4500ft.

Your calc will be 4500 / 30 = 150 + 866 = 1016.

That should give you the closest idea. According to the reverse calculation your field is 4650ft above sea level.

Hope this help.

Regards,
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MILO
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Re: Pressure - Windfinder vs W underground

Postby MILO » Wed Aug 06, 2014 10:27 am

:) :) Nice simple response. Thanks

One site is Witbank dam. The actual altitude is not shown on the sites.
Both sites are showing a different temperature so I can safely assume the elevations are not the same.

The "/30" portion of the calc is because pressure drops by 1hpa per 30 feet ?

Also on the Aquilla if I pressed and held the "-" on the instument panel until alt reads "0", the pressure should read QFE, in this case around 866 ?
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Re: Pressure - Windfinder vs W underground

Postby Wargames » Thu Aug 07, 2014 10:00 am

MILO wrote: The "/30" portion of the calc is because pressure drops by 1hpa per 30 feet ?
That is correct.
MILO wrote: Also on the Aquilla if I pressed and held the "-" on the instument panel until alt reads "0", the pressure should read QFE, in this case around 866 ?
Sharp Thanks.
Yes. QFE is more used commercially so that the pilot know the runway is at 0ft. Although it is not a term that is used a lot in south africa that I know off.
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