Windchill stored as heatindex

Hyrules Hyrules

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Oct 13, 2019, 2:37:50 PM10/13/19

to weewx-user

I have been reading about heat index and looking at the NOAA formula to calculate the heatindex but it seems there is something wrong in the archive table as it stores the heatindex as the windchill value. Is this a bug ? I'm using a vantage pro 2 and MySQL to store data. Thanks.

gjr80

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Oct 13, 2019, 6:44:34 PM10/13/19

to weewx-user

Hi,

Actually what you are seeing is not heatindex being stored as windchill but rather heatindex and windchill being set to outTemp. I am not intimately acquainted with the NOAA heat index and wind chill calculations (in Australia we tend to use apparent temperature) but you will find that the NOAA formulae have a caveat that they provide meaningless results outside certain meteorological conditions. For wind chill the calculation is only valid for temperatures <= 50F and wind speeds > 3mph, for heat index the calculation is only valid for temperatures >= 80F and humidity >= 40%.

In cases where wind chill and heat index calculations are invalid WeeWX uses the outTemp value. For the values provided heat index and wind chill calculations will be invalid and I strongly suspect WeeWX has used the outTemp value instead.

Gary

Hyrules Hyrules

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Oct 13, 2019, 6:50:29 PM10/13/19

to weewx-user

Your are absolutely right. The value is indeed the outside temp. I know that the value are only really valid when they meet specific criteria but could it be possible to add an option in the future to still add them even if invalid in weewx.conf. I`m currently trying to move a template from weatherlink and the app still calculate the value even if bad. For the time being I will create a method in my template to calculate it as it is easy.

Thanks again Gary.

Hyrules Hyrules

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Oct 13, 2019, 7:14:56 PM10/13/19

to weewx-user

Or simpler is there a way for weewx to just take the value from the weather station as it is in the loop data ?

Hyrules Hyrules

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Oct 13, 2019, 7:23:16 PM10/13/19

to weewx-user

well it's in the loop2 packet not the loop...

gjr80

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Oct 13, 2019, 7:35:00 PM10/13/19

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windchill and heatindex are only available in the LOOP2 loop packets and LOOP2 support will not be available until v4.0. Under 4.0 if you set WeeWX to use LOOP2 packets and set record_generation = software under [StdArchive] in weewx.conf you should be able to get heatindex and windchill from the station. But I would argue whether you are any better off, the station derives windchill and heatindex (just as WeeWX does) and if you look at Davis Application Note 28 you see the station exhibits similar behaviour as WeeWX for conditions outside certain limits (albeit different to NOAA).

Gary

Hyrules Hyrules

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Oct 14, 2019, 9:52:25 AM10/14/19

to weewx-user

Thanks Gary I hope to see 4.0 soon.

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