You don't have to modify any code, just change the calculation formula by
setting this option in weewx.conf
Look for the [StdWXCalculate] section, then beneath add
[[WXXTypes]]
[[[heatindex]]]
algorithm = old
That put it back to the "old" method for me where heat index is calculated
above 80F.
On Saturday, March 13, 2021 at 6:11:21 PM UTC-5 ln77 wrote:
> First, I misspoke — the change is in heatindexF(), not dewpointF() (duh!)
> In the weewx software directory (/usr/share/weewx when installed as a
> Debian package), there is a file weewx/wxformulas.py. Look for the line
> "if T is None or R is None:” — it’s line 164 in the 4.4.0 version. Add “
> or T < 68”, so that the line becomes “if T is None or R is None or T < 68:”
> You could change 68 to 80 or whatever you want the low-temperature
> threshold to be.
>
> WARNING: this is the wrong way to maintain weewx software. The change
> will get overwritten when weewx is updated. There are better ways to
> implement this, but that would’ve taken me more time than I wanted to spend
> so I’m OK with the hack for my installation. But use at your own risk.
>
> -Les
>
>
>
> On 13 Mar 2021, at 14:03, [email protected] <[email protected]> wrote:
>
> Hi there Les - less than two days ago I updated my weewx from an older
> 4.xx version to the current one, and suddenly i was getting the same weird
> plot. Finally typed in "heat index wind chill" into the forum and out
> popped this thread. I want to feel better as well - where is the spot where
> you tweaked DewpointF()?
> Cheers - Jon
>
> On Friday, March 12, 2021 at 2:00:13 PM UTC-7 ln77 wrote:
>
>> It looks strange to see the heat index suddenly dropping a few degrees
>> below the temperature when the temperature gets above 40ºF. (I just saw
>> this this morning.) After looking through the thread on weewx’s new heat
>> index calculation, I noticed an oddity at the National Weather Service:
>> while their calculator calculates a heat index down to 40ºF, on the page
>> where they describe the HI equation
>> <https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml> they say
>> the “Rothfusz regression is not valid for extreme temperature and relative
>> humidity conditions beyond the range of data considered by Steadman.”
>> Looking at Steadman’s sultriness papers from 1979
>> <https://journals.ametsoc.org/search?f_0=author&q_0=R.+G.+Steadman> he
>> never looked at temperatures below 68ºF. I couldn’t find any reference to
>> indicate why they arbitrarily extended the javascript calculator down to
>> 40ºF. Other authors, and the NWS tables, suggest that the heat index isn’t
>> really of value below 80ºF.
>>
>> So I tweaked dewpointF() in my installation to return None if the
>> temperature is below 68, and now I feel better.
>>
>> -Les
>>
>>
>>
>> On 24 Feb 2021, at 12:54, gjr80 <[email protected]> wrote:
>>
>> Pat,
>>
>> v4.2.0 saw a new heatindex formula implemented, refer issue 601
>> <https://github.com/weewx/weewx/issues/601>. The new formula gives some
>> results in some circumstances that may appear counterintuitive. v4.3.0
>> added back the ‘old’ heatindex formula and gives the user the ability to
>> choose between the two algorithms . This is covered here
>> <http://www.weewx.com/docs/usersguide.htm#%5B%5BWXXTypes%5D%5D> in the
>> User’s Guide under [[[heatindex]]].
>>
>> By the looks of it you are seeing the ‘new’ algorithm.
>>
>> Gary
>>
>> On Thursday, 25 February 2021 at 06:36:38 UTC+10 [email protected]
>> wrote:
>>
>>> I thought heat index wasn't calculated unless the temperature was over
>>> 80F? Today it's about 49 F and I'm seeing heat index in my charts (which is
>>> reporting lower than temperature - I thought that was wind chill?). I don't
>>> recall seeing it before at this temperature range.
>>>
>>> I just want to make sure this is expected for temperatures so low to
>>> have a heat index reading?
>>>
>>> Top graph is Seasons skin and bottom is Belchertown skin. The values
>>> match across skins
>>>
>>
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>>
>>
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