After a few days of running with the heatindexF(), and with relatively low temps here in PHX, all was fine. Then yesterday it spiked up to 69+ deg F, and the ~ 2F difference came back for the duration of the heat wave %^)
Given how dry it is here, I think that I'll switch to the old method - at least *I'll* feel better %^) On Sunday, March 14, 2021 at 8:45:58 PM UTC-7 ln77 wrote: > That would be the sensible thing to do. But it’s a different algorithm, > not just a change in the low-temperature limit. The new algorithm is > closer to Steadman’s original concept of giving a perceived temperature > relative to an average humidity, so low humidities can result in a heat > index that’s lower than the ambient temperature, which will never happen > with the old algorithm. > > The other subtle difference is that my modification returns None when the > temperature is below threshold, rather than returning a heat index equal to > the temperature. That means there are no abrupt jumps in the value as > temperature changes slightly; the heat index just isn’t reported. > > -Les > > > > On 13 Mar 2021, at 17:16, Pat O'Brien <pobri...@gmail.com> wrote: > > 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, n7uv...@gmail.com <n7uv...@gmail.com> 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 <gjrod...@gmail.com> 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 pobri...@gmail.com >>> 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. 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