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. The values >> match across skins >> > > -- > You received this message because you are subscribed to the Google Groups > "weewx-user" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to weewx-user+...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/weewx-user/f46bd022-500d-414e-b96b-c2bc382d37b9n%40googlegroups.com > > <https://groups.google.com/d/msgid/weewx-user/f46bd022-500d-414e-b96b-c2bc382d37b9n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > > > -- You received this message because you are subscribed to the Google Groups "weewx-user" group. To unsubscribe from this group and stop receiving emails from it, send an email to weewx-user+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/weewx-user/c9ac3f2e-8a1b-4dc5-8168-74fff1cc44e9n%40googlegroups.com.
