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 <gjroder...@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+unsubscr...@googlegroups.com > <mailto:weewx-user+unsubscr...@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/23595CDF-1D28-49B5-AE8C-A108967A34AE%402pi.org.
