Growing Degree Day observation use

[Andrew McGinnis]

There's a dearth of information on Growing Degree Days (GDD) for weewx.  Hopefully someone out there has done it.

I see the [StdReport] [[Units]] [[[Groups]]] > 'group_degree_day', of which 'growdeg' is a member.  

https://weewx.com/docs/5.1/reference/units/?h=growdeg

And I see [StdReport] [[Units]] [[[DegreeDays]]] > 'growing_base = 50.0, degree_F'

https://weewx.com/docs/5.1/reference/skin-options/units/?h=grow#growing_base

And that's about it in terms of references to GDD.  

GDD is a pretty simple calculation, done either with the average of (daily High and daily Low) or the mean daily temperature, minus the base temperature of 32F/0C for cool season zones or 50F/10C for warm season zones. 

There's an If/Else logic required to handle GDD that would be negative, where IF(((HiF + LoF)/2)-50F) < 0, GDD = 0, ELSE GDD = ((HiF + LoF)/2)-50F) 

[ or alternately, just $day.outTemp.avg - growing_base ]

Within weewx, I can find $day.outTemp.avg, but how would I pull out GDD for use in any of my templates? Is 'growdeg' available as a software-derived observation and I'm just overlooking it, or is it something I have to make in say, [[StdCalibrate]] > [[[Corrections]]] ?

[jterr...@gmail.com]

There is an XType extension providing GDD and other related parameters : https://github.com/roe-dl/weewx-GTS 

[Chuck Rhode]

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On Mon, 17 Feb 2025 20:46:04 -0800 (PST)
Andrew McGinnis <andrewj...@gmail.com> wrote:

> There's a dearth of information on Growing Degree Days (GDD) for
> weewx. Hopefully someone out there has done it.

I publish a *WeeWX* extension that produces GDD graphs for control of
codling moth on apples in addition to other insect crop pests.

See the list of extensions at:

+ https://github.com/weewx/weewx/wiki

*weewx-phenology* is listed under XTypes and may be downloaded from the
page at:

+ https://lacusveris.com/Phenology/weewx_extension.shtml

Including a GDD XType in existing templates can be a bit fraught
because the calculation is a cumulative one that requires iteration
from some point in the past. It's not cheap, but neither is
*weewx-phenology*'s image generation.

At this time of year, there isn't much to see, but you can view a
place-holder graphic at:

+ https://lacusveris.com/Weather/phenology/

... which shows current conditions in my backyard.

The GDD report is at:

+ https://lacusveris.com/Weather/phenology/cumulative_growing_degree_days.html

Suppose you untarred the *weewx-phenology* archive to
~/weewx-phenology, then the template that produces the GDD report may
be seen at ~/weewx-phenology/skins/Phenology/cumulative_growing_degree_days.html.tmpl.

Be forewarned: It's messy. Mostly it's *python* variable assignments
and shows the day date looping required.

- --
.. Be Seeing You,
.. Chuck Rhode, Sheboygan, WI, USA
.. Weather: https://LacusVeris.com/Wx
.. -7° — Wind WNW at 15 mph. Sky clear.
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