Hi group,

Running weewx 4, Python 3, Mysql-database, Vantage Pro 2 with solar, UV.

Within Weewx, I want to calculate a derived value from two existing values 
that's provided by my weather station. To be more specific, in the 
Netherlands, the shortage of precipitation is calculated by the amount of 
rainfall minus the amount of evapotransipration (sum of max ET-values every 
day). During the period of april till september, it;'s a drought indicator.

My SQL-query to do show is, where 'tekort'  (Dutch) stands for 'shortage'. 

select abs(round(sum(archive_day_rain.sum)-sum(archive_day_ET.sum),0)) as 
tekort from archive_day_ET,archive_day_rain where 
from_unixtime(archive_day_rain.datetime,'%Y')=year(curdate()) and 
month(from_unixtime(archive_day_rain.datetime)) >= 4 and 
month(from_unixtime(archive_day_rain.datetime)) <= 9 and 
archive_day_rain.datetime=archive_day_ET.datetime;

What I am doing right now, is running a crontab job, and start my 
PHP-script every minute to calculate the outcome of the SQL-script above 
and writes the outcome to a plan txt-file. Next step is to grab this value 
from the txt-file and show it (javascript) on my website. See 
http://www.weerinwelsum.nl, below the webcam. It says ' Doorlopend 
neerslagtekort', [amount of millimeters]. This is the value I am talking 
about.

What I really want to achieve, is that I calculate this derived valu by 
WeeWX itself, and make this derived value available for everything that's 
wants to use is. For example, store this derived value in a database-field 
I am not using (for example, soilTemp4) and make this derived value 
available for whatever skin (in my case, Belchertown). In other words, 
Weewx calculates this new value and makes it available for the complete 
WeeWX-ecosystem (if you understand what I mean).

Am I clear in what I want to achieve? And is this possible, and what are 
the pointers to do so?

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