Understanding rain units and values - mainly wsum

[Tomasz Lewicki]

Further to my earlier thread about deleting or modifying values in the database (https://groups.google.com/g/weewx-user/c/EN_elfqKhlI), I would like to understand in what units precipitation is expressed. I am comparing the current and previous records of precipitation values in the station software and in Weewx, and I cannot reconcile them in any way. For 24 hours, my station recorded 15 mm of precipitation, while the wsum field in the archive_day_rain table shows 1588.2. What does this mean?

I found a thread titled "Weewx WD Import - Rain data units, values" (https://groups.google.com/g/weewx-user/c/Umc6L8NqXvc), but did not find an answer to my question there.

I summed the rainfall data for the whole day from the database using the query (1746223200 = 2025-05-03 00:00:00 localtime, 1746309600 = 2025-05-04 00:00:00 localtime) and exported it to a CSV file:

select datetime(dateTime,'unixepoch','localtime'),dateTime,rain from archive where dateTime < 1746309600 and dateTime >= 1746223200;

Another query:

select sum(rain) from archive where dateTime < 1746309600 and dateTime >= 1746223200;

Summing the rainfall both ways from the 'rain' column in the 'archive' array gave me the same value as 'sum' in the 'archive_day_rain' array. Where then did the value of 1588.2 in the wsum column come from?

[gjr80]

Short answer re rain units is it depends. Rain is recorded in the database using the rain units of the unit system used by the database. The unit system of the database used by a WeeWX instance is specified by the [StdConvert] target_unit setting in weewx.conf. Possible unit systems are US (the default), Metric or MetricWx. The US unit system records rain in inches, Metric records rain in cm and MetricWx records rain in mm. If you have a WeeWX database but don't have the corresponding weewx.conf used with the database you can look at the usUnits column in the archive table in the database. If each row contains the value 1 the database uses the US unit system, a value of 16 means the database uses the Metric unit system and a value of 17 means the database uses the MetricWx unit system.

The wsum column in the archive_day_rain table contains the weighted sum of all archive record rainfall values for the day concerned (probably better described as the sum of all weighted archive record rainfall values for the day concerned). The weighted rainfall for an archive interval is obtained by multiplying the rainfall recorded in the archive interval by the archive interval. The wsum column is used when calculating aggregates for observations when the archive interval is not constant (some history; initially WeeWX required all archive records to have the same archive interval, later this restriction was relaxed and the wsum column was implemented to ensure meaningful aggregates could still be calculated). The wsum column is typically used for observations where you are interested in the daily average, eg daily average temperature, humidity etc where your archive table consists of records with differing archive intervals (if you did not use a weighted sum short period high or low values could skew the aggregate). For rainfall the typical aggregate of interest is the sum which is independent of the archive interval and hence the wsum column in itself is largely meaningless and can be ignored. By the sounds of your investigation the sum values are correct.

Gary

[Tomasz Lewicki]

@gjr80 Thank you for the in-depth explanation, now I understand the issue better. Although it did not bring me any closer to solving the problem. I left most of the settings in weewx.conf at default. The unit in my weewx.conf is metricwx. In weewx.sdb, in the ‘archive’ table, in the usUnits column, I have a value of 1.

In the other thread (https://groups.google.com/g/weewx-user/c/EN_elfqKhlI) I wrote back to @michael.k that rebuilding the daily summaries did nothing, although the rain data looks reasonable in my opinion.

Translated with DeepL.com (free version)

[Tomasz Lewicki]

Problem is solved - I described my way in other thread (https://groups.google.com/g/weewx-user/c/EN_elfqKhlI). Thank you Gary!