My start
Familiar structure, difficult entry point
I considered Poland a suitable next country. At first glance, the data looked clean and, above all, easy to use. The system reminded me of the German Weather Service, so I assumed I would only need to make minor adjustments to my downloader and mapper.
During research, however, the first familiar hurdle quickly appeared: translations were missing, so I repeatedly had to work through the Polish descriptions with a translator. The real problem, though, was not the language but the way the basic station information was distributed.
Station metadata
Basic station data as a puzzle
The metadata for the Polish weather stations was spread across several sources. Three available PDF reports contain different groups of stations along with their short KOD identifiers. The climate stations are listed in mapa_zawartosci_klimat.pdf, the precipitation stations in mapa_zawartosci_opad.pdf and the synoptic stations in mapa_zawartosci_synop.pdf. None of these sources alone provided all the information I needed for Climate Chronicle.
In addition, wykaz_stacji.csv links the long nine-digit station ID to the short codes, while kody_stacji.csv contains further details such as elevation and founding year. I therefore had to piece the information together like a puzzle and additionally research individual details online. No previously processed country had kept me occupied for so long just gathering basic station information.
With the help of AI, the scattered information could eventually be merged comparatively well. The download area below therefore now offers a largely merged station list for free use. About 1000 elevation values are still missing, however, since I could not find an easily usable and at the same time reliable complete data set for this.
In addition, the collected station data is available without headers. I first had to find a usable way to process this too - solvable in principle, but actually unnecessary extra work, and especially hindering given the research and the language barrier.
Daily data and monthly values
Once the monthly data disappeared, processing became truly complex
Another problem concerns Poland's monthly data: it has no longer been available since 2024; according to the available information, there are technical issues. The problems appear to be serious, which is also visible in the station-count chart, which first dropped sharply in 2015 and, from 2024 onward, was partly unavailable altogether.
This forced me to tackle an area I had initially wanted to avoid:
calculating monthly values myself from the daily data. The data volumes are large, the calculations have to run automatically, and, as I gradually discovered, several "nasty" special rules apply to the Polish files at the macro level for programming purposes. I got stuck at this level for quite a while, since I needed my own additional log scripts to validate the correctness of the data, but unfortunately new errors kept turning up :D.
Empty values and the associated status fields are particularly important in the Polish data. Poland has documented a change to the data format and, for missing measurements or phenomena, removed the previous zero entry from the value field while keeping the status fields intact.
A later check of the actual daily files for all three IMGW areas showed:
status 8 means a missing measurement. Status 9, on the other hand, can mean a valid value of 0 for precipitation, sunshine hours, and snow and event values. For temperatures, the same reinterpretation would be professionally incorrect. Snow cover days were additionally tricky, because no status 9 occurred in the files examined, but there were many genuine status-8 gaps.
The mapper therefore no longer reads just one value column; instead, it assigns the value and the corresponding W status field as a pair for each file schema. Only after a specific evaluation of the status are daily values aggregated into monthly values: status 8 always yields null, while status 9 produces a valid zero value only for matching measurands. This prevents an incomplete daily series from appearing unnoticed as complete due to a misinterpreted status.
Current status
The Climate Chronicle mapper now knows the Polish special rules
The basic station information has now been merged to a point where I can work with it. What remains open are mainly numerous elevation values. At the same time, the mapper can now handle the headerless data files and the field-dependent interpretation of the status values.
When converting the daily values, I rely on a detailed calculation overview available with the measurement data. I am not an expert in every technical aggregation rule; that is why I researched the calculations and implement the documented rules automatically as soon as daily data is available. If technical objections or errors turn up, I adjust the calculation accordingly.
Poland thus turned out to be significantly more work than expected at the start. The combination of scattered station metadata, missing monthly data and measurand-dependent status rules in particular made the integration very difficult for me. The decisive progress is that this data can now be processed automatically going forward, and that I learned a great deal along the way. I hope Poland doesn't change its data format again anytime soon ;D.
Sources and working material