What goes up must come down (maybe)
First let me note that measuring rainfall has one important difference from measuring temperature. Rainfall is much more variable than temperature both temporarily and spatially. A significant depth of rain can fall in a few minutes. Most places on earth can sometimes go for days with no rain. Similarly, it can rain heavily on one side of town and a few kilometres away there be no rain at all. Getting an accurate estimate of rainfall over a given area requires a number of raingauges.
The few major data bases of rainfall have stopped updating. The most recent update of one major rainfall data base is “GHCN Version 2 Precipitation Version 2”. The main file with data, V2.PRCP, is dated 8 November 2019. The equivalent for temperature is now up to version 4 and is updated on a regular basis.
As can be see the decline has been severe. The data were only analysed from 1970 onwards. The drop in observing stations was steady and effected all parts of the world. Turkey went from 214 stations to 28, Angola from 92 to 0, Australia from 1851 to 39 and the USA from 2016 to 13. There was a sharp fall from 2004 to 2008 when, for example, Bolivia went from 18 stations to 0 and Indonesia from 28 stations to 0.
The data base included some stations with data going back over a hundred years. Out of the total of 234 stations which have appeared in the data base, only 19 had stopped reporting by 1970. Before the data base ceased updating in 2019 this had increased to 96 sites with no new rainfall measurements.
Another major rainfall data base is produced by the Climatic Research Unit of the University of Norwich in the UK. The following chart shows the number of rainguages in their data base at 10 year intervals since 1900.
As can be seen the number of stations in their data base has also fallen dramatically since 1970 – but at least they are still trying.
Rain has not stopped falling so how does the climate change industry manage to report on rainfall. They complement raingauge data with satellite-based estimates.
The following chart is taken from A Review of Global Precipitation Data Sets: Data Sources, Estimation, and Intercomparisons, Qiaohong Sun et al, Reviews of Geophysics, 56, Jan 2018. It compares different estimates of precipitation over land on a global and tropical basis. The main point these charts is making is that different estimates of precipitation produce very different results. The global annual average varies from around 600 mm/year to 850 mm/year.
In short, we have no way of knowing how much rain is falling on a global basis. As I said in my introduction, the impact of climate change will be more severe if evaporation increases relative to rainfall. Is it? Nobody seems to know and, as long as they are doing nothing, nobody seems to care.