GPCC Full Data Monthly Product Version 2018, precipitation in mm/month, 2.5 degree combined with the GPCC monitoring product
X axis: whole world in 144 2.50° steps, first point at 178.75° W, last point at 178.75° E
Y axis: regular grid with 72 -2.50° steps, first point at 88.75° N, last point at 88.75° S

Monthly data available from Jan1891 to Oct2018 (1534 months)
Variable precip (gpcc full data monthly product version 2018, precipitation per grid) in mm/month
Full metadata.

Get grid points, average area or generate subset

Mask:	no mask mask3.txt   add a mask to the list
Latitude:	°N - °N
Longitude:	°E - °E
Boundaries:	halfway grid points interpolated
Make:	average max min set of grid points subset of the field
Demand at least:	% valid points in this region
Units:	convert to mm/day leave in mm/month

Create a field with lower time resolution

New time scale:	annual (Jul-Jun) annual (Jan-Dec) biannual seasonal
New variable:	mean sd sum sum above sum below number min time of min max time of max first time last time of precipitation
Threshold:	no cut less than greater than mm/month percentile normal 1971-2000
Minimum:	% valid data
First apply:	-month running mean
Missing data:	ignore, climatology, trend, persistence.

Create a field with lower spatial resolution

Average over blocks of longitude and latitude grid cells

Construct a statistical forecast model from this field

Please consider downloading this field from the authoritative site.
If you really want to get it here, GPCC V8+monitoring precipitation is available as a netcdf file (size 25.5145 MB).