Other weather sources
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Measurement Data Sources
From in-situ monitoring equipment. If properly maintained and quality-controlled, these are the highest quality source of weather information. However, the coverage depends on a weather station having been set up in advance for the location and times of interest. There are datasets such as BSRN and SURFRAD which make their measurement data publicly available.


Numerical Weather Prediction (NWP) Data Sources
These are mathematical simulations of weather systems. The data quality is much lower than that of measurements, owing in part to coarser spatial and temporal resolution, as well as many models not being optimised for solar irradiance for PV applications. On the plus side, these models typically have worldwide coverage, with some regional models (e.g. HRRR) sacrifice global coverage for somewhat higher spatial and temporal resolution. Various forecast (e.g. GFS, ECMWF, ICON) and reanalysis sources (ERA5, MERRA2) exist.


Satellite Data Sources
These sources process satellite imagery (typically from geostationary satellites) to identify and classify clouds, and combine this with solar irradiance models to produce irradiance estimates. The quality is generally much higher than NWP, but still not as good as a well-maintained weather station. They have high spatial and temporal resolution corresponding to the source satellite imagery, and are generally optimised to estimate solar irradiance for PV applications. Free sources such as PVGIS are available, and commerical sources such as SolarAnywhere, Solcast and Solargis provide paid options though often have free trials.