A reanalysis produces a complete, coherent global weather record by processing historical observations (stations, radiosondes, satellites, buoys) with a fixed modern numerical model. The most widely used are ERA5 (ECMWF, from 1940), JRA-55, and MERRA-2.
Utility
Unlike raw station data (with gaps and instrument changes), reanalysis provides gap-free, complete fields for any point on the planet and atmospheric level. It underpins climate change studies, normal calculations, and trend verification. ERA5 has 31 km resolution and hourly updates.