ERA-40 is an ECMWF re-analysis of the global atmosphere and surface conditions for 45-years, over the period from September 1957 through August 2002 by ECMWF. Many sources of the meteorological observations were used, including radiosondes, balloons, aircraft, buoyes, satellites, scatterometers. This data was run through the ECMWF computer model at a 40 km resolution. As the ECMWF's computer model is one of the more highly regarded in the field of forecasting, many scientists take its reanalysis to have similar merit. The data is stored in GRIB format. The reanalysis was done in an effort to improve the accuracy of historical weather maps and aid in a more detailed analysis of various weather systems through a period that was severely lacking in computerized data. With the data from reanalyses such as this, many of the more modern computerized tools for analyzing storm systems can be utilized, at least in part, because of this access to a computerized simulation of the atmospheric state.