ECMWF data can be differentiated into 3 main categories: analyses, "instantaneous" forecasts and accumulated forecasts.

• Analyses are produced by combining short-range forecast data with observations to produce the best fit to both. The data are available a few times per day.
• "Instantaneous" forecast data are produced by the forecast model, starting from an analysis, and are available at various forecast steps (hours) from the analysis date/time. These data are relevant to a particular validity date/time (forecast start date/time plus step), see the article on Parameters valid at the specified time.
• Accumulated forecast parameters are accumulated over particular periods. In some datasets the period will be from the beginning of the forecast (eg in ERA-Interim), while in ERA5 the period is since the previous post processing (archiving). You can divide values by the length of the accumulation period to calculate averages over the accumulation period.
• Minimum or maximum forecast parameters are the minimum or maximum during a particular period.

Some parameters are only analysed, some are only forecast (e.g. radiative fluxes) and some are both analysed and forecast (e.g. temperature, winds and pressure).

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