Some ECMWF parameters are described as being valid at the specified time, the validity time. Sometimes they are referred to as "instantaneous" parameters, and may even have "instantaneous" in their name. Parameters such as these have a designated validity time eg 12:00 UTC. However, they are not really instantaneous because they represent an average over the model time step. Some forecast and most, if not all, analysed parameters fall into this category.
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Some forecast parameters are not strictly valid at the specified validity time, but over are in fact valid throughout a processing period. During this period, the parameter could be accumulated, averaged or the minimum/maximum could be calculated, from the values at all the model time steps during the processing period. For more information, see ERA5 terminology: analysis and forecast; time and steps; instantaneous and accumulated and mean rates and min/max parameters
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