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Such instantaneous parameters may, or may not, have undergone post processing, such as being been averaged in time. For example, hourly instantaneous values might be averaged over a month, in which case they would be archived in a monthly mean "stream". This would be indicated in other metadata in the GRIB header of the data, as would details of the averaging frequency and period. Even in the case of a monthly mean, these parameters would still be referred to as "instantaneous".
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