Analysis Times
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Currently the traditional times for launching sondes are
00Z, 06Z, 12Z and 18Z with the 00Z and 12Z having the most 
launches.  However, prior to June 1957, the launches occured 
at 03Z, 09Z, 15Z, 21Z which most being at the 03Z and 15Z 
hours.  These "off-time" launches had the potential of reducing
the accuracy of the reanalyses.  So to avoid this situation,
the analysis times were changed from 00/06/12/18Z to 03/09/15/21Z
for the period before June 1957.  In order to make life easy
for all the users of the reanalysis, 00/06/12/18Z analyses were
made from the 3 hours forecasts.  The 0-6 hour forecast averages
(fluxes) were calculated from the 3-6 hour and the following 0-3
hour forecasts to produce averages from the traditional 6 hour 
periods.