!!! WARNING !!! Please read this before you start investigating the archived data.
Archived data are only available for spots on the main list (not for custom spots). For most spots the archive starts when the spot was added to the list.
Archived data are not based on real measured values at the location! It only comes from data produced by the forecast model. Every model cycle (every 6 hours) the archive is filled with the nearest forecast values. Every night the statistics are regenerated based on the archive data.