GRIB files and time zones

An incomprehensible malfunction occurred recently for a Weather4D user, sailing in a “lost hole” on the edge of the Indian Ocean, namely the Cocos Islands, challenged me to the point of getting lost in conjectures. A GFS file due ten days ago categorically refused to be displayed in the application with the date, damn !

This unfortunate sailor - for which I would like to thank him - forced me to make a determined search for the cause of the problem.. After much procrastination, explorations of settings that may be faulty, since no other user had reported such a problem to us, the light finally came out !

Time zones

All “world tour” navigators have consulted a map of the time zones of our planet. But they don't have, for the most part — like me, shame on my ignorance — probably not explored all the subtleties.

From the Greenwich meridian, official time zones used to calculate time differences (well known to aviators) are numbered from 0h. to -12h. towards the West, and of 0h to +12h towards the East, so one time zone for each of the 24 hours of the day. Depending on their meridian position, States choose to associate a time zone, or several for larger ones, by possibly adding a winter/summer time change for the sake – they say – of saving energy. So far nothing new, we learn that at school, or later for dunces like me. But this does not apply everywhere on the globe !

“Exotic” time zones

There actually exists 38 time zones, thus 14 more than the 24 conventional time zones. Among them, 10 “Exotic” time zones are used by maritime countries. What's special, for 8 of them, is that they don't use a full-time hour, but a fraction of an hour.

And that's where the problem lies !

GRIB files

Weather and ocean forecasts in GRIB format are given by hourly time steps (*) to 1 minimum hour, therefore incompatible with fractions of an hour. If you have set the time on your tablet/smartphone or computer to local time while passing through one of the regions above, applications like Weather4D will be unable to translate round hours from GRIB files into time zone fractions, and therefore the display will be corrupted.. This is what happened to our unfortunate user in the Cocos Islands with his time zone of +6h30.

The solution is, without changing the settings of your devices, to switch the time of your application — Weather4D in this case — in UTC which immediately solves the problem.

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(*) With a few rare exceptions for some models with metric resolutions, not used by navigation applications.
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