[R-sig-Geo] Weird experimental 3D spatiotemporal variogram

[Piero Campalani]

Dear list,

I am predicting PM measurements on a spatiotemporal grid with monthly
intervals in time.
At modeling time, I am looking at the experimental 3D variograms
(`wireframes`) but I see that weird decreasing behavior in time (see
wireframes_2008-1.eps for January 2008): there is a peak at 0 time lags,
then correlation in time is much higher over different days.
How can I interpret such variogram?
Would it mean that there is a very high spatial variability for values on
the same day, whereas temporal variability is significantly lower?

Thanks for any hint,
(I can provide implementation details in case of need)

Piero
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