... So I would expect the skywave plot to be elongated to the east through south at the 0.25 mV/m contour line. It appears that the data was generated using a signal strength prediction program as "50% of the time" was used in the qualifying remarks (50% implies a median value - which likely comes from our monthly median model of the ionosphere). But the prediction program didn't look at ground around the antenna.
The link below leads to the FCC methodology and formulae used to determine the skywave signals of AM broadcast stations.
http://www.gpo.gov/fdsys/pkg/CFR-2012-title47-vol4/pdf/CFR-2012-title47-vol4-sec73-189.pdf
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