There was a recent discussion in the ON4KST “kHz” logger about why some station seem to have more consistent success with the path from the “lower-48” to Alaska on 630-meters. Laurence, KL7L / WE2XPQ, ran some very nice path plots as well as terrain profiles using Google Maps from his QTH to the QTH’s of many of the most active stations that are heard in KL7. Not surprising, the stations who have access to the salt water path are heard most often.
The salt water path is far more efficient than paths that cross even the best ground conditions. Terrain variations tend to result in the diffusion of signal and results in additional absorption. Some discussion suggests that duct mode might be a far more likely path for stations whose paths predominate over land. Furthermore, discussions with Merv, K9FD/KH6 / WH2XCR, suggest a very strong correlation of enhanced conditions to approaching frontal boundaries, much as is seen on 160-meters. Other theories involve the recent discoveries of plasma tubes in Australia but that work is in the very early stages.