I'm trying to figure out helicopter hover in ground effect and out of ground effect specs in the thousands of feet. I can make sense of ground effect within a wing- or rotor span of the ground, but not a hover out of ground effect spec lower than a corresponding hover in ground effect. This is the closest I found to a meaningful discussion but it doesn't connect in my mind -

http://www.airliners.net/aviation-forums/general_aviation/read.main/781418/

What is the significance of hover out of ground effect of 6,000 feet and hover in ground effect of 10,000 feet for the same helicopter?

Thanks,
Sixto