10K at or near the equator, Tropic of Cancer to Tropic of Capricorn. Florida(31 degree north latitude)maxs at about 8,800. I am at the 45th parallel and never have gotten a reading over 7,500 at the peak of summer, mid June to early July. Its a magical band around the globe between the 40th and 50th parallels, give or take. Its where light and climate meet to grow some of the worlds most amazing plants...
Edit: My numbers I pulled from memory and figure they are pretty close. What I am sure of is max Zenith(10,000 fc)of the suns rays is only possible within a defined band around the globe.