I'm not a scientist, nor a balloonist, however as a pilot of small aircraft and having some aero-dynamics training; I feel that colder climates are going to provide the best lift and safety of flight. not necessarily arctic temps, but the cooler the better. Denser Air provides better lift, and I would expect better control of lighter than air machines, lots of interesting reading on blimps and balloons as far back as the past centuries in Europe. another thing to consider is terrain, and obstruction avoidance, as well as presence of high wires. One also would like to have accessible terrain to recover the craft on intentional or emergency landing.