When I lived on Saipan in the Pacific and visited my parents in Breckenridge I noted that my 10k  times were just as good at 9000 ft with humidity around 27% and temperatures in the 70’s as at sea level with 80% humidity and temperatures in the 80’s. Last month I had the same experience, in reverse: living at high altitude and visiting Saipan. Reading our blog on asthma, I attribute that to the lower viscosity of air and lower air pressure in the mountains compared to the high density of water vapor in the islands.  Both locations are beautiful and inspiring places to run!