Chase Field is air conditioned. But we also see almost over 100 days here of 100+ degree weather. They routinely play night games with the roof open (I believe 95 degrees is the cutoff). The entire seating bowl is air conditioned. They generally cool the stadium several hours before the game to bring it down to 70 degrees inside. As for Miller Park, I just cant see it being cost effective for the team to spend the money to air condition the stadium for the handful of games where it is oppressively hot outside.