Originally Posted by tango3

Sounds like the APU was inoperative and they needed to use the Ground Power Unit. From what I hear/understand, the GPU does not generate sufficient power for both aircraft systems and air conditioning, so it might appear that the AC was rather weak (likely this is why dedicated air conditioning units are used during aircraft static displays at air shows). Once the engines have started and the aircraft has taken off, the AC would then be more effective in cooling the aircraft.