Air carrier First Officer reported there were ATC ground delays; the aircraft had one air conditioning pack inoperative; the cabin temperature was at 90 degrees; therefore the Captain refused the aircraft.

Narrative

During the preflight after boarding was complete. The aircraft was operating with the left air conditioning pack inoperative on MEL XX-XX. The Flight Attendants informed the Captain that several passengers were complaining about the excessive temperature in the cabin. They also stated that it was unbearable in the aft galley. On the overhead panel the air temperature reading in the aft galley was 90 degrees Fahrenheit. The First Officer checked with ATC about delays getting to the destination due the the impending line of thunderstorms approaching the airfield. ATC stated that the possibility of ground delays was high due to the weather. Due to the single air conditioning pack being unable to keep the cabin temperatures at a safe level and the impending weather delays the Captain refused the aircraft and made a maintenance write up in the logbook.Cause: The aircraft was operating on a single air conditioning pack and was unable to maintain safe cabin temperatures with a full flight. Suggestion: Air conditioning packs must be repaired and not placed on MEL during the summer months. If need to get the aircraft to a maintenance base it should be operated in cooler environments where safe cabin temperatures can be maintained.