best data I've seen so far indicated that humidity from 40-80% was good for community spread. Obviously the high, dry temperatures help AZ, but you also have the community to consider. A lot of 'indoor' activities, a lot of elderly population, a lot of poor immigrants who are packed in their apartment instead of out working...
A virus that was overly sensitive to heat or humidity wouldn't have as much transmission success as this virus has enjoyed. The 'summer lull' was always based on irrational denial or optimism.