The urban heat island effect isn't to do with AC specifically. It's a mix of tarmac absorbing heat during the day then re-radiating it at night, a higher population density and a higher density of machinery.
The night time re-radiation has the effect of bringing the min closer to the max readings, and so making it look as if temperatures are going up, even when max temps aren't changing much. The same effect can't be seen in rural places where there's less tarmac.
