
Originally Posted by
attikus
Without any googling, it probably has something to do with the locations of the official weather reporting locations (most likely, the airports). AC's airport is several miles inland, whereas OC's is just across the water from Assateague. Given the ridiculous heat we've had all summer and the cooling effect from being on the water, a 4 - 5 degree difference doesn't seem out of whack.
I know that Annapolis and Reagan Airport in DC -- both on the water -- are consistently several degrees cooler in the summer than BWI and Dulles for this reason.