The excruciating heat that smothered the mid-Atlantic under triple-digit temperatures came to an end Sunday, leaving in its wake buckled roadways, kinked train tracks, withered yards and a newfound sense of what it means to be hot. A weak cold front moving into northern Maryland should make for “relatively cooler temperatures,” National Weather Service meteorologist Andy Woodcock said, which “is a joke to say the upper 80s.” “The average maximum temperature for July is supposed to be 87 or 88 degrees,” Mr. Woodcock said. “We’re going to cool down to the norm.”