The mechanism is dull, and it runs all summer. Warm wet air meets a cool surface, water condenses, and anything with dust or paper backing on it starts to grow. Average relative humidity at the airport runs 81% in August. Mean daily high dew points hit 77.3°F in July. Those are ordinary summer readings here, not storm readings.
Air conditioning does the rest. NOAA’s 30-year climate normals for the North Myrtle Beach station show about 2,003 cooling degree days a year, and 98.6% of them fall between April and October. Seven months of cooling means seven months of cold coils, cold supply ducts and cold registers sitting in wet air. Each one is a surface that water can find.
Winter never resets it. The normal January high in Myrtle Beach is 55.5°F, no month of the year averages a high below that, and the area only drops to freezing about 38 nights a year. A crawl space or a closed-up condo here never gets the long dry cold that stalls mold further north. Spring adds sea fog, when warm air drifts over an ocean still cold from winter and hangs moisture on everything for days at a time.
Rain stacks on top of all that. September is the wettest month on the long-run normal at 6.77 inches, with July at 6.61 and August at 6.27, and those are the same months the Atlantic season is busiest between its 1 June and 30 November bookends (NWS Wilmington is the forecast office that covers Myrtle Beach). A house that takes water in September then has the warmest, wettest air of the year sitting inside it while it tries to dry out.
