Humidity. The relative humidity typically ranges from 57% (mildly humid) to 90% (very humid) over the course of a typical September, rarely dropping below 38% (comfortable) and reaching as high as 99% (very humid).he air is driest around September 23, at which time the relative humidity drops below 66% (mildly humid) three days out of four; it is most humid around September 1, rising above 87% (very humid) three days out of four.