A clear sky is generally a function of how dry the atmosphere is, and the source region of the airmass in place. New England has seen fairly frequent depressions cross through this summer, and as the cold front passes by the skies clear and air from Central Quebec or northeastern Ontario moves in. These are basically unpopulated areas with few sources of pollution so visibility becomes very good. When you have clear skies at night, any surface can radiate its long-wave radiation (warmth) into space very efficiently (clouds act like a blanket, and a reflector, and slow the loss down) and so water bodies cool off. As the days get shorter, the energy balance of the water body changes -its loosing more energy at night than it receives during the day and so you get a steady net loss of energy and thus temperature - a process that accellerates through the late summer and early fall.