Outside air enters a ram air scoop on an aircraft frame, and is ducted to a ram air control valve. The air control valve outputs a desired air mass flow to a cyclone air-water separator which removes moisture and produces a dry air flow. A heater assembly heats the dry air to a desired temperature and directs the heated air into an equipment bay enclosure on the aircraft. The relative humidity of the heated air is sensed by an air moisture sensor which produces a corresponding signal. Other sensors disposed near a payload in the bay enclosure produce corresponding air temperature and air pressure signals. All the sensor signals are input to a processor or controller configured to activate the air control valve and the heater assembly according to set points for temperature and humidity that are specified for the payload in the bay enclosure.