A dual cap for protecting a humidity sensor of a radiosonde of an aerological observation system includes an outer cap and an inner cap. The dual cap surrounds the humidity sensor. The outer cap has an outer ventilation hole formed in its top, and an outer air circulation opening formed in its side periphery. The side periphery of the outer cap also has a first wall that acts as a barrier to air flow. The inner cap is affixed to and is disposed inside of the outer cap. A ceiling of the inner cap faces the outer ventilation hole so as to impose a barrier to air flow. The inner cap further has an inner ventilation hole in its top and an inner air circulation opening formed in its side periphery. The inner air circulation opening faces the first wall of the outer cap.