Problem
A weather balloon that carries instruments to measure atmospheric temperature and pressure is launched from a measurement station. Attached to the weather balloon is an 'air core'. This is a relatively new technique for atmospheric gas sampling. An air core is a thin-walled (and therefore, light) but long (100 m or more) metal tube wrapped into a coil. The tube is open at one end and closed at the other. As the air core rises with the balloon, the air in the tubing is passively drawn out of the tube. The tube is released at a height of about 30 km and descends (using a parachute) back to Earth. As it does so, it refills with the surrounding air, therefore sampling the atmospheric profile. When the air core reaches the surface it is collected, the tube is shut off and it is then taken to a gas analyser. The air is then pumped gently and slowly out of the tube and through the gas analyser, providing a record of how the concentration of various gases change with height.
Why does the air in the tube passively flow out of the tube as it rises with the balloon, and the surrounding air passively refills the tube as it descends?