チューブのように、開口部を通る流れの量を測定しようとしている場合、「空気の断面」は空気を通り抜ける中空の断面を指すことができます。オブジェクトが大気中にある場合の空気の質量。
If you were attempting to measure the amount of flow through an opening, like in a tube, the "cross-section of air" could refer to the hollow cross section, through which air can travel and which at any given moment is occupied by some mass of air if the object is in an atmosphere.