One way to hold this word: a pocket. Not the air itself — the little closed space that holds it. A pocket on a coat is a small room shut off from everything around it, and that is what the word does to air: it marks off one small bit of air from the rest. Sometimes that space is real and still, a bubble sitting inside wet cement. Sometimes it is only what it feels like: the plane drops, and the passengers say the air had a hole in it.
Two branches
被關住的一小包空氣
這一支最貼近口袋的原意：東西合起來，把一口空氣包在裡面，出不去。看得到、摸得到、敲得出聲音。
noun · B2
A small space inside or under something that has filled with air, usually because the air was shut in and could not get out — for example a bubble left in wet cement, or the air caught under an upside-down boat.
氣泡／空氣夾層
空中的那個洞：飛機掉一下
同一個「一小塊跟周圍不一樣的空氣」，只是這一塊在天上、而且在動。乘客感覺到的就是口袋那個畫面：空中有個凹下去的地方，飛機踩空了。
noun · B2
A patch of air that is moving downwards, or where the pressure is lower, which makes a plane drop suddenly for a moment as it flies through.
亂流（使飛機突然下沉）