It appears most often during rush hour, reading the carriage’s flow and shaping breezes from a whisper to a brisk draft. When crowds make the air stagnate, it slips in from the end of the car, threads through the middle, and carves a path that compensates for weak air conditioning. Odors are trapped in small vortices and vented outside the instant the doors open at the next station. It lingers beside acts of kindness, tying coolness at a passenger’s shoulder. For nuisances, it pricks the nape with a single cold point, and gently thins excessive sweat or perfume to preserve everyone’s dignity. At times it nudges ventilation buttons and AC settings as a playful “wind’s trick,” aiding the conductor’s judgment. On stormy days it avoids overblowing so hats and papers stay put. On the last train it evens the breath of sleepers and sands down harsh drunkenness to head off scuffles.