So i noticed after a couple of times that when i opened my window the wind from outside only blew in through the bottom of the window but by holding a lit lighter near the top of the window the air from inside was going outside(the flame was pointing outside vs when i held it at the bottom of the window it was pointing in the opposite direction)
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Air travels from hot area to cold area. Since your room must have been hotter than the outside cool air rushed inside while hot air went out. Not since hot air is lighter than cold air it goes out through the upper part of the window.. Hope this answers your question.