Geographical problem: the flame at the door of the classroom in autumn

In late autumn, because there are many students in the classroom, people's breath releases heat, so the temperature in the classroom is higher. The temperature outside the classroom is very low. If the door is slightly open during class, the warmer air in the classroom will spread out from above the door. If you put a lighter by the door, the indoor wind will blow out the flame. The cold air outside the classroom diffuses into the classroom from the lower part of the door, so put the lighter on the lower edge of the door and the wind blows the flame into the room.

A is equivalent to the lower edge of the classroom door, and B is equivalent to the upper edge of the classroom door. Pay attention to the direction of the arrow.