The refrigerator has a freezer near its top. This is because the frezeer at the top makes the air near it cool. The cold air being more dense, comes down, while the less dense warm air rises from the lower part. Thus the convection currents formed in a refrigerator keeps the lower part of the refrigerator cool. . . .
The air conditioner are fixed at a certain height. This is because the cold air coming out from the air conditioner is denser and thus moves down. On the other hand, the warm air in the room is less dense and thus moves upwards. Thus, the convection currents formed, keep the room cool. . . .