Question

Why is the cooling unit of a refrigerator made at the top?

Answer 1

When the air becomes hot it becomes less dense and it shifts upward. The upper portion of the freezer works on convection current. When the hot air reaches to the top from lower areas and cool air being more dense shifts downward so this shifting result in a current of air called a convection current.

Complete answer:
We know that hot air has a lesser density than cold air. So it always rises in an upward direction. Now the freezer cools the hot air, thus increasing its density. This cold air descends to the lower part, on the way down it absorbs the heat from the food items. As it reaches down its temperature increases and again it rises in an upward direction. The refrigerator is a very good example of a convection phenomenon. If the freezer was placed at the bottom of the fridge (case opposite to one, given in question) then this cycle would not occur. The cold air would get accumulated in the lower portion of the fridge. This would cause unevenness in the temperature of the air inside the refrigerator and there won't be any circulation.
So basically, the reason behind it is convection current. The convention is the process in which heat is transferred through a material medium, due to the actual migration of the particles of the medium from one place to another.

Note:
Convection is a way of heat transfer like conduction and radiation are. Transfer of heat through liquids and gases takes place by convection. Compared to conduction, convection is a faster mode of heat transfer. The phenomenon of the convention is used in producing circulation of air in the cinema hall too.