These 2 photos were taken a NTUC hypermart at Nex shopping centre. There are many such freezers or merely insulated containers chilled by ice. But wouldn’t it be better to have a cover or lid to prevent thermal energy to getting the food?
First concept to know, thermal energy flows from a region of hot to cold. As the NTUC is air-conditioned, the temperature difference is not too big. Hence the gain in thermal energy by the food and ice will not be great.
Secondly, in convention of fluid, warm air rises as it is less dense and cool air sinks as it is denser. So for this setting, the cool air around the food and the ice is denser. So the cool air will sink and stay inside the container, keeping the food cold and the ice will not melt rapidly.
Thirdly, a more practical reason, which is for the convenience for the customers to have access to the food items easily so as to promote sale. Especially for fast moving items or promotion items.
Now you understand?