Describe why sleeping on the top bunk, in a bunk bed, would be a great choice in the winter but a bad choice for the summer.

Heat rises. So in the cold winter you want to sleep higher up and stay warm. In the summer, when it is really hot, and heat rises, you are going to be really uncomfortable.

Since i have slept on a bunk bed for the past 3 years in a dorm i have a lot of knowledge about it. Personally sleeping on the top was great for the cold. Mainly because of the heater ventilation was near me -.- 

but if you want to go all scientific i can go that way.  

When you heat air it expands.Its molecules are further apart, so it takes up more room. Its going become less dense than the surrounding air around it resulting in it to rise. That is what makes hot air lighter than cold air.