The intro physics book claims warm air raises. Why? Is it because we usually make our observations on ground level and warm air will go anywhere and there is more chance of it going up than down as there is more room at the top (from our point of view) .So we observe it going downwards. Or is it because warm air is able to go against gravity more than cooler air. It doesn't all go up but it can go up so even though its motion is random, it has the ability to go up relative to cool air. We observe an overall shift upwards of those warm air so we say warm air raises. The book then suggests that the fact that warm air rises is an explanation for why the freezing compartment of the fridge is at the top. I don't understand why this is the case. How are they related?