To maximize convection cooling in a rack, does it make a difference whether the hottest items are at the top or bottom? If so, which produces the most cooling and why? Please limit answers to ones based on facts or industry-standard design principles rather than speculation, opinion, or personal choice.

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Please note for this question I want to focus purely on the effect of the placement of the items on convection-only cooling. Please ignore other considerations, such as weight distribution or use of fans.