How do the positions of the hot and cold water affect the speed of convection

Convection is the process in which heat is transferred by movement of a heated fluid such as air or water. Natural convection results from the tendency of most fluids to expand when heated so it becomes less dense and rise above.  

For example- In a pot of water on a stove the liquid t bottom is heated by conduction. The hot water has lower density and it rises above pushing the cold water downwards so it gets heated.

Explanation:

Convection is defined as a process in which a fluid (liquid or gas) when heated then cooler substance (more dense) will sink to the bottom whereas hotter substance (less dense) will rise to the top.

This means that if warmer substance is placed below colder substance then speed of molecules of warmer substance will be in the upward direction and molecules of colder substance will move in the downward direction.