Final answer:
When the hotter plate is on the top of a horizontal enclosure, convection currents develop as the hot fluid rises and cooler fluid descends.
Step-by-step explanation:
In the context of heat transfer through a horizontal enclosure, when the hotter plate is at the top, convection develops in the enclosure. This natural convection is due to the hotter fluid (or gas) that is in contact with the hot plate expanding, becoming less dense, and rising. The cooler fluid sinks to the bottom, creating a convection current that facilitates the heat transfer from the hot top plate to the cooler bottom regions. This process is analogous to what happens inside a pot of water on a stove, where the water at the bottom, heated by the stove, rises to the top as it expands and becomes less dense, while the cooler water descends.