A solar pond that consists of three main layers; the top one is near ambient temperature and has low salt content, while the bottom one is very hot and salty and is lined with a dark-colored material.

The middle (gradient) zone acts as a transparent insulator, permitting sunlight to be trapped in the bottom layer (from which useful heat is withdrawn).

This middle layer, which increases in brine density with depth, counteracts the tendency of the warmer water below to rise to the surface and lose heat in the air.