Page "Trombe wall" Paragraph 7

Because temperature variations tend to propagate through dense masonry materials ( thermal diffusion ) at a rate of approximately 1 inch per hour, daytime heat gain will be available at the interior surface of the thermal mass in the early evening when it's needed.

This time lag property of thermal mass, combined with its thermal decrement ( dampening of temperature variations ), allows the use of fluctuating daytime solar energy as a more uniform night-time heat source.