Thermal mass' describes a material's capacity to absorb, store and release heat. Dense materials like water and concrete have a high capacity to store heat and are referred to as 'high thermal mass' materials. Thermal masses are often employed in greenhouses to equalize temperatures between daytime and nighttime. A dark colored water barrel can represent thermal mass, absorbing heat in the daytime and then releasing it in the cooler night.
Water barrels are a popular choice not only for their ability to store heat, but also for their multiple usabilities within a greenhouse. These are cheap, and useful for storing water that can be utilized in the growing process itself.
