Cacti’s corrugated shapes create pockets of shaded, cooler air that help them shed body heat.
Introduction
Prickly spines and soaring human-like arms have become instantly recognizable signs of cactus-ness, but they are not the only amazing of these desert dwellers.
Growing under the scorching sun, cacti have had to evolve ways to keep cool and save water. For many species, bulbous stems and alternating ridges are critical parts of this survival kit.
Many cacti exteriors have ribs that give them peaks and troughs. The peaks shade the trough areas from sunlight, keeping them cooler.
This infrared image of a cactus shows how the peaks of their ribs shade the trough areas, so that temperatures in the troughs are cooler.
The Strategy
The peaks-and-troughs pattern that encircles the thick stems or branches of many cacti work in several ways to protect cacti from getting too hot and dry. For starters, the peaks provide shade for the troughs, reducing the amount of solar heat that gets into them.
But that doesn’t prevent cacti from still absorbing a lot of heat, so the ribs help in another way. Because the troughs are shaded, the air in them is cooler. It can then absorb more heat from the body of the cactus than warmer air could. As it warms, the air rises out of the troughs, upward to the peaks, where winds waft it away.
Additionally, the bumpy ribs disrupt airflow around cacti much more than if the cacti’s surfaces were smooth. That allows more airflow to whip away more heat. And a surface with many folds has much more surface area than a flat one of the same diameter, so the ribs also offer more surface area to dissipate heat.
To conserve scant water, cactus ribs have another secret at work. Plants in general have pores called stomata, usually located in leaves, through which water can evaporate, carrying heat with it, and cooling down the plants. Cacti cannot afford to lose much water, so the sun-drenched peaks of the ribs have fewer and smaller stomata than the shaded troughs.
Finally, when the heat and sun of the desert give way to rainfall, cacti’s folded surfaces transform. As the plant takes in refreshment, the ribs and troughs can unfold, expanding sort of like an accordion to store the watery bounty.
The Potential
Architects have already been incorporating cactus-inspired designs into buildings—particularly in hot, dry regions—to reduce the amount of heat absorbed by building surfaces and/or maximize the heat they can radiate. But similar structural innovations could work on smaller scales and other materials—for cars, clothing, cooling devices, and food and beverage packaging, for example.
The Minister of Municipal Affairs & Agriculture building in Doha, Qatar, was designed to mimic the shading properties of cacti and regulate the amount of sunlight and heat transferred into the building.
