Los Angeles has coated its reservoirs in millions of black plastic balls. But why are they a heat-absorbing black instead of light-reflecting white? Because they’re shade balls, and their purpose has nothing to do with the drought.

Switch between wearing a dark shirt and a light shirt on a sunny day and you’ll quickly be reminded that dark colours absorb light and heat up faster than lighter colours that reflect light. But the purpose of those balls has nothing to do with keeping the reservoirs cool. The balls are coated in carbon black, a food-safe pigment with an albedo near zero. Both these characteristics are vital in explaining why the balls are a dull light-absorbing black instead of a shiny light-reflecting white.

The purpose of shade balls is to provide shade, not to prevent evaporation. They block sunlight, so the ultraviolet light doesn’t catalyze nasty chemical reactions. Chlorine can accelerate reactions leading to bromate when exposed to UV light, which is a carcinogen that really should violate drinking water standards instead of merely being a thing it’s nice to minimize. It is a secondary benefit that the shade also reduces evaporation, a small but important smidgeon of water savings during the ongoing extreme drought.

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A small portion of the 90 million black plastic balls added to the Los Angeles Reservoir on August 12, 2015. Image credit: AP/Damian Dovarganes

Carbon black is nearly-pure elemental carbon produced by burning hydrocarbons in an air-poor environment. It’s mainly used in rubbers (like car tires), but about 9% of it is used as a pigment in plastics and paints. Carbon black can pose a health risk when it’s a powder by irritating lungs, but as a pigment it’s locked safely away. It is used worldwide in food packaging, and meets the NSF/ANSI 61 standards for materials that come in contact with drinking water. This means the balls won’t do anything nasty to the water supply they are protecting.

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What does it mean that carbon black has an albedo near zero? Basically, it’s incredibly dark and reflects almost no light. That in turn means that any sunlight is absorbed by the ball, not reflected or refracted. For thin plastics, black is opaque while white is translucent. Cheap, thin-walled black balls still provide actual shade while lighter colours permit sunlight to penetrate into the water. Blocking as much sunlight as possible is the name of the game for shade balls, so black is the better colour. In an interview with Mashable, a spokesperson from the ball-manufacturing company XavierC explains that this theory is backed up by their testing, “After decades of testing, black has been deemed the color that provides the best protection.”

Black will also help the balls survive longer. Ultraviolet light, the component of sunlight that gives us sunburns, is very effective at breaking down plastics. This is problematic by reducing the lifespan of the balls, and thus increasing the expense of replacing them. Carbon black will prevent the plastic from breaking down in UV light, giving them a multi-decade lifespan.

Floating plastic balls cast shade to protect water quality; any reduction in evaporation is just a bonus. Image credit: AP/Damian Dovarganes

Black balls will heat up more than white balls would, and might even bump the temperature in the reservoir. They also might not. Sunlight heats an uncovered reservoir, not by directly heating individual water molecules, but by heating the bottom and that heat transferring to water through conduction. The warm, less-dense water on the bottom rises, and fresh, cooler, water sinks down to be heated in turn. In a covered reservoir, sunlight heats the top surface of floating balls instead. Water is still heated by conduction, but it stays at the top instead of circulating. That might slow down heat transfer. But either way, it isn’t very important.

As a final bonus, while black heats up faster than white, it also cools down faster. Anything that absorbs light readily emits it equally-readily, so when the sun sets a reservoir coated in black balls is going to cool down rapidly as heat is radiated into the night.

Top image: Four inch diameter black plastic shade ball. Credit: AP/Damian Dovarganes