Why Are 96,000,000 Black Balls on This Reservoir?
Why Hacker News recommends it
Referenced as the explanation of shade balls and used in discussions of evaporation and floating covers.
What it is
Veritasium rides a boat across the Los Angeles reservoir covered in 96 million black plastic balls. He explains why they were really introduced, which was not primarily to reduce evaporation.
Recommended by 2 HN commenters across 2 different months, 2019 to 2025.
What Hacker News said
Recommends el_benhameen · 2019-05 · on “A Norwegian town built a giant mirror to deflect the sun (2017)”
In a sort of related/inverse vein, I thought the engineering at play here was pretty interesting. 96 million shade balls on a public water supply reservoir: [this video]
Recommends Tyrannosaur · 2022-11 · on “Floating solar panels that track the Sun”
[this video] Veritasium video about Los Angeles' reservoir black spheres project. They used black because - The goal was light-blocking more than anything else; black worked best - The black pigment held up to the environment (especially UV degradation) better than anything else they tried I expect the second point was the most important thing. Black plastic is very stable.
Cites guerrilla · 2021-11 · on “A man who shoots at birds to keep them off a toxic pit [video]”
Or float those plastic balls [1] on it, like they do for some reservoirs to stop evaporation. It'd take a lot of them, but that'd work and could be both cheaper and more permanent. 1. [this video]
Cites wolfram74 · 2022-06 · on “US Army deploys its first floating solar array”
The article describes artificial reservoirs as the water body of choice, which are less complex (partly because they already destroyed the ecosystem that was there) than naturally forming lakes, and has the added benefit of reducing solar driven evaporation in the same vein as this[0] [0] [this video]
Cites thamer · 2023-12 · on “Largest Floating Solar Power Plant Taking Shape on Hydropower Plant”
Shading is used in California to prevent evaporation, see for example the well-documented use of "shade balls": [LINK] Their use in Los Angeles was covered in a Veritasium video that was all over YouTube for a while (97M views): [this video] The estimate given when they were added in 2015 – in the middle of a record drought – was that they would prevent the loss of 300 million gallons of water each year.
Cites vanderZwan · 2025-11 · on “Mixing Is the Heartbeat of Deep Lakes. At Crater Lake, It's Slowing Down”
I think that's the wrong way round: climate change causes longer summers and shorter winters, so the problem is one of cooling, not heating. Shade balls[0] could work, but then they'd have to cover part of the lake with that. EDIT: And of course, that also comes with a reduction in total light reaching the lake, which may have different side effects beyond temperature alone. [0] [this video]