Hacker News Reader

Comment by hervature | Hacker News Reader

Genuine question for people in the field. My understanding is that the cooling effect of trees is primarily driven by evaporative cooling. That is, the shade effect only really exists because the plant does not shrivel up and die due to storing water. How much more effective are trees vs. big swamp coolers? Even in this article, they admit that daytime cooling of half a degree requires 3 times more water.

View on HN

There is something to be said for the parts about shading the surface too though. You're unlikely to cool an entire desert a significant amount with water you bring in but if that's something that happens as part of keeping the actual surfaces in the city cooler during peak heat times on top of the air cooling effects of evaporation then the sum result is greater than the parts in terms of effect.

Of course it's Vegas, I wouldn't be surprised if we decided to make the downtown completely indoors so we could just run AC in the streets too. It's not exactly the city of practicality.

They do outdoor AC in [some places in some] parks in Qatar.

Sounds like Qatar. Innovative and expensive.

Qatar is basically Vegas if it had more money.

Under the sand on the beach, too!

>> the downtown completely indoors so we could just run AC in the streets too.

So ... a shopping mall? Many cities do this already, linking various public indoor spaces by walkways/tunnels. Also those cities where the air outside is too cold. A few canadian universities link buildings with tunnels so students can avoid going outside.

Also: Toronto's PATH - https://www.toronto.ca/explore-enjoy/visitor-toronto/path-to...

Cooling the streets is nothing like a mall or growing tbe walkway connections, those are much too sensible.

If you cover the roof in solar, it will generate more than sufficient power to air condition it.

>on top of the air cooling effects of evaporation

I think you meant to say *on top of shade* because blocking the sun is the main effect here (the geometry makes more sense too!)

Less confusing phrase from abstract which implies that the 0.5deg evaporative cooling is almost a rounding error:

  during the day, trees provide significant shade by intercepting solar radiation, reducing mean radiant temperature (up to 16 °C)

   during the day, trees provide significant shade by intercepting solar radiation, reducing mean radiant temperature (up to 16 °C)

Sadly they work the opposite way at night, they don't allow SPACE to cool the ground. A textil/plastic plate moved automatically can provide more cold, if that's the goal.

To me what seems too good to be possible is the triple combo of shade, radiative cooling, and water harvesting

Only ambient sunlight needed here, but they'd have to make the panels horizontal (indeed, curtained at night like you say) somehow:

https://www.thebrighterside.news/post/mits-high-tech-hydroge...

Edit I guess the low-maintenance option might be some kind of giant variegated Madagascan succulent

It really depends on how broad and high the tree canopy is; you don’t need super high tree density to provide a lot of shade.

The main benefit over other solutions is that properly selected trees require very little upkeep from humans, unlike anything mechanical or requiring movement. Though I imagine that is less the case in Vegas.

Trees excel at harvesting water. When you water a tree, there is evaporative cooling like an artificial cooler but during the night, the dew falls back on the leaves and the ground where some of it finds its way back to the trees (possibly via an invertebrate first). Also the reflectiveness of leaves helps. Then there's the soil where layers of dead leaves, wood and others accumulate, sequester CO2 and create a sponge. Finally, by virtue of making the region cooler, rain is more likely to fall. Humans can probably engineer something better but the bar is high.

For reader clarification: accumulating carbon in soil from decaying plant matter still leaves it part of short term carbon cycling, not to be confused with geological-time carbon sequestration. As you know.

Not that simple. When carbon in the soil rises beyond a certain percentage (a tree falls) it'll convert back to CO2 because of the bugs eating it. But if the percentage starts at zero (in a desert) and an ecosystem settles there, the percentage will rise and stay above zero for as long as the ecosystem survives. Basically the plants generate matter as fast as wildlife eats it.

This sequestration can be long term if things go well or it can be short term if plants die in large numbers (because of climate change, diseases...).

>Trees excel at harvesting water.

Specifically, some kind of giant succulent would be best for arid climes

https://en.wikipedia.org/wiki/Aloidendron_dichotomum

But they probably don't evaporate water and cool as much as other plants and trees.

Right, but as pointed out in TFA & below the cooling from evaporation is almost negligible compared to that from the shade. Maybe Vegas would be okay with conserving their water too

It's counterintuitive..

In the abstract it discusses that most of the effect (16deg Celsius) is from reduced radiative heating and only a few degrees from evaporation.

Mostly the benefit is instead of having the concrete under you absorb and emit the sun, the leaves above you do.

This dramatically reduces the heat we feel at human height.

> the effect (16deg Celsius)

Did I read that right? 16°C seems like an enormous effect.

Seems like trees would be a small investment to effectively get "outdoor AC-ish"?

EDIT: for those of us who are more comfortable with Freedom Units, that's like going from 104°F to 75°F!

Obligatory response to 'Freedom Units' for those who haven't seen it: https://www.youtube.com/watch?v=JYqfVE-fykk

Yes, and cities with lots of trees are way more livable due to this. Planners in our town seem to hate trees with a passion, thank god we‘re moving away from this concrete desert.

Trees in cities are expensive to maintain, which is why they're often on the chopping block when budgets get tight. This is especially true in places like Las Vegas where there is little natural tree cover due to the climate. You have to have a staff of arborists to keep the trees alive in such a harsh environment.

Indeed, the town I‘m living in was 80% destroyed during WWII and that still shows in its finances. It’s amazing how long major disaster affects a region. Big drug issues, highest cancer rate in the country etc

The problem is that to get these effects you need large canopies of trees, and to get that to happen the trees have to take the space of something else. For street trees it takes away land from parking or traffic lanes; for properties it occupies both horizontal and vertical square footage since the sky above the tree needs to be clear. These are unpopular with some political affiliations and interest groups.

Replacing inefficient street parking and wide roads with trees is a straightforward win.

It is like Sim City 1 where crossroads generated traffic, so you'd replace them with parks.

Even without any evaporative effect, the air cooling of leaves (at least bringing them to the surrounding air temperature) happens more easily than that of concrete pavement due to height and larger surface area. The concrete can easily get heated much hotter than the air at even 10-20ft.

Wrt. water consumption - Mediterranean species like say olive trees are kind of optimized for low water consumption, by for example having leaves covered with wax-like stuff decreasing evaporation.

MIT should deploy their desert water harvesting tech in LV[0]

they just need to figure a way to reorient their panels to provide shade?

[0] only sunlight needed

https://www.thebrighterside.news/post/mits-high-tech-hydroge...

Re: olives, hopefully the terpenes can also help cloud formation

Trees are also vertical structures. Any vertical structure will absorb some of the light, turning it into heat, then be cooled by rising air. This keeps the heat from getting to the ground, with or without evaporation. In other words, instead of the sidewalk getting hot, something 20+ feet in the air get hot. Hot air rises and the air near the ground stays cooler.

The increased water usage is tough because we're serious about water reclamation here in Vegas, but you can't reclaim water lost to evaporation, which is why there are policies (and serious fines) around excessive landscape watering. It might not be a worthwhile tradeoff, especially if there are alternate cooling methods that don't involve water loss.

Trees are great, but ultimately a pretty ridiculous idea if the goal is to create shade, even if you're not worried about water consumption. Avoiding concrete walls or overhangs is smart because you don't want the thermal mass.. but of course you can build these things out of fabric or thin metal.

The funny thing is, if you build a wall or canopy to avoid the water consumption plus literally waiting a decade for a tree to get tall.. now you're probably in violation of your HOA height restrictions, etc. Desert cities need to basically drop the idea of conforming to the typical expectations of visitors and newcomers by trying to add greenery. It's better to add shade, dig underground, build wind-catchers[1], salsabils[2]. There's tons of basic things like making sure roof surfaces are more reflective, and more strategic architectural things[3] that can be done to improve things and the techniques have been used forever

[1] https://en.wikipedia.org/wiki/Windcatcher [2] https://en.wikipedia.org/wiki/Salsabil_(fountain) [3] https://en.wikipedia.org/wiki/Passive_cooling

You know how a lot of classic midcentury modern houses (which you can still find in downtown Vegas!) have those kind of lattice-y decorative walls and panels? Are those actually functional for reducing heat and not just cool-looking?

How to Dress and Undress your Home | LOW←TECH MAGAZINE

https://solar.lowtechmagazine.com/2025/06/dressing-and-undre...

The perforated architectural elements designed to filter sunlight are referred to as brise-soleil (sun-breaker in french)

https://en.wikipedia.org/wiki/Brise_soleil

part of the cooling effect that trees produce is from photosynthesis, the percentage of light converted to plant matter can be as high as 1.5% more will be reflected, and the shaded area will of course,be shaded, and then there is transpiration of water, which varys greatly with species.other effects will be due to the built environment, where a lot of asphalt and concrete could mostly obliterate the real effects of a few trees. from wiki "The average rate of energy captured by global photosynthesis is approximately 130 terawatts, which is about eight times the total power consumption of human civilization"

https://en.wikipedia.org/wiki/Photosynthesis

It's confusing phrasing.

Increasing *evaporative* cooling by 0.5deg requires 3x more water, but shade alone is the *main* mechanism,it doesn't require water.

>during the day, trees provide significant shade by intercepting solar radiation, reducing mean radiant temperature (up to 16 °C)

The point of that article is that in many places the evaporative cooling is the main thing but in Vegas the water situation is such that it's more about the shade so the optimal tree is something that gives shade but doesn't need a lot of water.

Right where I am sitting now I have an LED strip above my desk and when I have my shirt off (right now) I can very definitely feel the radiant energy when it is on, so if it is really hot I either turn it off or switch it to green because the eye is most sensitive to green light. In fact, as I'm writing this, I just set the backlight on the 55-inch TV I use as a computer monitor down so I'd feel more comfortable.

So an umbrella?

Yeah, but like an umbrella made by something like Drexler's nanotechology.

How about something more ... Realistic

https://www.thebrighterside.news/post/mits-high-tech-hydroge...

That's the opposite of a tree, which turns drinking water into nothing

"Water harvesting" has been around for a long time. I think I first saw a water harvesting device in the U.S. Army Survival manual in the 1980s. It's an active research area and devices continue to improve.

Passive? Hydrophobic+Dessicant? dew collector? In the desert?

Dew collector seems to make the most sense for 1980

If we're being condescending: 'So, a parasol?'.

I wasn't trying to be condescending, just trying to point out that trees might not be the most viable structure. Last time I was in southern Arizona when I wanted some relief I'd just find a solar panel installation in a parking lot and hang out under that.

Swamp coolers can't generally provide shade.

I got a great business idea now.

Trees acts for their little thermal mass and large surface exposed to air, essentially IR radiation from the Sun can't much reach the ground or humans under trees (if they are large and dense enough) and the part of the radiation touching trees get quickly dispersed in the air (climbing the atmosphere).

The limited effect is that cities are dense and can't be made as forest so trees can't do nothing for buildings taller them them.

> trees can't do nothing for buildings taller them them.

Plant trees on top!

Still not useful: facades remain exposed and so the largest thermal mass of the building.

Some of it is converted to chemical energy used for the tree’s growth etc chatgpt says 30-40%

There is literally no difference. Water is water, and its specific heat of evaporation doesn't depend on the way it's evaporated.

Reply on news.ycombinator.com