[throwaway3334]

For now, everyone is free to pump out kilowatts of heat with their roofs, parking lots, air conditioners into the surrounding air. It is not considered pollution, even when it is already 100 °F outside.

On the days when there is an inversion in the atmosphere, the hot air stays trapped. People have to run ACs in their houses, stores and offices. And that results in even more heat routed to the place, with ACs units, literally pumping energy from the solar arrays somewhere in the desert into the city.

[8bitsrule]

At this time last year, this region was subjected to a 'heat dome'. Luckily this building has a 'cool dome' in the basement. Spent a few days listening, reading and solving down there (12-16°C cooler).

Much cooler temperatures (13°C) are found just a few feet below ground. (Memorable to those who've visited 'root cellars.) Before long those mysterious 'underground cities' in Anatolia may not seem so mysterious. It's a low-cost, low-tech, zero-energy solution, used by native communities in many regions of the world.

[wcoenen]

> Much cooler temperatures (13°C) are found just a few feet below ground. (Memorable to those who've visited 'root cellars.) Before long those mysterious 'underground cities' in Anatolia may not seem so mysterious.

When the London metro was built (over a century ago), the clay was at 14 degrees C. But over time, it heated up from all the power dissipated by the metro system. Now it is around 20 to 25 degrees C and doesn't really act as a heat sink anymore. The same happened in the NY subway.

So soil can act as a great heat sink for a while, but not forever. I suspect the same would happen if you'd put a modern city underground and relied on the temperature of the soil instead of pumping heat out somehow.

[smaudet]

This and geo-thermal. The crazy megalomaniacs pushing solar are as bad as the rest of oil/coal kingpins, unsustainable and another ecological train-wreck in progress.

[rocqua]

My intuition says that the volume of inside vs outside air is so big that it has little effect.

[throwaway3334]

You can look at how much space roofs cover in the suburb in the Bay Area. It'd be roughly 25% streets, 40% roofs, 30% backyard, 5% front-yard. Adding white reflective coating increases "Solar Reflectance" and decreases "Thermal Emittance" and absorption. Most dark roof materials reflect 5 to 20% of incoming sunlight, while light-colored roof materials typically reflect 55 to 90%. A white roof coating that you can purchase at your regular home improvement store would do that 90%.

So effectively, in the suburban area there is an opportunity to change our average solar reflectance by ~32% [(90% - 10%) * 40%]. If we just abandon the idea that a good house should look like a house in Normandy and have a black roof.

[edit: I've opened a section of sat map (in Sunnyvale, near Reed Ave/Sequoia Dr.) and estimated roof/street/backyard/front-yard by area, for a lot of 2 houses and adjacent street. The numbers above are from that estimate. Heat and noise of ACs is somewhat local (urban heat island effect, etc), so that local estimate is what counts. It is not surprising that we can have 40% roofs. Land is expensive in the suburb like that.]

As per: https://en.m.wikipedia.org/wiki/Reflective_surfaces_(climate... - "If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 gigatonnes of greenhouse gas emissions, or equivalent to taking 300 million cars off the road for 20 years. So it seems, there is not only local/urban heat island effect, but even some global effect. To put this into perspective, Tesla sold 2 million vehicles. No idea, if the Wiki numbers are correct...

[uxp100]

I suspect that there are good stats somewhere, but I think that 40% roofs might be a pretty high over estimate in say, Sunnyvale, or even San Jose.

[rch]

I'm not aware of any stats, but I've experienced the difference between hot neighborhoods and cooler neighborhoods, right next to each other in Houston. It would be great to have a quantitative measure for this effect, similar to a walk score (which isn't perfect, but can be occasionally useful).

[kolinko]

Are you sure it’s because of AC and not the amount of concrete pathways compared to green areas, vegetation and shade from trees? Also various thermal properties of the ground?

Even if houses were 50% of the land area, they take up a miniscule amount of air volume. Also, hot air floats up, so there is little chance you could feel the heat from an AC outside. On the other hand, if the earth is hot because you have few trees, you will feel it everywhere when walking on a sidewalk.

In Europe at least it’s common knowledge that you need to have trees and greenery everywhere to make heat bearable outside. If you replace trees and grass with concrete you get an island of heat that will be unbearable during summer to walk through.

edit: thermal camera image of a walkway with trees in summer https://images.app.goo.gl/m2XN7rqodavDcwoE9