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May 17

What would be cooler, under the sun, a wet box painted black or a wet box painted white?

If the walls of the box are of the same material, but different color, and in both cases soaked in water. The interior of the boxes is the same as the exterior. Does it matter what type of material, if both white and black are the same material?

I assume that the best option is having internal walls that are black, but external that are white. Bumptump (talk) 09:08, 17 May 2023 (UTC)[reply]

White on the outside will keep the box cooler. The colour of interior walls is indifferent, since virtually all internal heat transfer is by conduction. --Lambiam 10:34, 17 May 2023 (UTC)[reply]

And even if the internal heat transfer is by radiation, that must be infrared radiation, and there's no reason to assume the white paint is also light in the infrared. In fact, it's unlikely. Water is also quite dark in the infrared. PiusImpavidus (talk) 17:11, 17 May 2023 (UTC)[reply]

Sinking cities

Is it possible for some modern cities to sink into the ground due to their weight? I've read about Jakarta, and now there's this article about New York, too. [1] Are there any other cities at risk, and are these scenarios realistic or is it just clickbait? 2A02:1210:82E8:200:69F8:D9F1:FA9D:B735 (talk) 23:03, 17 May 2023 (UTC)[reply]

Yes. Mexico city. Bazza (talk) 08:38, 18 May 2023 (UTC)[reply]

Many cities sink: Jakarta, Shanghai, Venice, Amsterdam, New Orleans. The weight of the city may play a role, by accelerating soil compaction, but may not be the only or most important factor. Other factors:

Extraction of natural resources, like water, oil, gas, salt, coal or ores from underneath the city can cause the ground to sink, sometimes several metres.
Lowering the groundwater level or the water level in lakes near the city to prevent flooding may allow oxygen to enter the soil, leading to oxidation of peat. As the peat goes away, the ground level sinks, restoring the flood risk (and releasing greenhouse gasses in the process). What's more, not only the peat oxidises, but also the centuries-old wooden pilings under the buildings. It's turning into a big problem in Amsterdam and surrounding cities. Hardening the land surface with buildings and roads causes rainwater to flow directly to surrounding surface water, instead of first supplementing groundwater. This also lowers the groundwater level.
Sometimes the soil sinks naturally, due to tectonics, compaction or apparently by sea level rise (the soil doesn't go down, but the reference goes up). In the natural situation, this is compensated by the accumulation of sediments. Where a city is built, no more sediments accumulate, so the sinking turns into a problem.

Cities built on sediments are most at risk, like in river deltas and some coastal locations. PiusImpavidus (talk) 09:17, 18 May 2023 (UTC)[reply]

For completeness, another minor factor can be Post-glacial rebound, where land rising due to the removal of the weight of glacial ice can result in pivoting so that other land sinks – this is a factor in the British Isles where the once-glaciated North is (slowly) rising but the (formerly unglaciated) South is correspondingly sinking. {The poster formerly known as 87.81.230.195} 90.199.210.77 (talk) 12:22, 18 May 2023 (UTC)[reply]