So why a surface sand silo instead of going down and using the soil/clay/bedrock/whatever is there? Ease of installation and maintenance?
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It'll just be cheaper to build it on the ground than to dig a big hole and then build it in the hole.
the reason we wear coats has more to do with convection than the heat conduction of air
Air is a pretty good insulator, we wear coats to prevent air from moving around. How do most coats keep you warm? Trapped air.
The London underground is hot because the ground is an okay-ish heat conductor: it carries heat away well enough that for a century it essentially acted as an unlimited heat sink, so during its initial construction they never bothered to build proper ventilation. In fact, it was advertised as the perfect place to stay cool during the summer! But this has gone on for long enough that a significant area around the tubes has gradually warmed up from 14C to 19C-26C, and the smaller temperature gradient from tube-to-ground means less heat is carried away, which means the tubes now stay hotter.
Had ground been a great insulator the Victorians would've had to install a proper ventilation system from the start, and they wouldn't been having this issue right now. On the other hand, had ground been a great conductor this issue would've taken far longer to pop up as the heat would've spread through the ground faster.