How to cool a stone bench in the sun: the physics of stored heat
Each kilo of granite needs 800 joules to rise one degree. Here's how to cool a stone bench without waiting two hours.
English · Original discussion in Spanish · Published
Picture a stone bench in the full July sun. When the shade finally reaches it, the relief is brief: the stone is still burning hot. Anyone who sits down discovers something that physics explains with cold, hard numbers.
The stone's thermal inertia is enormous. Each kilo of granite needs about 800 joules (0.22 watt-hours) to rise one degree Celsius. A typical bench can weigh a hundred kilos or more, and the heat from a morning of sunshine stays trapped for hours.
Does pouring water work?
Yes, but with caveats. Water draws heat away by conduction and, as it evaporates, speeds up the cooling. On smooth surfaces, a quick splash creates a layer of steam that insulates and slows heat loss. The most effective method is to apply a cloth soaked in cold water, let it do its job, and repeat. If you also force evaporation with a fan, the effect multiplies.
The limits of home engineering
The real problem is that the heat isn't just at the surface. In a thick bench, the interior stays hot and will reheat the surface minutes after you cool it. The only definitive solution, as the person who did the math admits, is not to sit there at three in the afternoon.
Physics is stubborn: against the stone's thermal inertia, a bucket of water and a cloth are the best that household science can offer. That, and waiting for the sun to go down.
The stone's thermal inertia is enormous. Each kilo of granite needs about 800 joules (0.22 watt-hours) to rise one degree Celsius. A typical bench can weigh a hundred kilos or more, and the heat from a morning of sunshine stays trapped for hours.
Does pouring water work?
Yes, but with caveats. Water draws heat away by conduction and, as it evaporates, speeds up the cooling. On smooth surfaces, a quick splash creates a layer of steam that insulates and slows heat loss. The most effective method is to apply a cloth soaked in cold water, let it do its job, and repeat. If you also force evaporation with a fan, the effect multiplies.
The limits of home engineering
The real problem is that the heat isn't just at the surface. In a thick bench, the interior stays hot and will reheat the surface minutes after you cool it. The only definitive solution, as the person who did the math admits, is not to sit there at three in the afternoon.
Physics is stubborn: against the stone's thermal inertia, a bucket of water and a cloth are the best that household science can offer. That, and waiting for the sun to go down.
Also available in: Deutsch
Summary of a discussion on Burbuja.info - Foro de economía, actualidad y política., translated from Spanish and reviewed before publication. Read the full discussion (21 replies).
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