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Why does a 104°F pool/tub feel boiling hot, whereas a pot/cup of water at the same temperature does not feel hot at all.

(Normally a pot/cup of water won't be hot enough to cause one to immediately remove ones finger from the water till it's around 165-175°F).

Probably same conductivity, specific heat, BTU's (since the amount of BTU's needed to raise 1°F is proportion to volume of water. Meaning, 1 BTU is needed per pound, so no matter what the volume is it will contain the same BTU's per pound).

Perhaps there's much more "heat" (BTU's/Joules) available in a tub/pool to "refill" the spot which transferred into ones body (perhaps through conductivity) not allowing the area of water touched by ones body in the tub/pool to cool off fast enough.

Another possible factor might be that perhaps there's an increase in convection; not sure if the higher the volume of water, the higher the convection.

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  • $\begingroup$ 104F is 40C, and I would never enter a tub at 40c, it feels the same to my fingers, So it depends on personal reactions which depend on our biological and psychological state. I am voting to close . $\endgroup$
    – anna v
    Jan 13 at 7:31
  • $\begingroup$ Thanks to the two that answered. $\endgroup$
    – Albert
    Jan 15 at 19:27
  • $\begingroup$ to show your thanks , you could vote (it is the up arrow on the left to give a positive vote) $\endgroup$
    – anna v
    Jan 16 at 6:43

2 Answers 2

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It depends on the surface area of your body that is exposed to the hot water. If you get into a hot tub, most of your body is exposed to the heat, whereas dipping your fingers into the water only exposes a small surface area.

The sensitivity of different parts of your body will also play a part. Your fingers and hands are less sensitive to heat than other parts of your body. This also applies to the soles of your feet, where the skin is thicker. Which is why a hot water bottle that feels pleasantly warm under your feet will feel much hotter against your legs. This is also why mothers with young babies are advised to test the temperature of their baby’s bath water with their elbow, which is more sensitive to heat than their hand.

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The difference in temperature perception between a 104°F pool/tub and a pot/cup of water at the same temperature is likely due to physical phenomenon like thermal mass, convection, heat transfer rate and sensing area (finger vs whole body).

The larger volume of water in a pool or tub means a higher thermal mass that help to maintain the temperature of the water. Also, the surface area of the water in contact with your skin is much larger in a pool or tub than it would be in a cup or pot, allowing for more heat transfer to occur.

Convection is the transfer of heat by the movement of a fluid or gas. In the case of a pool or tub, convection occurs when the warmer water at the surface sinks and is replaced by cooler water from the bottom. This creates a circulating pattern, where warm water is continually brought to the surface and cooled water sinks to the bottom, promoting the transfer of heat throughout the water. This process increases the heat transfer rate to the skin, hence making you feel hot.

In a pot or cup, convection is less likely to occur because the water is confined in a small space, so the heat transfer rate is slower.

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  • $\begingroup$ Thanks for your great answer! Just trying to understand what it means that "convection is less likely to occur" in a pot or cup. Doesn't a pot of water get heated with convection (the fire heats up the water on the bottom which rises to the top, the colder water gets heated and rises to the top etc.). - So why should a pot that's 104 deg F on a stove (or just off the fire) not have convection? $\endgroup$
    – Albert
    Jan 20 at 17:45
  • $\begingroup$ @Albert Convection does occur in heated pot. However there is no significant difference in temperature between the bottom and top layers of water. This lack of a temperature gradient means that there is less energy driving convection, making it less likely to occur. In this case, the water is not circulating much and the heat is being transferred mainly by conduction $\endgroup$
    – Haris
    Jan 21 at 2:14

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