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I've heard somewhere that during summer the temperature of ground water is lower than that on the Earth while during winter the temperature of ground water is higher. How is this possible ?

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Soil is a bad conductor of heat. So, the layers of the top surface of earth act as insulators.

During summer, they insulate the underground water and soil from the heat radiation from the sun, and from the general hotter atmosphere, hence underground water is colder.

In winter, they again act as insulating layer and prevent the heat from underground to escape into the cold outside air. Thus, underground water can be warmer than outside.

One noteworthy point is that the underground water temperature remains fairly constant throughout the year, (for a given geographical region), because the water table is quite below the ground, and hence it is insulated from the outside atmosphere.

It feels colder during summer, because outside water is hotter and it feels warmer during winter because outside water is colder .

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  • $\begingroup$ I always thought the answer must contain the $4^{\circ}$C density anomaly. If water would become denser the colder it gets, wouldn't the warmest water always migrate to the top to minimize buoyancy forces? $\endgroup$
    – Koschi
    Aug 1, 2021 at 12:56
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    $\begingroup$ @Koschi This is only relevant for shallow depths and cold climates. A few meters below the surface, the temperature is more or less constant and equal to the average surface temperature over a full year. At lower depths, the temperature rises because of the internal heat generation in the earth. Note, is very different from ocean temperatures, because there are convection currents driven by temperature in the oceans, but not in rocks. $\endgroup$
    – alephzero
    Aug 1, 2021 at 13:33

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