When I wade into the sea or immerse myself in an outdoor swimming pool, why does the water always seem coldest when it reaches my midriff?
When we feel hot or cold, what we actually perceive is the difference in temperature between ourselves and another object.
Take three buckets and fill one with freezing cold water, one with lukewarm water and one with hot (not scalding) water. Then simultaneously plunge your left hand into the cold-water bucket and your right into the hot-water bucket and leave them there for 30 seconds before removing both of your hands and putting them into the third bucket containing the lukewarm water. Your cold left hand will feel that the lukewarm water is hot and your hot right hand will feel that the same lukewarm water is cold.
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So in the original scenario the reader is really describing a greater temperature difference between tummy and sea than between legs and sea. Therefore the question now becomes 鈥渨hy is a person鈥檚 tummy area warmer than a person鈥檚 legs?鈥�
I would postulate that the difference comes from heat generated by the stomach and intestines as they digest meals, a process which continues for a long time after food is eaten. This would give a localised increase in skin temperature in the tummy area and cause the sea to feel coldest when it reaches that level.
Oonagh Griffith, Lisburn, County Antrim, UK
Two factors operate when we enter the sea. Our skin temperature is cooler than our core temperature 鈥� that of our trunk organs, heart, liver and stomach. This is because we circulate warm blood through cool skin to lose body heat.
Typically, our limbs are cooler than our trunk because we operate a countercurrent heat exchanger to limit the loss of body heat. The arteries that supply a limb run close to the veins returning from this limb. Warm arterial blood flowing to the limbs is cooled by venous blood returning to the heart. This means that our feet and hands are almost always cooler than the skin of our trunk.
Wearing a bathing costume makes the midriff even warmer than it would be if you were a naturist. Both of these mean that the difference between skin temperature and water temperature becomes greater as you immerse yourself. The bigger the difference, the greater the discomfort.
Peter Bursztyn, Barrie, Ontario, Canada
Several of our male correspondents point out that the shock is worst when cold water reaches their testicles. This is odd because these are cooler than the abdomen, with less of a temperature difference between them and the legs. Sexual organs are, of course, richly supplied with sensory nerves, so it would seem as though temperature difference is not the whole story 鈥� Ed