A summer swim can sometimes bring a little surprise: after enjoying a pool warmed by the sun, diving into the sea, equally under the sun, can deliver a real thermal shock. Even in intense heat, saltwater always seems cooler than the fresh water in pools. It's not just an impression: several physical and environmental factors explain this difference.
The first reason lies in the water mass. Oceans and seas contain volumes of water vastly greater than those of a pool. Water has a very high heat capacity, meaning it takes a long time to warm up or cool down. In summer, the sea warms slowly and maintains a fairly stable temperature, often lower than that of the air. In contrast, a shallow pool exposed to the sun sees its temperature rise rapidly.
Illustration image from Unsplash
Next, there's the influence of currents. In the sea, constant water movements mix surface layers with deeper, colder water masses. Tides, wind, and marine currents bring water from offshore or depths up to the surface, preventing the temperature from rising as much as in a pool. This phenomenon, called upwelling in some regions, is particularly noticeable on certain coasts where the water stays cool even in midsummer.
Salinity also plays a role, but more subtly. Saltwater has a lower freezing point than freshwater, slightly altering its thermal properties. However, this isn't the direct factor explaining the sensation of coolness: it's rather the density and thermal conductivity of seawater that intensify the feeling of cold on the skin.
We must also consider the psychological and sensory effect. The sea is often choppy, and its movements create constant contact with new water masses, always cooler than the thin layer warmed by body contact. In a pool, on the contrary, the water is calmer, and a small "warm bubble" forms around the swimmer, reducing the sensation of cold.
Finally, the environment around the sea amplifies this impression. Sea air is often cooler and more humid, and the coastal breeze accentuates body heat loss through evaporation. Even if the actual temperature difference between sea and pool is only a few degrees, the perceived difference can be much more pronounced.