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Why do the water temperatures differ from one beach to another?


19 ° C in the water in Fouesnant or Douarnenez this Thursday, July 22, 18 ° C in Concarneau or even 17 ° C in Camaret-sur-Mer. In Finistère, in the same geographical area which is a priori restricted, the temperature differences observed in the sea can sometimes prove to be significant depending on the beaches. How to explain these changes which push us to favor one spot rather than another to refresh us during the summer period?

Upwelling phenomenon

Jérôme Fournier, CNRS researcher based at the Concarneau Marine Biological Station (29), explains that several parameters must be taken into account. Among these criteria, we can find local currents (study of sea currents), with the presence of more or less active “upwelling”. A phenomenon where the wind pushes back the hottest surface waters off the coast, at the same time bringing up the cold waters that are at depth.

The “felt” temperature can vary greatly depending on the cloud cover, the wind, the air temperature, etc.

This element is far from being the only one to integrate: bathymetric gradient (more or less pronounced slope), nature of the seabed (rocky, sandy, muddy) and their color (dark, light), rate of suspended matter (possibly salinity) or algal blanket also explain why water temperatures change from one place to another. “Note that the“ felt ”temperature can vary greatly depending on cloud cover, wind, air temperature, etc. », Specifies Jérôme Fournier.

Temperature differences in the same bath

Okay for the beaches, but who hasn’t already said to a loved one during a swim: “Come here, it’s better! “? Because the scientist reminds us: “The temperature of the water also varies within the water column (vertical stratification) of the same beach. Example: the bather’s feet are in water of a temperature lower than that of his waist ”.

Two parameters are important here: the very local currents linked to the movements of swells where the waves will generate “a stratification of the water masses and therefore of the temperature”. But also local meteorological conditions “such as the absence of wind and high air temperature”. Clearly, “the hot air heats up a thin layer of seawater (a few decimeters) and in the absence of wind, this heated body of water remains on the surface (less dense), while the layer of water lower has remained cooler and is therefore more dense, ”summarizes Jérôme Fournier. So, still up for a dip?

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