Spring temperature

March, 21st marks the beginning of spring on the calendar, but it also marks the beginning of the swimming season in the biological pool.

After years of experience, it became clear that, in the South of Portugal, the water temperature in the biological pool reaches values ​​above 20 ºC on this date, making it ideal for the first bath.

This phenomenon is so consistent that one can adjust the watch: the 21st is synonymous with mild bathing water temperature. Interestingly, water temperatures invariably rise in the second half of March, regardless of the weather conditions of the previous days or weeks, as shown in the remote measurement graph in the picture. But why?

Since the weather does not seem to significantly influence this phenomenon, we must look to the sun. At the vernal equinox, the sun always reaches the same position, resulting in a more direct angle of incidence of the sun’s rays on the water’s surface. This greater incidence of solar energy heats the water more efficiently. Furthermore, global solar radiation, which penetrates clouds, contributes to this warming, making it relatively independent of weather.