Water is a very special liquid. For example, the energy retention of water is very good, while it takes a lot of energy to heat water.

These photos from a thermal camera day and night show the comparatively cold water (dark blue) in the biological pool (during the day) and the high temperatures, at night, of the same body of water and on the same day.

From the perspective of the user of a biological pool, this means that a biological pool reaches the highest water temperatures in the late afternoon. And, some biological pools in the South of the country can be used in the autumn until Saint Martin, because the strong solar radiation in combination with the good energy retention capacity of the aquatic element allows even baths at the late end of the year.