Having a swimming pool close by in the summer makes the season enjoyable. The only cause for concern might be the coldness of the water if you jump in first thing in the morning. Usually, the water feels the warmest after the sun has set, and sometimes it is actually warmer than earlier in the day when the sun was at its highest.
Night Air is Cooler
When the evening air cools down from the heat of the sun, the difference between the air temperature and the pool water changes. Sometime, the air temperature will drop even lower than the pool water. Since the pool water temperature will not change that much within one or two hours, the cooler air makes the water feel warmer than it was previously.
Day Heat Warms the Water
Sunshine warms the water starting in the later morning until the late afternoon so that by nightfall, the water is at its highest temperature. This heat not only warms the water but the materials holding the water, including the ground under the water.
Fresh Water Cools the Pool
One of the reasons that pool water feels warmer in the night is because in the morning, after backwashing the pool, many owners turn on fresh water to top off the water levels. This fresh water Is much colder than the warm water of the pool and drops the temperature of the water by several degrees.
Heat Dissipates During the Night
Without the sunlight adding heat to the pool overnight, the temperature of the water will drop by several degrees by morning. The rate of decrease will also increase significantly if the night temperatures drop by 20 or 30 degrees from the daytime temperatures.