Why is water in a swimming pool cooler than the surrounding sidewalk?

This statement is not always true and depends on various factors. In many cases, the water in a swimming pool may feel cooler than the surrounding sidewalk due to the following reasons:

1. Evaporation and Convection: When water evaporates from the pool's surface, it takes heat energy away, causing the water to cool. Additionally, water near the surface experiences heat loss through convection, where the warmer water rises, and cooler water sinks.

2. Thermal Mass: The sidewalk is often made of materials like concrete or asphalt, which have a high thermal mass. This means they absorb and store heat during the day, especially in direct sunlight. The swimming pool water, on the other hand, has a lower thermal mass and can release heat more quickly.

3. Depth of the Pool: The deeper portions of a swimming pool tend to be cooler because the water has more volume and takes longer to heat up. Shallow areas, especially at the edges of the pool, may experience higher temperatures due to direct sunlight exposure.

4. Shade: If the pool is located in a shaded area, it will receive less direct sunlight, which contributes to the cooling effect.

However, it's worth noting that under different circumstances, the water in a swimming pool can be warmer than the surrounding sidewalk, especially during hot weather or in regions with high ambient temperatures. Factors like continuous sunlight exposure, shallow pool depth, and the use of pool heaters can influence the water's temperature.