After a hot sunny day, people sprinkle water on the roof-top because :

Sprinkling water on a roof-top after a hot sunny day utilizes the principle of evaporative cooling. Water has a large latent heat of vaporisation, meaning it requires a significant amount of energy to transform from a liquid to a gaseous state [2]. When water is sprinkled on the hot roof, it absorbs this necessary thermal energy (latent heat) directly from the roof's surface and the surrounding air to evaporate [1]. This process effectively removes heat from the structure, leading to a substantial drop in the roof's temperature [1]. While water does have a high specific heat capacity, the primary cooling mechanism in this scenario is the phase change (evaporation), which exploits the fact that water absorbs approximately 2,260 kJ/kg (or 540 cal/g) to vaporize, providing a much more efficient cooling effect than simple conduction or convection [2].

Sources

1. [1] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 10: Water in the Atmosphere > EVAPORATION AND CONDENSATION > p. 86
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > Explanation > p. 295