After a hot sunny day, people sprinkle water on the rooftop because water has large latent heat of vapourisation. Latent heat of vapourisation is a physical property of a substance. When a material in liquid state is given energy, it changes its phase from liquid to vapour; the energy absorbed in this process is called heat of vapourisation. When water is sprinkled on the rooftop or ground, it evaporates because of the latent heat of vapourisation leaving behind a cooling effect. In case of ground, this evaporation causes cooling of the surrounding area after some time, whereas in case of rooftop, the room beneath is cooled.