This is because Uchimizu is not effective if water is sprinkled on an asphalt-paved road in summer when air temperature is high since water vaporizes soon after sprinkled and the effect of inhibiting temperature rise by the heat of vaporization is minimal.
夏の気温が高い状態でアスファルトに打ち水をしても、水はすぐに蒸発してしまい、気化熱による気温上昇の抑制効果が得にくいためである。