This work examines the heating and cooling of an urban rooftop using radiative and meteorological measurements plus numerical simulations for the months June through September. Longwave atmospheric radiation accounts for 63.6% of the radiant energy absorbed by the roof, compared to a solar contribution of 36.4%. Changes in incident solar and longwave irradiance in response to varying cloudiness have opposite signs. As a result, the mean radiative heating on clear days differs by only 3.35% from that on cloudy days with precipitation. In response to radiative forcing, the roof reaches a temperature that provides the thermal emission, sensible heat transport to the atmosphere and heat conduction into the interior required to balance the heating. These three processes offset 79.4%, 18.4% and 2.2%, respectively, of the average heating for dry days. After precipitation, evaporation provides a 24 h average cooling of 36.4 W m−2. Evaporative cooling comes primarily at the expense of sensible heat transport and is accompanied by smaller surface-to-air temperature contrasts than exist during dry periods. Comparison of measurements and simulations for wet days shows that evaporative cooling leads to a 24 h average drop in surface temperature of 1.2–1.3 K, with larger instantaneous reductions during daylight.

Areas of Focus: Energy Markets
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Energy Markets
Well-functioning markets are essential for providing access to reliable, affordable energy. EPIC research is uncovering the policies, prices and information needed to help energy markets work efficiently.
Electric Power
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Electric Power
As the electric power system faces new pressures and opportunities, EPIC research is working to identify the mix of policies needed to accelerate the global transition to clean, reliable, affordable...
Energy Efficiency
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Energy Efficiency
Improving energy efficiency is lauded as a promising way reduce emissions and lower energy costs. Yet, a robust body of research demonstrates that not all efficiency investments deliver. EPIC faculty...
Renewable Energy
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Renewable Energy
Lower technology costs and supportive public policies are driving an increase in renewable energy in markets around the world. EPIC research is assessing the costs, benefits, and efficiency of policies...