Summary of a presentation to the American Physical Society, Atlanta, GA, March 1999.

Melvin Pomerantz, Hashem Akbari, Paul Berdahl, Haider G. Taha,
Lawrence Berkeley National Laboratory, Berkeley, CA 94720
and Arthur H. Rosenfeld, U. S. Dept. of Energy, Washington, DC 20460

It is a well-documented experience that cities are warmer than their rural surroundings ("Heat Islands").

Figure 1

In an extreme case, Chicago in 1995, the indoor heat killed 700 elderly people. The additional temperature always has the damaging consequences of increasing demand for electricity (for air conditioning) and increasing smog. Causes of the problem include the use of structural materials that are black. The physics is simply that materials are black because they absorb sunlight strongly. They can become very hot (as much as 70 °F above the air temperature). The hot surfaces then heat the air, which causes discomfort. The chief culprits are dark roofs and asphalt pavements. Hot pavements may be good for frying eggs, but they are a bother in almost all other ways. The way to fix the problem is to make surfaces whiter, so they reflect the sunlight and stay cooler .

Figure 2

The physics part of our work is to measure the reflectivity of conventional roofing and paving materials, and try to find alternate materials that are more reflective. We also measure the emissivities. A material stays cooler if it is able to reradiate excess energy. Metals are poor emitters, so the title "Cat on a Hot Tin Roof" is good physics. A cooler roof on a building benefits it directly and immediately, and might have prevented some of the deaths in Chicago. If all roofs were cooler the energy savings in many cities could be tens of millions of dollars a year.

Figure 3

The costs of extra heating in winter are less than the summer savings in cooling as far north as New York City. Further, if enough of the surfaces are cooler, the whole city stays cooler. Lower temperatures would decrease the production of smog, which has many cities out of compliance with clean-air standards. We do computer simulations to estimate the city-wide lowering of temperature and reduction of smog that deployment of whiter materials may achieve. If all possible surfaces nation-wide were cooler, we estimate that the energy and smog savings would be worth about $10 billion per year.

Figure 4

The public policy aspect of our work is to inform people that their discomfort and expense during the summer can be relieved by as much as 4°F if everyone installed whiter materials during new constructions and resurfacings. Governments can help by revising building codes and specifications for pavements. We recommend that engineering and building standards adopt cooler materials. More reflective, and thus cooler, pavements have benefits in addition to reduced energy and smog. The use of cooler pavements will likely lead to longer life of the pavements themselves. When pavements are softened at high temperatures they are more easily damaged by traffic. This requires more frequent maintenance; it can be avoided by reflecting sunlight before it heats the pavement. Whiter pavements will reflect street-lamp light onto pedestrians and signs, helping to avoid accidents. With an increasing population of automobile drivers who are elderly - and whose vision is not what it used to be - we need all the illumination we can get.

Supported by the U.S. Department of Energy and the U.S. Environmental Protection Agency under contract No. AC03-76SF00098.

More information can be found on our website - http://EETD.LBL.gov/HeatIsland.