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Cool Roof Coatings >> Cool Roofs Reduce Urban Heat Islands

Cool Roofs Reduce Urban Heat Islands

Dark colored materials absorb more heat from the sun.  Good example of this can be noticed from wearing dark colored or black clothes, on a hot sunny day.  Tests have shown that black surfaces in the sun can become up to 70°F (40°C) hotter than the most reflective white surfaces. If those dark surfaces are roofs, some of the heat collected by the roof is transferred inside the building.

Black Shingle

White Shingle

White Shingle

Reflectance = 5%
Temperature = 180°F

Reflectance = 29%
Temperature = 157°F

Reflectance = 60%
Temperature = 128°F

Shingles with lighter/whiter color or roofing granules reflect the solar heat.

Staying comfortable in under a dark shingle roof often means more air conditioning and higher utility bills. These roofs also heat the air around them, contributing to the heat island effect.  Cool roofs can reduce the heat island effect and save energy.

In a study funded by the U.S. EPA, the Heat Island Group,, carried out a detailed analysis of energy-saving potentials of light-colored roofs in 11 U.S. metropolitan areas. About ten residential and commercial building prototypes in each area were simulated.  Both the savings in cooling and penalties in heating were considered.  Estimated saving potentials of about $175 million per year for the 11 cities. Extrapolated national energy savings were about $750 million per year.

Potential net energy savings from changing roof reflectivity. Savings are measured in dollars. Note that the net savings are the savings of cooling energy use less the penalties of heating energy use.

The Heat Island Group has also monitored buildings in Sacramento with lightly colored, more reflective roofs. They found that these buildings used up to 40% less energy for cooling than buildings with darker roofs. The Florida Solar Energy Center performed a similar study, also showing up to 40% cooling energy savings.

Urban Heat Islands Consume Energy and Contribute to Increased Pollution and Environmental Damage
Urban heat islands are dark areas like parking lots, highways, and commercial roofs.  Because cities have higher concentrations of buildings, roads and pavement, most urban heat islands are largest and most costly in these areas.  Higher temperatures in urban heat islands directly translate to increased energy use, mostly due to a greater demand for air conditioning.  As increased air conditioner use takes place, power plants burn more fossil fuels.  This in turn increases both the pollution level and energy costs.

For example, on warm afternoons in Los Angeles the demand for electric power rises nearly 2% for every degree Fahrenheit the daily maximum temperature rises.  In total, it is estimated that about 1-1.5 gigawatts of power are used to compensate the impact of the LA heat island. This increased power costs the Los Angeles ratepayers about $100,000 per hour, about $100 million per year.

An additional consequence is that the probability of smog also increases by 3-5% for every degree °F rise in daily maximum temperature above 70°F in our cities.

The impact of these pollution levels is also seen in smog. The formation of smog is highly sensitive to temperatures; the higher the temperature, the higher the formation and, hence, the concentration of smog.  In Los Angeles for example, at temperatures below 70°F, the concentration of smog (measured as ozone) is below the national standard.  At temperatures of about 95°F all days are smoggy. Cooling the city by about 5°F would have a dramatic impact on smog concentration.

Los Angeles Urban Heat Island Study
Cities all over the world have been warming up in the summer over the years. Los Angeles, is a striking example of how a city was transformed into an urban heat island.

In the 1930s, Los Angeles was an area covered with irrigated orchards. The high temperature in the summer of 1934 was 97°F. Then, as pavement, commercial buildings, and homes replaced trees, Los Angeles warmed steadily, reaching 105°F and higher in the 1990s.

Currently, about 40% of the area in the LA basin is covered by buildings and roads which could realistically be made 30% more reflective during their next resurfacing. If this were done, summer temperatures in LA at 3 p.m. on an average sunny day could become 5 to 9°F (or 3 to 5°C) lower. LA would then consume 1/2 to 1 GW less in peak power, energy worth at least $100,000 per hour. Most areas would also have improved air quality, and the population-weighted average predicts an ozone reduction of 10 to 20% overall.

Because the rate of smog formation depends on temperature, this same model was used to estimate the effect on the region's smog, taking into consideration wind patterns, moisture, and other factors specific to the area. The results showed an overall reduction in smog by about 10%, the equivalent of removing three to five million cars from the roads.

Similar modeling studies been performed for Houston, Dallas, Chicago, Atlanta, Washington D.C., Baltimore, Philadelphia, New York City, Miami, Phoenix, and Tucson.

Sacramento, California (1998)
The image below shows how California's capital city of Sacramento glows in its own summer heat in this false-color infrared image taken as part of the interagency Urban Heat Island Pilot Project (UHIPP). Sacramento was the second of three cities surveyed in UHIPP. Similar studies have been conducted for cities all across the nation.

 In this "quick look" image - which has not been calibrated or corrected - white and red are hot, and blue and green are cool. North is up. The hottest spots are buildings, seen as white rectangles of various sizes, and a rail yard (orange) to the top right (east) of the Sacramento River flowing from top to bottom (the American River cuts across the top right corner). The state capital is the red spot in the green rectangle to the right center of the river. A significant number of cool areas are present, including the American River Parkway at top right. Interstate 5, running north to south along the East Side of the Sacramento River, and US 50, from left to bottom right are also visible.

The image was taken Monday, June 29, 1998 at 1 p.m. local time by the ATLAS imager aboard a NASA Lear 23 jet equipped with various sensors and cameras for UHIPP.

Credit: NASA/Marshall Space Flight Center and
Global Hydrology and Climate Center

From surface temperature estimates, the white areas are about 60 degrees C (140 degrees F), said Dr. Jeff Luvall, the principal investigator at NASA's Marshall Space Flight Center. Dark areas (vegetation) are approximately 29 to 36 degrees C (85-96 degrees F). Since the image has not been calibrated, absolute temperatures will change after calibration, but the relative temperature differences between surface types will not.

Atmospheric profiles of temperature, relative humidity, and pressure were measured with a balloonborne instrument package called a radiosonde to calibrate the ATLAS measured surface temperatures. Additional roof surface temperatures were taken with a handheld "heat spy," an infrared thermometer to help calibrate the ATLAS thermal measurements. Scattered around the city on three rooftops were instruments which measured the visibility or transmissivity of the atmosphere to aid in the calibration of the visible data.

The image clearly demonstrates the principle behind UHIPP, that the differences in cooling and heating between the natural and manmade surfaces can affect city temperatures.

"Urban forests are important to keeping cities cool," Luvall said. "What's important are both the extent and arrangement of these forests."  Conversely, the hot cities actually keep the forests greener longer.  See the NASA study on How Urban Heat Islands Make Cities Greener

Luvall said that it is important to note that this is a quick-look image that has not be corrected for atmospheric interference or fully calibrated with ground sensor data.

In addition to ATLAS and the "heat spy" instruments on the ground, the study uses images from a 23x23 cm (9x9 in) film camera aboard the Lear 23 and sensors aboard weather satellites.

The Global Hydrology and Climate Center (GHCC) in Huntsville, Ala., working with the U.S. Environmental Protection Agency and several local governments, is conducting UHIPP. The GHCC is a joint venture by NASA/Marshall, the Universities Space Research Association, and the Space Science and Technology Alliance of the State of Alabama. UHIPP follows the successful Urban Heat Island Experiment in Atlanta in May 1998.