If you haven't heard, LA is painting its streets white to keep the city cool. But why white? Why not green. At least with green we can call this idea what it really is, Greenwashing.
So here is the pitch: We've all heard about the heat island effect (UHI) where an urban area is significantly warmer than its surrounding rural areas due to dark surfaces, such as asphalt, absorbing the sun's energy, retaining the heat and raising the temperature of the surrounding area. So what do we do? Paint the streets white of course! This will reduce the surface temperature by 10 degrees as claimed by the city of LA and result in surrounding air temperature being reduced by 3 degrees. Or maybe only 1 degree per a study by the EPA (Cool Pavements, 2012). Articles around the Internets claim America's streets could soon be white as California's cool pavements reduce the extreme heat effects of climate change. Mission Accomplished.
Not really.
Now, I can't speak towards how glaringly less safe a more reflective road will be. Or how over time car and truck tires will make a white surface turn, well, not white. I also can't find any specifics on the materials used in the paint so I can't say for sure if VOCs or heavy metals will contaminate stormwater as it breaks down over time. I can say that the paint manufacturer is poised to make a lot of money if this goes through. LA has 6,500 miles of streets. At 60,000 square feet per mile of road and 30 gallons required per 1000 sq ft. that is ~12 million gallons of paint required for this effort. Also at $40k per mile that is $260M dollars. Is manufacturing 12 million gallons of paint good for the environment? I can't say. So lets put these items aside and look at the physics and thermodynamics of the idea.
It is common sense that dark objects are hotter than light objects in the sun. Color effects surface temperature because a dark object's materials adsorb the electromagnetic waves of light from the sun (as radiation). Since energy is never created or destroyed, just converted, the radiated energy contained in these adsorbed waves gets converted into heat (as convection) and measured as an increase in surface temperature. On the other hand, a lighter object, like a white street, appears cooler because it reflects the electromagnetic radiant energy, converting less heat to convection.
So how are they backing their claim that a painted street is 10 degrees cooler? Using an IR or infrared thermometer. The problem is an infrared thermometer does not directly measure temperature (gasp!). An IR thermometer measures infrared radiation (hence the name) and them multiplies it by emissivity. Emissivity is a value from 0-1 which measures how well an object emits infrared energy, or heat. Reflective surfaces (like a white street) have a low emissivity ratings, because they reflect ambient infrared energy not their own emitting IR. Unless the IR thermometer has been calibrated to the different surface emissivity, its readings will be at a lower temperature than actual for the more reflective surface.
Back to the first law of thermodynamics which says energy can not be created or destroyed, only converted. The sunlight energy that hits the earth's surface, or in this case, the streets of LA, is not destroyed by the white street. It is reflected, and the energy will go somewhere. That somewhere is the surrounding areas, buildings, trees, and back up in the sky and atmosphere. While the street itself may be cooler, the assumption that the heat will just disappear is questionable. A 2012 study found that white roofs reflect IR energy back up to the sky where they will hit clouds and reduced cloudiness, allowing more sunlight to reach the ground (Journal of Climate, Feb. 2012) . The reflected light energy will also hit the airborne smog and gasses surrounding the city, increasing the air temperature, just at a higher altitude. I know what you are saying, what about snowball earth? The difference here is the snowball earth reflection scenario didn't have the LA air pollution to contend with.
So are white roofs as bad as a white street? No. The benefit of a white roof is the decrease in air conditioning needed inside the building. As an HVAC engineer I believe the energy savings of lower internal building temperatures due to conduction through the roof are better for the environment than the reflective heat. But a street does not house occupants in a conditioned space below it.
In the end, I'm very skeptical of this white street movement. I would like to see comparative data from embedded temperature sensors in the pavement and sensors measuring air temperature above before passing judgment. IR measured surface temperature is a poor indicator of the true performance of this product. And if white streets are not the answer then what is? Shade trees and solar panels. Both adsorb light energy and turn it into good (oxygen and electricity) not bad (more heat).
Alex Mathers P.E.