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Why Downtown Is Usually Hotter Than Surrounding Neighborhoods, And What We Can Do About It

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(Photo by Karol Franks via the LAist Featured Photos pool on Flickr)
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As we enter the throes of another heat wave (one that could possibly break records), the L.A. area will see scorching temps at every turn. Even coastal Santa Monica and Venice will hit the mid-80s on Saturday, and that's actually considered to be tepid. Elsewhere, you'll see 102 degrees up in Burbank, and 94 degrees out in Echo Park.

Of course, the sea breeze coming in from the coast has a cooling effect, meaning the further you head out east, the warmer it tends to get. But there's an anomaly in certain pockets; some neighborhoods tend to be cooler than the ones that immediately surround it. South Pasadena, for instance, is expected be a couple degrees cooler on Saturday compared to its neighbors Pasadena (to the north) and Alhambra (to the south). And as noted at the L.A. Times, the inverse happens in downtown L.A., which tends to face higher temperatures compared to the areas surrounding it.

What's going on is what researchers dub the "urban heat island effect," a phenomenon in which the air temperature in an urban city is higher than that of surrounding areas'. This is thanks to the sidewalks, the pavement, the buildings, and many of the other defining attributes of a city.

“The urban heat island effect occurs for three primary reasons. The first is that cities have a lot of dark materials like dark pavements and dark roofs and those dark materials absorb a lot of sunlight," George Ban-Weiss, a professor of environmental engineering at USC, said in a USC release about the summer heat. Ban-Weiss added that, "Another reason is that cities generally don’t have a whole lot of vegetation. Vegetation transpires and evaporates water and thus the evaporative cooling can act as sort of an air conditioner." This is partly why South Pasadena, which has an abundance of lawns and tall trees, is somewhat cooler than its neighbors.

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The "heat island" effect is even more pronounced in parts of America where small cities sit next to rural spots. According to the Environmental Protection Agency, the difference in temperature between a city and an adjacent rural area can be as large as 22 degrees in the evening. As the EPA notes, the phenomenon is generally terrible for communities in a variety of ways. High heat leads to more energy consumption as we poor mortals attempt to cool down. And the more power and electricity we use, the more emissions we put into the air, as the companies that supply electricity often rely on fossil fuels. Another (not so expected) consequence of heat? Water pollution. Heated stormwater generally becomes runoff, spilling into storm sewers and finding its way into rivers and ponds. And all this goes without saying that the heat itself is a hazard; it can lead to heatstroke and exacerbate existing health conditions. According to data from the Centers for Disease Control and Prevention, an average of 658 Americans die each yeah of heat-related causes.

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(Photo by Lara23 via the LAist Featured Photos pool on Flickr)
Taking these factors into consideration, along with the fact that the Earth has broken temp records for the past three years, and that experts say that L.A. will see triple the number of "excess heat" days by 2060, the City of L.A. has moved to help the city cool down. Mayor Eric Garcetti had pledged in 2015 to lower the city's temperature by 3 degrees over the next two decades. The pledge came as part of a report—“the pLAn”—which suggests that the mayor's goal is attainable if the city starts planting more trees and installing more "cool roofs" that absorb less heat.

The initiative began to take shape in July 2016, when Matt Petersen, former Chief Sustainability Officer for the Mayor's office, gathered a group of about 20 civil servants and university scientists to outline a way to draw back L.A.'s rising temperatures. “What we are trying to do is create a research collective to help us reach our target,” Petersen told the Times. “It’s a huge challenge.”

Ban-Weiss, who's a part of the city's cool-down team, told the Times that the solution would likely require a multi-faceted approach. “The heat island effect is a regional phenomenon, and the way you choose your mitigation strategy could vary block to block,” Ban-Weiss said.

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While there are different ways to mitigate the heat, the solution seems to round back to the use of reflective street pavement, reflective roofs, and the planting of more vegetation, especially trees. This May, L.A. City Councilman Bob Blumenfield helped launch an experiment in which a street in Canoga Park was coated with a seal (CoolSeal) that reflects most of the sun's rays. As reported at Los Angeles Daily News, nearby residents said that the cooling effects could be felt within days. “It’s cooler,” resident Maria Jimenez told the Daily News. “Last year, it was horrible.”

As Natasha Jenkins, director of communications at GuardTop (the company that produces the seal), told LAist, a newly sealed street will see a dramatic difference in surface temperature. "You might get a 20 degree drop when compared to the regular pavement. And, depending on the time of day, you can see anything from a 10 to 40 degree difference," said Jenkins. She adds that the seal (made from a proprietary material) is non-toxic and non-carcinogenic.

According to Jenkins, GuardTop has been part of a city-wide test program in which CoolSeal is applied to a new street in a different council district every Saturday since May. Aside from Canoga Park, CoolSeal has also been used in areas in Culver City and West L.A. This Saturday, the seal will be applied to a section of 77th Street between Cowan and Beland avenues in the Westchester area. These CoolSeal applications will happen weekly up until mid-August, after which city planners will assess the usefulness of the sealed streets. These gray streets, it should be noted, cost between $25,000 and $40,000 per mile, and may require some maintenance work after seven years. "It depends on where the street is. There's more wear and tear in highly-trafficked areas, of course. Around schools, parks and community centers, we don't expect as much wear," said Jenkins.

As promising as the solutions may sound, we are still a ways off from producing real results. As the Times reports, the mayor's team is still in the research phase, identifying which cities in L.A. should be targeted for cooling measures, and determining which strategies would suit a certain neighborhood. The office expects to have a more defined set of ideas by 2019.