Why does the Valley get so hot?

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On a July afternoon, a garden cart loaded with dozens of sensors cruised around some of the Valley’s hottest streets. The cart, known as Marty, scooted through neighborhoods in Pacoima and Sun Valley, taking measurements on some of the residential cul-de-sacs that have recently been painted as part of LA’s cool pavement pilot.

The city’s urban cooling efforts have gone viral, thanks to imagery of street crews spreading oozing glops of white paint onto LA’s wide roads. One video that netted 20 million views worldwide was cited in a recent Urban Land Institute report on urban heating. It gained the program international recognition—and lots of curious visitors.

Researchers from Arizona State University brought Marty—which stands for mean radiant temperature—to the Valley to help the city measure what’s called thermal comfort: how hot the treated roadways, combined with air temperature, wind speeds, and humidity, feel to the humans who are using them.

Prioritizing human comfort is changing the way the city thinks about streets, says Greg Spotts, assistant director of the city’s Bureau of Street Services. “Urban cooling can help give a framework for us to think with more sophistication about the right-of-way, which has been mostly about moving traffic.”


On a 99-degree day in Canoga Park, the sidewalk is 106 degrees Fahrenheit.

Because roads get so hot and take up so much space—they cover 15 percent of the city’s land area—they provide the best opportunity to cool the city down. And there’s no better place to test new ideas for LA’s scorching streets than the San Fernando Valley

LA as a whole is facing a future of warmer temperatures, but within 20 to 30 years, parts of the Valley will experience more than 120 days that are 95 degrees or hotter. That’s one-third of the year when it will be potentially dangerous for humans to be outdoors.

“Being the Valley is like being in a hot bowl,” says Karina Jiménez, a student at Cal State Northridge who grew up in Canoga Park.

As an intern at Climate Resolve, Jiménez is part of an outreach team that’s talking to Orange Line passengers at her local Sherman Way station, trying to figure out which interventions—from cool pavement to patio misters to shade structures—would most cool down their car-free commutes.

What people complain about most is “definitely the pavement,” she says. “You can feel it radiating beneath your feet, like it could melt your shoes.”

The Valley is consistently one of the warmest places in the Los Angeles region, mostly thanks to that pavement. Not only do the 3,000-foot peaks of the Santa Monica Mountains block cooling coastal breezes, the presence of so many other hills and mountains creates an additional heating phenomenon called downslope warming. The hottest temperature ever recorded in LA County was observed here—a record-shattering 119 degrees in Woodland Hills on July 22, 2006.

Now add in urban heat island effect—where heat is absorbed by hardscape surfaces that make it hotter than surrounding rural areas—which is particularly bad in the Valley.

“It’s not as much the buildings, it really is the pavement itself,” says climate scientist Daniel Swain. “The whole San Fernando Valley has completely urbanized; only the hills really have open spaces. There’s not a lot of breaks in the pavement, so there’s a significant urban heat island effect.”


The Valley’s black asphalt streets can register as 142 degrees—or hotter.

With the cool pavement program, LA’s Bureau of Street Services is trying to boost LA’s albedo, a measure of solar reflectiveness. For effective urban cooling, hardscape materials like roofs and walls and roads can be painted light colors to create higher-albedo surfaces.

There’s no question that the light gray coating applied by the Bureau of Street Services cools the surfaces of streets—they’re up to 10 degrees cooler than the black asphalt streets, as confirmed by a temperature gun used by Curbed. But questions remain about how effective these efforts can be, especially if they’re only applied a few blocks at a time.

“If you’re going around doing cool pavement, that’s not primarily what will improve your thermal comfort,” says Kelly Turner, assistant professor of urban planning at UCLA’s Luskin Center, who studies urban heat mitigation. “Shade will determine whether you’re comfortable or not.”

According to the Los Angeles Urban Cooling Collaborative, a coalition of groups working on cooling solutions, a combination of increased tree canopy, cool pavements, and reflective roofs can work together to improve thermal comfort—enough to reduce heat-related deaths by more than 25 percent.

Planting more trees is by far the most effective and efficient way to cool a city—in addition to providing many more public health and ecological benefits. Cool roofs—which LA mandates on all new structures—have proven benefits without adding too much to construction costs.

But cool pavements, which are more expensive and energy-intensive to install and maintain, don’t do much for people using sidewalks. And the city can’t yet put cool pavements on big streets served by transit like Sepulveda, which is, at times, up to 10 lanes wide in the Valley.

“The cool pavements aren’t going to save everybody,” says Molly Peterson, an environmental reporter for KQED who has spent the past several years studying the effect of heat on residents of low-income Valley neighborhoods. “It’s great for side streets, but it’s not enough.”

Peterson has spent some of LA’s hottest days at unsheltered bus stops in the Valley, handing out water and talking to Metro riders. “When I talked to some people, they said, ‘I physically cannot wait 40 minutes in the sun.’”

What Peterson would like to see from the city is as much energy and attention as the cool streets have gotten being devoted to creating a comprehensive shading program.

“Other cities are focusing on getting shade into public places, putting the shade in places where people are outside,” she says. “We may have a mandate that serves the past, we don’t have a mandate that serves the future.”


An unsheltered bus bench in West Hills is 130 degrees at noon.

A sun-baked community in the northeast corner of the Valley might offer a glimpse of that future: San Fernando, a city of 24,000 people. Heading north from the city’s pretty, historic downtown, there are not only well-maintained street trees, but every third parking spot has been torn out and replaced with another tree, doubling the shading effect on the sidewalk.

Nineteen years ago, San Fernando Mayor Cindy Montañez used streetscape improvement dollars from Metro to plant the trees in the street.

“We weren’t doing it under this idea of climate change,” she says. “We were just saying, ‘Damn it’s hot, we need more trees.’ Eventually that became this idea of urban cooling.”

“More trees” has now become almost the unofficial city motto for San Fernando. Montañez, who is now CEO of TreePeople, championed an effort to canvass the community in the name of cooling, clustering groves of trees near schools and senior centers, giving away hundreds of fruit trees at events, and knocking on doors where people didn’t have street trees and offering to plant them.

The idea of swapping pavement for tree-planting might resonate with LA’s newly appointed city forest officer, Rachel Malarich, who is an alumna of TreePeople. It also might be the only real chance the city has to cool itself down, says Swain, the climate scientist.

“At some point, someone is going to have to look at what to do with the fact that LA has so much pavement,” he says. “It’s hard to envision LA not being a car city, but on the other hand, it’s hard to imagine a future where things remain the same. How can LA rethink its pavement in terms of, will there be a need for 10-lane roads in 30 years?”

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