What Is a 'Heat Island'? You Might Be Living Inside One in the Bay Area

T

he Earth baked under its hottest July in recorded history, according to the latest data from the National Oceanic and Atmospheric Administration (NOAA). That same agency also confirmed that July was the warmest month on record for California.

But heat does not impact everybody the same way. In a state as unequal as California — where families at the top 10th percentile make ten times more than families at the bottom 10th percentile — there’s also a striking inequality at play when it comes to which Californians are bearing the brunt of rising temperatures.

I’ve reported on how workers experience extreme heat — both indoors and outdoors — but in conversation after conversation with folks, the obvious kept coming up: where we work is only one place where we experience heat. Excessive heat follows us where we live, eat, play and rest. Place helps define our relationship with heat. But we all don’t have access to the same places.

In a region as expensive — and historically segregated — as the Bay Area, how much economic, political and social power each of us has gives us access to different spaces. The way our cities are built reflects that and as human-made climate change intensifies, certain cities and neighborhoods — along with their residents — will experience heat more intensely than others.

To better understand what’s in store for the future, I spoke to researchers, community organizers and residents to learn how heat is unevenly impacting the Bay Area now. What Bay Area cities and neighborhoods are feeling more of the heat now? And what actions are residents and officials taking to make these communities more heat resilient as we approach a much hotter future?

Searching for heat islands

Researchers use the term “urban heat island” to describe when urban areas experience warmer temperatures than what’s around them. The heat island “effect” is defined as “the difference between the temperature of the air in the city and some reference that you have outside,” said Ronnen Levinson, a Lawrence Berkeley National Lab scientist who has studied the impact of heat in cities for decades.

Cities absorb heat from the sun. Miles and miles of concrete soak up heat during the day and release it at night. During heat waves, places like Los Angeles or Houston clearly show much higher temperatures than surrounding areas.

Much of that is due to exposed concrete surfaces like highways, parking lots and large-scale developments.

However, Levinson said that just comparing the air temperature inside and outside an urban area doesn’t always give us a complete picture of how heat builds up.

In the Bay Area, the region’s geography — cities freckled with hills, year-round fog rolling in from the Pacific Ocean, and the Bay itself — naturally produces different microclimates. It wouldn’t make sense to compare the air temperature in San José, a very developed urban area located in a valley, to that of a coastal forest somewhere in San Mateo County.

“In my mind, the question is really: Is the temperature inside the city higher than you want it to be?” Levinson said.

That’s why researchers have started to zoom in: instead of comparing metro areas with what’s outside, they’re now looking at the temperature differences between different neighborhoods — and even street blocks.

How heat moves within Bay Area cities

In 2018, NOAA began a multi-year campaign to map out urban heat islands across major U.S. cities, including San Francisco, San José, Oakland, along with parts of the East Bay. Researchers weren’t comparing these cities to each other. Instead, in each city, they recruited residents and asked them on a hot summer day to drive around at different points with special sensors on top of their cars.

The sensors were consistently measuring the air temperature — what’s called a “temperature transect.”

By collecting temperature transects, researchers  identified which neighborhoods experience warmer temperatures in comparison to other parts of the same city. NOAA published the results as separate Heat Watch reports for each city.

“If you were to look at a city from afar and you see that it’s got a lot of green spaces, that’s probably a good thing.” said Levinson from the Lawrence Berkeley National Lab who has reviewed the Heat Watch reports.

“If you are seeing a lot of pavement, almost all pavement is quite hot. The same goes for roofs,” he added, “especially on flat buildings if you see dark roofs. With a few exceptions, those are all quite hot.”

In each Bay Area city included in the study, we see how heat responds to both geography and human activity — both past and present.

In the East Bay, volunteers drove different routes that crossed Oakland, Berkeley, Emeryville, Piedmont and Albany. Neighborhoods between International Boulevard and the 580 freeway, such as Saint Elizabeth, Jefferson and Maxwell Park were consistently the warmest areas in Oakland, in comparison to the rest of the city. Meanwhile residential areas at higher elevations, like Montclair, stayed much cooler.

Below is a map that uses NOAA data to visualize temperature ranges collected by volunteers in the afternoon of September 17, 2019. Neighborhoods that are shown to be warmer are also home to thousands of Black and Latino families and also have less available tree canopy than the rest of Oakland, according to city data. West Oakland and neighborhoods near the Coliseum still have a lot of industry present, which means large tracts of exposed concrete.

Source: NOAA Climate Program Office and CAPA Strategies/University of Richmond.
Map Produced by Matthew Green

How historic racism parallels heat outcomes today

Oakland’s current demographic distribution has roots in an urban segregation policy that began in the 1930s known as redlining, where banks would deny mortgages to families of color — specifically Black families — in certain neighborhoods of U.S. cities. Families of color were then only able to buy homes in different areas, usually near freeways, industrial zones or airports.

Thanks to University of Richmond researchers who created the spatial data project, Mapping Inequality: Redlining in New Deal America, we have access to maps created by the Home Owners’ Loan Corporation (HOLC) in the 1930s that divide many American cities into areas by colors — with blue representing places that were “best” for investment and red meaning “hazardous,” usually defined as neighborhoods where it was in fact possible for families of color to secure a housing loan.

In all the maps of Bay Area cities included in this story, we’ve added the option to see which neighborhoods were redlined and how that compares to heat distribution in these cities — just choose between the different layers listed on the upper left side of the maps.

Over in the South Bay, volunteers crisscrossed San José on August 26, 2020 and tracked higher temperatures in the northern and eastern half of the city during the morning commute, while in the afternoon, they found warmer temperatures in southern and eastern neighborhoods, like Alum Rock, Evergreen, Edenvale and Santa Teresa. The report points out that neighborhoods with more roads, highways and parking lots “appear to create hotspots that affect nearby residential developments.”

Source: NOAA Climate Program Office and CAPA Strategies/University of Richmond.
Map Produced by Matthew Green

When we compare this Heat Watch Report to 2020 Census data available for San José, we find that the warmest neighborhoods are also the ones with some of the highest levels of Latino and Asian residents. According to city data, some of the neighborhoods with the highest levels of poverty are also those with the lowest levels of canopy cover — most of them located east of Highway 101.

A 1937 redlining map for the city is also available but it only shows central San José, and unlike Oakland, the parallels between past urban planning decisions and current heat outcomes aren’t as clearcut.

It’s also important to point out the role geography plays in the way heat moves through the South Bay. San José sits in a valley — the Santa Clara Valley, or also known by the industry that calls this place home, Silicon Valley — where the Santa Cruz Mountains sit on the west and the Diablo Range on the east. With the Pacific Ocean flanking its side, the Santa Cruz Mountains take in a lot more humidity, which helps keep communities on the western side of the valley cooler than those on the east.

Source: NOAA Climate Program Office and CAPA Strategies/University of Richmond.
Map Produced by Matthew Green

Two years later, on September 2, 2022, neighborhoods in San Francisco found warmer afternoon temperatures in the eastern half of the city, including the Tenderloin, Chinatown, Mission, Bayview, South of Market, Mission Bay and Treasure Island.

Like in the East Bay and San José, San Francisco’s warmer neighborhoods are home to very diverse communities, including the bulk of the city’s Black and Latino populations. San Francisco’s Heat Watch Report points out that the built environment, like having large amounts of impervious surfaces exposed to the sun, is a factor in these communities being warmer — after all, San Francisco still has industrial areas, and they’re overwhelmingly concentrated in the Bayview District, where one in five of the city’s Black residents live.

We can also compare the Heat Watch data to San Francisco’s own 1937 redlining map. Almost all areas marked favorable for white residents are located in the city’s westside and northern neighborhoods immediately near the Presidio.

And similarly to San José, geography is also a factor to the way heat moves around the city. While western neighborhoods like the Sunset or Richmond District don’t have large industrial areas, they do have four very large parks — Golden Gate, the Presidio, Lincoln and Lake Merced — nearby and are the most exposed to the fog that rolls in from the Pacific Ocean, which helps cool these parts significantly.

What heat island maps don’t tell us

At a quick glance, San Francisco’s Heat Watch Report seems to tell a straightforward story: eastern neighborhoods are hotter than western ones due to differences in their built environment. But like all things in the Bay Area, things are more complicated than what they seem.

“If you take the Heat Watch Report and put it on top of health impact data, it’s not necessarily one to one,” said Matt Wolff, who manages the climate and health program with the San Francisco Department of Public Health. He, along with city staff from different agencies, developed San Francisco’s very first Heat and Air Quality Resilience (HAQR) Project (PDF), which defines potential strategies the city can take to adapt to more intense heat and wildfire smoke brought upon by climate change.

As part of that project, Wolff and his team looked at the temperature differences between different neighborhoods, but also at other factors, like data on heat-related 911 calls and emergency room visits. The team soon realized that just mapping out heat islands wasn’t the best way to identify who is the most vulnerable to extreme heat.

“There are vulnerable people in every neighborhood,” said Alex Morrison, who’s a geographic information system analyst with the city’s Office of Resilience and Capital Planning and along with Wolff, helped lead the project.

Sometimes, Morrison said, looking at a map through too many different layers of information can oversimplify where vulnerable populations are. “If you have a map that shows vulnerability is concentrated in this [one] neighborhood, what you’re doing is you’re not telling the story of the older adult who maybe lives alone in the Sunset District and is vulnerable because they live in a house without air conditioning.”

“The one day it gets to 100 degrees, that person’s vulnerable too.”

When it’s really hot around us, it becomes harder for our body to regulate its internal temperature. Our body has certain tools, like sweating, to help deal with excess heat but there is a point when it becomes overwhelmed — that’s called heat illness.

Painful muscle cramps and fainting are both symptoms of heat illness, which could potentially lead to heatstroke, where the body’s internal temperature continues to rise causing serious damage to the brain and other vital organs. Anyone can develop heat illness, but there’s factors, like age and pre-existing medical conditions, that put certain folks at greater risk.

One more element the team looked at, Wolff said, is a person’s capacity to adapt to extreme heat. “Do you have access to the economic, political and social power to prepare for and respond to heat?” he explained.

How do we close the heat gap?

There isn’t just one solution to heat inequality.

In their report, San Francisco’s team makes several recommendations for city leadership (PDF), which include setting up incentives for property owners to adapt their buildings for stronger heat waves. Of San Franciscans, 65% rent their homes (PDFs) and the report acknowledges that in many situations, tenants don’t have the capacity to install things like heat pumps or high albedo roofs that help cool down a building’s temperature by reflecting back sunlight. By covering the costs of these improvements, officials may be able to encourage hesitant landlords.

“There’s a saying in the field that ‘every heat death is a preventable death,’” Morrison said. If someone is experiencing extreme heat,  city officials have the responsibility to make sure that they are taken care of as soon as possible, he added.

“The ability of our public health sector in emergency response to scale with these [extreme heat] events is essential to preventing a lot of those potential deaths,” he said.

One more tool cities have to help cool down warmer neighborhoods: green infrastructure. If things like large parking lots, freeways and industrial areas — what comes to mind when we talk about infrastructure — absorb heat really well, cities can move away from this type of development and push for structures that reflect off heat instead.

In the heat watchmaps for San Francisco, the East Bay and San José, neighborhoods with more green areas tended to stay cooler throughout the day than other parts of the city. With this in mind, Bay Area cities are heavily investing in developing stronger urban forests and giving priority to neighborhoods that have less trees and parks.

“Trees are green infrastructure,” said David Moore, senior tree supervisor with Oakland’s Department of Public Works. Moore and his team have been working for a few years on developing the city’s Draft Urban Forest Plan, which acknowledges that factors like historical redlining policies and income inequality across the city are why street trees are not evenly distributed throughout the city.

“Trees are also infrastructure that appreciates value over time,” he said. “Instead of building something that soon after starts rusting and falling apart, you plant this little thing and it gets bigger and bigger and provides more value over time.”

The draft plan states that having more tree canopy cover and green spaces in areas where people live and work can help close the gap in which neighborhoods experience more heat, along with providing relief to those who are most at risk during heat waves.

“The cliché would be, ‘Well, the government should just plant more trees in these areas,’” Moore said. But something important to keep in mind, Moore said, is that when the city plants a tree in front of a family’s house, that family is now responsible for maintaining it. And as the tree grows, that responsibility also grows with it.

In 2008, Oakland eliminated its street tree pruning and watering program due to budget cuts. There are now trees that the city planted in front of homes that the city hasn’t given care to in over 15 years, Moore added.

“There’s a feeling of frustration that the city hasn’t taken responsibility for these amazing trees — which could be great assets — but end up getting a bad rap when we look at them,” he said. “So we can’t assume that people necessarily want a street tree in front of their house, and if they do want it, it might not be from us.”

City officials have partnered with local nonprofits Common Vision and the Oakland Parks and Recreation Foundation in order to identify community members interested in having trees planted by their homes and deciding, as a collective, what type of tree to plant and how to share the responsibility of caring for it.

And there’s money to back up these efforts: Last year, the U.S. Forest Service awarded the city $8 million as part of the agency’s Urban & Community Forestry Inflation Reduction Act Grants. The funds will be used to increase tree canopy on public and private land, boost maintenance for existing trees and create jobs to support these efforts — all of this in priority neighborhoods in the city’s downtown, east side and Fruitvale.

Several other cities in the region, including San Francisco, San José, Berkeley, Vallejo, Petaluma, Hayward and Concord, have also been awarded grants from the Forest Service. In San José, $5.5 million will be designated for existing tree management and plant thousands of new trees in disadvantaged census tracts, most of them located in the city’s east side.

“This is an engineered environment and we have the ability to engineer it so that it is lush and has a canopy cover across the whole city,” said Colin Heyne, public information officer for the San José Department of Transportation, which created the city’s Community Forest Management Plan.

The power community has to resist heat

Every public official KQED spoke to for this story agreed on one thing: heat mitigation strategies are most effective when community members take the lead and cities follow.

In San Francisco’s Mission District, In Chan Kaajal Park is one clear example of how residents in a neighborhood with greater exposure to heat organized to transform their built environment.

In 2008, the corner of 17th and South Van Ness streets was a parking lot. Families in the area, with the support of environmental justice group PODER, had pushed the city for a new park in that part of the neighborhood for years — and community members wanted to transform this space with affordable housing alongside it. In 2012, the city finally approved a plan for the park, while at the same time, residents worked with PODER and the Mission Economic Development Agency on a fully affordable housing development next door.

Almost a decade later, In Chan Kaajal — which translates to “My Little Town” in Maya Yucateco, spoken by many of the Indigenous families in the area — opened in 2017.

“In our organizing work, so much of it is about stopping gentrification, displacement, speculation,” said Jacqui Gutiérrez, organizer and educator with PODER. She herself was not involved directly in the push for In Chan Kaajal, but those efforts have served as a blueprint for other organizing efforts she’s helped lead to expand green spaces in the rest of the Mission and Excelsior District. “We also need to envision and build what we need to thrive. Green space is non-negotiable in a climate crisis.”

“In Chan Kaajal is a result of that. It’s a tangible kind of win in environmental justice because people use the park every single day,” she added.

Organizing for more green spaces and protections against extreme heat can happen anywhere, she added.

“First map out where the existing gathering places in your community already are, whether they’re inside or outdoors, whether they’re green or not,” she said. “Often the list of resources available for our communities don’t exist. When we know what we have, and don’t have, that’s our starting point.”

Understanding your own heat risks — and the protections you have

Before the next heat wave rolls through California, take some time to assess where extreme heat impacts you and your loved ones the most.

Find out: Is your family’s health particularly vulnerable to heat?

San Francisco’s heat team found that it’s indoors where many residents experience heat — given the fact that San Francisco has the lowest rate of air conditioning in the country. Certain types of homes in San Francisco and the rest of the Bay Area are very effective at trapping heat during the day, but aren’t as well designed to let that heat out at night.

There’s several chronic health conditions that can intensify during a heat wave, including diabetes, certain types of heart disease, asthma and other lung conditions, and even some mental health disorders.  State officials have created an interactive guide that helps you create your own extreme heat plan, which also points out which risk factors you should take into account.

Remember: your workplace should be protecting you from high temperatures

Another place where many Californians experience extreme heat during the day: their workplace.

If you work indoors and your job doesn’t have a working A/C system, state officials announced earlier this year new heat protections for indoor workers. Once it’s 82 degrees or hotter in your workplace, your employer needs to provide you and all your coworkers with water, cool-down breaks and training to prevent heat illness. This includes restaurants, office buildings, warehouses and almost every other indoor workplace.

If you work outdoors — whether that’s in agriculture, construction, deliveries or another job where you’re exposed to the sun — your employer is also required to provide you with similar protections during heat waves.

Any person in California is entitled to these rights, regardless of immigration or contract status.