Indoor air temperatures in apartments in the Harlem section of Manhattan were up to 7 degrees hotter this summer than outdoor temperatures, creating hidden dangers for residents, according to field data gathered by AdaptNY’s Harlem Heat Project reporting initiative.
In New York City this July and August, the average outdoor temperature in the area was 83 degrees Fahrenheit. But during that same period, average indoor temperatures at the Harlem residences reached over 90 degrees.
That’s per City College researcher scientists Prathap Ramamurthy and Brian Vant-Hull, who shared the findings at a community workshop on Oct. 15.
The data was gathered as part of the summer-long initiative in which community-based citizen scientists placed digital sensors in 30 apartments around northern Manhattan starting in July. Thousands of data points were collected, with temperatures and relative humidity measured in each residence every 15 minutes.
Health impacts of heat the big worry
The project was designed to measure the health impacts on Harlem residents of what is known as the urban heat island effect. Unlike people, who can cool off by sweating, so-called evaporative cooling, buildings and roads absorb heat during the day and store it overnight, causing cities like New York to grow extremely hot in the summer months.
Even worse, while the city itself starts to cools down after the sun sets, its buildings typically retain the heat they’ve absorbed during the day. So at night, indoor temperatures don’t drop as quickly or as dramatically as outdoor ones.
All this leaves Harlem especially vulnerable, with the highest rate of heat stress-related emergency room visits in the city.
Part of that vulnerability is geographic. When it’s hot, buildings in midtown and downtown bake in the sun, and air conditioners take heat from inside apartments and redistribute it outside. Wind patterns then push this heat north through the city into upper Manhattan neighborhoods. Harlem ends up, for instance, with the highest rate
But it’s more geographic vulnerabilities: Harlem is home to a larger percentage of low-income residents, whose homes are often low-quality, high density, poorly ventilated, and lacking open space and shade, and whose access to social services is limited. The area has the fewest AC units.
In the summer, heat is often a leading trigger for mortality and illness, especially for the elderly, the young, and the sick. Already, 80% of the New Yorkers who die of heat die inside their own homes. This is because indoor heat waves are often more intense than their outdoor counterparts.
The Harlem Heat Project is one of the first experiments to measure this distinction, by collecting data from inside real people’s homes.
South-facing top floor apartments at most risk
When installing the heat sensors, researchers recorded information like the orientation, angle, and floor level of each apartment, and the location of the sensor within.
Apartments at the highest risk for overheating were found to be southwest- or southeast-facing units on the top floor of five-story buildings. Because of the angle of the sun in Manhattan, those apartments start getting direct radiation through the windows in the morning and continue to absorb the heat all day.
“If you are on the upper floor, have no AC, and are exposed to sun all day, you experience maybe 8 degrees hotter temperatures,” Vant-Hull explained.
Of course, there are variables that can alleviate this extreme heat. If your apartment is shaded by trees or taller buildings, the orientation and level won’t matter as much. And small behavioral changes like installing and closing blinds to change sun exposure can reduce indoor temperatures by several degrees.
“I’m not surprised by the results, really. You could feel it,” said Harlem resident Anthony Carrion, whose hot Harlem apartment was among those monitored this summer. “But I’m glad the data reflected it. I’m looking forward to potential policy changes.”
Indoor high temperatures lag after heat waves
Overall, nearly two dozen homes tracked in the study registered higher indoor heat index compared to the outside. But during heat waves – three days or more with temperatures at above 90 degrees – these numbers were much higher.
In periods such as these, researchers also measured consistently higher indoor temperatures than outdoor temperatures—in some homes, almost 12 degrees hotter.
The reason? Because heat is stored in buildings, there’s a lag effect: indoor heat waves start later, but last longer. The rate of increase of temperature indoors is much higher than that of the outdoor temperature, and even after the heat wave ends, the indoor temperature is consistently up to 10 degrees warmer.
Predicting outdoor heat waves is relatively easy: If we know today’s temperature we can predict tomorrow’s. It turns out that indoor heat waves can be predicted much in the same way, according to the researchers, by combining the sensor’s data on indoor temperature with data on Harlem’s outdoor temperatures. Because people are consistent in their patterns of air conditioning use, these models work even in homes with an AC unit.
“When forecasters look at [the temperature], they think ‘Oh, the outside is cooling down right now.’,” said Ramamurthy. “But the indoor temperatures are not cooling fast enough. Even in houses where the temperature peaks are much lower during the day, the nighttime peaks are much higher.”
Ultimately, it’s dangerous to focus on outdoor heat wave projections while ignoring indoor ones, they maintained. “Residents without AC are exposed to extreme heat conditions for longer than you might think if you were only looking at outdoor heat,” said Vant-Hull.
As a result, heat in urban areas should be treated as a natural disaster, the researchers argued, and response should be bold and immediate.
That kind of response may become even more necessary in coming years, as climate change exacerbates urban heat effects, making heat waves hotter, more frequent, and more intense. By the 2080s, heat waves in New York are expected to triple in number.
The Harlem Heat Project is a collaboration between Harlem residents, and journalists and researchers from AdaptNY, New York’s flagship public radio station WNYC, community climate and weather journal ISeeChange, and the community-based organization WE ACT for Environmental Justice.