"Take a look over your data sheet and make sure you know what all the words mean," Sarah Slack tells her class of eighth graders. The students stop talking to each other and turn toward their science teacher, who is standing in front of a digital blackboard holding up a sample data sheet. "We're going to be not only collecting temperatures in the sun and in the shade, but you're going to choose some words to describe the weather."

Slack's students do a lot of hands-on science. Inside their Brooklyn, N.Y. junior high school, J.H.S. 223, she's built a lab that is a small ecosystem itself. On one end of the room hydroponic gardens spritz water over patches of arugula, kale, cucumbers, peppers, and tomatoes, the grow lights casting a bright white glow across the room. In a back corner, tiny baby trout swim in a bubbling aquarium, the filter humming quietly. And on Slack's desk common mollies, another small fish, dart around a tank with basil growing out of the top. Each nourishes the other in a symbiotic cycle.

But on this early November day, the school playground will serve as the students' lab. "I think we're ready, everybody," Slack says, opening the classroom door. "You're going to need to bring three things with you – something to write with, your notebook, and you're going to need an infrared surface thermometer."

The students surround a milk crate at the center of the room to grab handheld thermometers, which they quickly begin testing on everything around them – the desks, the chairs, and their bodies – before following their teacher out the door.

While this temperature-taking lesson will provide basic practice in data collection, Slack has a much bigger lesson in mind.

She'll upload the measurements these students collect from playground surfaces to a database made available to climate scientists through NASA. Slack aims to help her eighth graders understand climate change, and also how to use science to mitigate the tangible impacts of the warming world. Especially right here, in at their school and in their neighborhood.

Students measure surface temps in the schoolyard

Slack reminds them what surface temperatures they'll be investigating. "We're measuring pavement, concrete, dirt, and grass. Which one of these is going to be the hardest to find out there?" she asks.

It's an easy question. "Grass," several students mutter at once.

The playground they're heading toward is similar to many in New York and other urban areas. It's made of – and surrounded by – asphalt and concrete, materials that heat up quickly but cool off slowly.

To get there, the students have to go through a tunnel of metal construction scaffolding that surrounds their school. Some run and skip; others amble slowly, gossiping with friends. They all spill out onto a wide open blacktop. The only plant life here is a handful of spindly trees, leaves brown and falling, spaced out along the playground's perimeter in small dirt plots.

Students split up to take surface temperatures of different materials in the yard. Most scurry around the asphalt pavement, pointing their thermometers toward the ground. Others start collecting temperature data from the small dirt patches under the trees, trying to find both sunny and shady spots to measure. A handful of students head for a bright, colorful mural painted on a concrete wall that serves as one of the borders of the playground.

One eighth grader, Li Qing, wearing pastel pink sweats and puffy lavender jacket, points her handheld thermometer at part of the large mural. "It's technically hotter than the dirt," she notes. "I guess the cement absorbs more heat, or something," she thinks out loud.

She points her thermometer at a flower painted in the mural.

"The white one is only 70 degrees," she says, before pivoting to a part of the mural inches away, showing a black bicycle tire.

"Oh my God. 89!" she says.

In short order, most students had measured and recorded temperatures of three of the four surfaces Slack asked them to measure.

Just one left to find. Grass.

Urban schools can be neighborhood hot spots

This hard surface playground is tucked between the four-story brick school building on one side and the concrete wall of a New York townhouse – with the mural – on another. Two tall chain link fences edge the rest of the playground. Through the metal links, blacktop streets lined with brick houses stretch as far as the eye can see.

City data ranks this neighborhood, Borough Park, high on its "heat vulnerability" index, which shows the relative impact of extreme heat events, such as deaths, on different communities. Surface temperatures are one big indicator of risk, and as Slack's students discover, the materials surrounding their school – asphalt and concrete – get hot.

When scientists look at a bunch of neighborhoods like Borough Park together, they see what's called an "urban heat island" – places where urban centers are significantly warmer than their suburban and rural neighbors. Within cities, different neighborhoods, blocks, and even buildings can have different surface temperatures.

Historic inequity plays a significant role: neighborhoods that have long been low-income and majority people of color often lack parks, lush green lawns, and trees that can help cool off a neighborhood.

"Formerly redlined neighborhoods are also those that tend to have fewer trees, and also more heat so that those patterns all connect to environmental justice issues," Slack says.

Borough Park has a long immigrant history and high rates of poverty and renters compared to the average in New York City. The two parks nearest J.H.S 223 are both a five minute walk from the school. Although they provide some shade and greenery, they are also part of the urban infrastructure, with leafy trees tucked around paved play areas. One park is right next to an elevated train line.

Slack hopes that teaching her students to think like scientists can be a catalyst for change right in this neighborhood.

"If we had more trees or more grass around us in Brooklyn, then people walking to the store on a hot day would be more protected from those hot temperatures," Slack says. "I think the most important thing that would come out of doing this kind of work is for students to realize that they don't have to fix climate change, but they have the ability to make things better in their own communities."

The data from their school yard might contribute to doing both.

Students' measurements help fill gaps in global data

The official temperature for Borough Park the fall day Slack's students did this outdoor lab was an unseasonably warm 68 degrees. Around the playground, students clocked temperatures lower and much higher than that.

"53 point 6," one student reported to Slack, measuring the pavement in the shade of the school building.

"Hold up, I got 79," another student told her group of friends as they stood in a circle in the sun comparing readings. "I got 81," one of her friends responded, pointing her thermometer toward her feet on the hot asphalt.

One patch of the playground asphalt pavement isn't black. It's has a grid of six circles, each a different color of the rainbow. Two years ago, Slack and her students painted this, as a way to test what a completely different schoolyard might feel like – a playground that was yellow, or blue, or purple instead of black. "We measure the temperature there to see what would happen if we painted the whole yard a color," Slack says.

The circles are faded from two years of sun, sneakers, and bouncing balls. But they're still cooler than the surrounding blacktop.

"Purple was the hottest years ago, but it's so faded now I didn't know if it would still be the hottest one," Slack says, reviewing one student's data sheet – which showed that even faded, the purple circle still absorbs the most heat.

Later, students will punch numbers from their scribbled sheets into calculators to determine average temperatures for different surfaces. Slack combines data from all her classes and adds it to a national collection via a NASA app called GLOBE.

Climate scientists have used a mixture of satellite measurements and on-the-ground temperature recordings to document urban heat islands. While satellites are much more convenient, since they can capture the temperature of a large area very quickly, they do not entirely reflect what people feel on the street.

"We have to remember that [a satellite is] going to pick up what it hits first," says Jennifer Vanos, a climate scientist at Arizona State University who studies extreme heat and schools. "So the top of the tree, the top of the building, the top of some shade canopy, which means it's not going to get what's kind of under that which is valuable information we do miss and if we're thinking about what's at the level of where human is, we might miss some of that surface temperature information."

Things like the construction scaffolding circling much of J.H.S. 223 and the awnings of nearby bodegas would block satellites from collecting street-level temperatures around the school and neighborhood.

Satellites also average temperature data over 30 square meter chunks, which is helpful in studying larger trends between neighborhoods but leaves out a lot of detail, such as how hot one building is compared to another.

University of Toledo geographer Kevin Czajkowski is helping NASA coordinate projects like Slack's with schools across the country.

Czajkowski says that efforts to study the urban heat island effect have picked up in the past several years. "GLOBE is facilitating studying the urban heat island. I think that's helped," he says.

In addition, this street-level data alongside the satellite readings could help local governments decide where to plant trees, set up cooling centers, or make future schools and playgrounds more resilient to heat – and safer for kids.

Hot playgrounds can be dangerous

Even within a dense city like New York, schools can stick out as hotspots if they lack shade or plants.

"If a school yard is dark asphalt and rubber or artificial turf, or concrete and there's not much shade or vegetation, then yes, you will end up seeing that those pop out on urban [heat] maps," says Vanos.

She also says heat trapped and amplified by schools and playgrounds can become dangerous for kids.

"If they are starting to experience the signs of heat illness, a lot of times kids may not notice that and just kind of keep playing," she says. "We want them to play and that's the goal of playgrounds for sure, but we want to make sure that the environments that they're provided to play are conducive to play even when it's hot out."

NASA coordinator Czajkowski knows from experience how detrimental hot playgrounds can be to children. His youngest son came home one day suffering from heat illness after playing outside at school. "He was really sick," Czajkowski says

Plants can make a playground much cooler

As Slack's students wrap up collecting the temperatures of asphalt, concrete, and dirt, many flock to her. "Where is the grass?" they ask. Slack points them outside the playground, through the gate of a chain link fence, across the school parking lot, and out onto the sidewalk.

The young scientists crowd around a few meager patches of green rooted under a couple of trees in the street strip. They try to aim their thermometers to catch the temperature of the sparse blades.

The grass appears cooler than on the blacktop. "67, 69.9," one student called out numbers from his thermometer to others gathered around the tree.

But because this patch of green is so tiny, surrounded by a sea of heat-absorbing city, it can't help cool the area much. "Just because those two surface temperatures are different does not mean that the air temperature above them is different necessarily," Vanos, the climate scientist says. "When you start to have large areas of asphalt, or large parks, then you can start to see an influence on air temperature."

Slack hopes this lesson will empower her students to imagine a completely different type of playground, dominated by plants, trees and natural surfaces, that could serve as a safer place to play amid rising temperatures, and help cool off the surrounding neighborhood.

"I understand all the challenges of having plants in an area where 200 middle school students go outside for recess," Slack says. "But I see the overwhelming value of providing them with a space that's going to be cooler, and that's going to help make their entire neighborhood cooler."

Over the next year, Slack hopes to train other teachers in New York schools to do the same sorts of data collection she does with her students, both to add to global warming data and to help paint a picture of temperatures at schools across the city.

But most immediately, she hopes that students in this school walk away from her class inspired to help their own community.

"My students are a population that need to hear and understand that they're going to have to talk louder and wave their arms around more to get the attention that their community needs more than many other communities in the city," Slack says.

NPR's Lauren Migaki contributed reporting.