The Urban Heat Island Effect
in New York City

In New York City and across the country, more people die from extreme heat than from all other natural disasters combined. To understand how to keep New Yorkers safe from extreme heat, we studied neighborhood factors that affect temperature, and found that grass, shrubs, and trees can play a major role. Let’s take a look.

At 6.00 PM on June 30, 2018, the air temperature at the LaGuardia Airport Weather Station was 89° Fahrenheit (F). At the same time, the air temperature on a block in East Harlem (below left) was nearly 5° hotter than at the Weather Station, and the temperature on a block in Bed-Stuy (below right) was 3.5° hotter.

100th St, between Lexington and 3rd Ave
Manhattan: 93.9° F
Stuyvesant Ave, between Jefferson and Putnam
Brooklyn: 92.5° F

This is an example of the Urban Heat Island Effect: when some areas in a city are hotter than others. Temperatures can even vary from block to block.

LaGuardia Airport is an open space without many structures surrounding it. It doesn't trap heat the way other places in the city do, so we often use it as a reference temperature. It is almost always cooler than other parts of NYC, where building boilers and vehicle engines generate heat, manufactured surfaces like roofs and roads absorb heat, and there’s less vegetation to cool things down.

What cools your block?

Anyone who has looked for a shady spot under a tree on a hot sunny day knows that trees help to cool off a street.

But by how much? How effective are trees at cooling neighborhoods, and what other characteristics contribute to an area’s air temperature during hot weather?

To find out how trees and other characteristics affect "hyperlocal temperature" - block-by-block differences - we worked with the Department of Parks and Recreation and the Mayor's Office of Climate and Resiliency to install air temperature monitors around the city. This was part of a comprehensive approach to keep neighborhoods cooler during hot weather in NYC.

We reviewed land-use data and satellite imagery to build a profile of each block where we were monitoring temperature. We found that temperature could vary significantly depending on block characteristics. For example, the block we studied in Brownsville, Brooklyn, had very little vegetation (trees, grass, and shrubs), while the block in Edenwald, in the Bronx, had high grass and tree coverage. You can see the difference in vegetation coverage in these satellite images.

We found that the average temperature at the Edenwald spot was more than 2° cooler than the Brownsville spot.

Brownsville, Brooklyn
Measure:
Tree canopy Low
Grass/shrubs Low
Building height: Low
Average temp: 82°
Edenwald, the Bronx
Measure:
Tree canopy High
Grass/shrubs High
Building height: Medium
Average temp: 79.8°

These are the factors we explored that affect hyperlocal temperature:

Total tree canopy coverage is the percent of the sidewalk in the right of way of the monitored street that is shaded by trees. Trees provide shade and prevent the ground from heating up. They also can cool the air through evapotranspiration. Evapotranspiration happens when the sun’s rays hit plants, causing water to evaporate from the leaves. This change from liquid to vapor cools the plants – just as sweating cools our bodies – and uses energy, thereby reducing the amount of energy left to warm the air.

The percent of grass and shrubs in front of buildings on the monitored street. Grass and shrubs cool the air through shading and evapotranspiration. Evapotranspiration happens when the sun’s rays hit plants, causing water to evaporate from the leaves. This change from liquid to vapor cools the plants – just as sweating cools our bodies – and uses energy, thereby reducing the amount of energy left to warm the air.

Our model looks at the median building height (the middle of the range of sorted heights) of buildings on the block. Taller buildings can absorb and reflect more heat (increasing the street's temperature), but at about 3 or more floors, begin to cool the street by shading it.

Impervious surfaces are made from asphalt, concrete, astroturf, and even hard-packed dirt. These surfaces absorb sunlight, and reflect and retain heat. They increase the temperature of the surrounding area.

Other factors, like the orientation of the street (North/South or East/West), how close the area is to the ocean, the elevation of the area, daily wind speed and weather patterns, can also influence how hot a block is.

What factors matter the most?

We built a statistical model that predicts the average summer temperature on a hypothetical city block, based on the neighborhood factors we studied in the hyperlocal temperature study? We modeled the average temperature from 6 to 9PM, when the Urban Heat Island Effect is the greatest (the sun sets, but the city retains heat).

Our model looked at how much each factor affected temperature, and it showed that the most important factor for reducing temperature was green space: trees, grass, and shrubs reduce temperature. Having more surfaces that hold heat increases temperature.

See for yourself - drag the green slider back and forth to see what the temperature would be on a hypothetical block if there were more vegetation or more impervious surfaces like roadways, which absorb sunlight and retain heat.

Impervious surfaces
XX%
Change the ratio
Vegetative material
XX%
Temperature
XX deg F

A block covered in trees, grass, and shrubs might have an average summertime evening temperature of 77°, but one completely covered in impervious surfaces would be 79°. More vegetation means lower temperatures.

Cooling a block by 2° may not sound like much, but small differences in temperature - especially at peak summer temperatures - really matter. For example, the average temperature at Central Park increased 3.4° from 1900 to 2013. Cooling blocks by just a few degrees can help offset the effects of climate change at the local level.

As the climate changes, NYC faces rising average temperatures and heat waves that place New Yorkers at risk. Already more than 100 New Yorkers die each year due to extreme heat. Even modest cooling can save lives, and cooling neighborhoods with trees and other vegetation is a critical step we can take to protect people from hot weather. Even modest cooling can save lives.

More info:

Learn how other ways NYC is working to counter the risks of rising temperatures, such as by ensuring equitable access to air conditioning.

You can explore the areas where we measured temperatures, and their average summer temperature, using the map below.

         
Temperature sensor mounted on tree.




More about NYC's greening efforts

There are numerous efforts to increase tree canopy and green space overall in NYC. The City plants and cares for street trees along the right-of-way; forbs, shrubs, and trees in parkland; rain gardens and other green infrastructure to capture stormwater; and more. Many tree plantings have been concentrated in areas with higher than average rates of asthma and heat-vulnerable neighborhoods.

Trust for Public Land’s New York City Playgrounds program has transformed over 200 playgrounds from using asphalt to using cooler, greener materials. There are also forests in all five boroughs, and the Department of Parks and Recreation and the Natural Areas Conservancy have developed the Forest Management Framework for New York City, a 25-year plan that lays out the process, costs, best practices, and goals for caring for forests in the city.

You can get involved, too: