Maps of urban heat islands show where residents can find pockets of cooler air in Boston, Philadelphia, Richmond, Baltimore, and Washington, D.C.

A dangerous heat wave is sweeping across two-thirds of the U.S. this weekend, bringing some kind of heat watch or excessive heat warning to nearly 200 million people. Temperatures on Friday pushed into the 90s in several cities, from St. Louis and Chicago in the Midwest to Washington, D.C. and New York City along the East Coast.

With stifling humidity, heat index values in many areas soared past 100 degrees Fahrenheit as of 1 p.m. Friday, and several cities are expected to hit record highs, according to the National Weather Service.

While the heat is uncomfortable everywhere, it will be particularly dangerous for those who live in city centers, where the urban heat island effect makes it feel warmer than surrounding suburban and rural areas.

The annual mean air temperature of a city with at least 1 million people can be 2 to 5 degrees warmer than its surroundings, according to the EPA. In the evening, that temperature difference can be as high as 22 degrees. Experts chalk it up to the asphalt, steel, and concrete that trap the heat better than natural vegetation, as well as the disruption of airflow by the grid-like layout of cities.

Even within cities, there can be surprising differences in temperature from one area to the next. In 2017 the National Oceanic and Atmospheric Administration enlisted volunteers to map air temperatures throughout Richmond, Virginia, to find where the urban heat island effect is most extreme. It was part of the agency’s Urban Heat Mapping Project, and in the summer of 2018, citizen scientists did the same for Baltimore and D.C. This weekend, theyre measuring temperatures across Boston. Here’s how the heat island has been mapped across those cities:

Baltimore, Maryland

The hot spots of Baltimore, as mapped in the summer of 2018. (National Oceanic and Atmospheric Administration)

This map, made in 2018, reveals that Baltimore’s hottest spots lie along the car-intensive stretch of Route 40 that runs across the breadth of the city’s lower midtown area. While temperatures spiked to the low 100s in some of the city’s densest developments, green and shady areas like Leakin Park, the Cylburn Arboretum, and Druid Hill Park checked in as much as 16 degrees cooler.

That means visitors to Artscape, one of the nation’s largest outdoor arts festivals, won’t find much relief this weekend: The event is held in midtown Baltimore. The state of Maryland is under an excessive heat warning, with NWS warning that heat index values in the area could hit between 110 and 115 degrees.

Washington, D.C.

The hot spots of D.C., as mapped in the summer of 2018. (National Oceanic and Atmospheric Administration)

An hour away, in the nation’s capital, the warmest spots clocked in at 17 degrees higher than the coolest spots in August 2018. Unlike Baltimore, however, the hottest area wasn’t downtown, but in the neighborhoods farther north, which are dense residential areas full of row houses, apartments, and commercial centers.

The hottest spots weren’t far from some of the coolest, either: The Armed Forces Retirement Home, which sits near a large golf course, appears to be an island of relative cool surrounded by a sea of hot. And just west of all that is Rock Creek Park, a relative oasis with low elevations and thick trees—it posted temperatures similar to leafy, low-lying areas right along the Potomac River.

Some suburbs—including Silver Spring, Bethesda, Pentagon City, and Alexandria—are relatively hot spots as well, described by NOAA as “warm balloons tethered to the city by warm strings of roadway.”

Richmond, Virginia

This map of Richmond, Virginia, shows where afternoon temperatures were particularly high on July 13, 2017. (National Oceanic and Atmospheric Administration)

Whereas the heat island effect generally leads cities to be warmer than the areas outside it, the 2017 map of Richmond’s hotspots show how heat intensity can vary from neighborhood to neighborhood, depending on land use, how dark or light surfaces are, and how much shade there is.

The cooler area in the center of the map is anchored by a river, but the neighborhoods around it are notably greener on an overhead satellite image. They registered 87 degrees at the time they were measured, compared to 103 degrees in the extended band of dense development that represents the hottest areas in the middle of the city.

Boston, Massachusetts

The urban hotspots of Boston, based on 2015 data. (Trust for Public Land)

NOAA’s Urban Heat Mapping Project is using this sweltering weekend to make its own heat island map for Boston and Cambridge. Volunteers will fan out across the area to echo what’s already been done for D.C., Baltimore, and Richmond, and if temperatures reach 100 degrees on Saturday, they’ll be documenting the heat island on a record-setting day. (The current record for July 20 was set in 1991, according to Boston Magazine, and the city has only recorded triple-digit temperatures 25 times since records began in 1872.)

Meanwhile, researchers at the Trust for Public Land mapped out the range of land surface temperatures across the metro area using 2015 data to show where the hot spots are.

According to Boston Magazine, the area is expected to experience “island weather” thanks to the humid tropical that will get trapped in the city, making it a “hot, damp, and probably smelly convection oven instead of a tropical oasis.”

Philadelphia, Pennsylvania

The urban hotspots of Philadelphia, based on 2017 data. (Trust for Public Land)

Philadelphia, like many of its neighbors, is under an excessive heat warning, with temperatures hovering around 100 degrees and heat index values reaching as high as 115 degrees this weekend. Hot spots include South Philadelphia, where brick homes located in a “concrete checkerboard” stretch for miles. Warehouse districts also show warmer temperatures on the Trust for Public Land map of hotspots in 2017, as do areas with large parking lots surrounding attractions like the city’s stadiums.

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