Streets and other built-up areas in the region absorbed and retained heat long after the sun went down, and became red hot during the many days of persistent high temperatures. Researchers at NASA’s Jet Propulsion Laboratory have created a series of maps showing land surface temperatures in the Phoenix area in July 2023, when the city experienced record hot weather. The images reveal the cumulative effect overnight and over the month of relentless daytime heating.
The data were taken during the nighttime hours (around 2 a.m.) on several days in July by an instrument called the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the NOAA-NASA Suomi National Polar-orbiting Partnership (Suomi NPP) satellite managed by the National Oceanic and Atmospheric Administration and NASA . The images show how built-up surfaces roads, buildings, airport runways and the like retain heat, sometimes hovering around 100 degrees Fahrenheit (38 degrees Celsius) for hours after sunset.
From July 1 to July 19, built-up surfaces on the maps gradually warmed, likely from the combined effect of the intensifying heat wave and the cumulative heating of these human structures. Because of their high heat capacity, these surfaces didn’t cool completely overnight before the heat set in the next day, said Glynn Hulley, the JPL climate scientist who created the series.
In the center of the images is Sky Harbor International Airport in Phoenix, where VIIRS measured the hottest surface temperature in the city. The airport is also where Phoenix measures its official air temperature. According to those measurements, the city experienced its hottest month on record in July, including a record 31 consecutive days in which the temperature exceeded 110 degrees Fahrenheit (43.3 degrees Celsius). The previous record was 18 days.
Land surface temperatures in cities are usually higher than in rural and undeveloped areas due to human activity and materials used for construction. Streets seen as a grid pattern on these maps are often the hottest part of the built environment due to dark asphalt pavement that absorbs more sunlight than lighter surfaces; asphalt absorbs up to 95% of solar radiation. In the images, the mountains near Phoenix are also remarkably hot due to their angle to the Sun and greater soil exposure due to the lack of vegetation.
“Dark asphalt and concrete have a high heat capacity, so most of the heat they absorb during the day goes into storage underground,” Hulley said. “That heat is released slowly at night, so air temperatures are much higher at night in dense urban areas, creating the classic urban heat island.”
Hot surfaces in and around the city contrast with nearby irrigated surfaces such as agricultural fields, golf courses and parks, which dropped as low as 68 degrees Fahrenheit (18.9 degrees Celsius) overnight. The Verde River and other nearby waterways were also significantly cooler.
VIIRS is one of five instruments aboard the NOAA-NASA Suomi NPP satellite. Short for Suomi National Polar-orbiting Partnership, the spacecraft is one of several in the Joint Polar Satellite System. The images were created from the VNP21IMG Land Surface Temperature product available from NASA Land, Atmosphere Near-real-time Capability for EOS (LANCE).