NASA's Jet Propulsion Laboratory (JPL) unveiled a series of detailed maps showcasing the extent of land-surface temperature variations in Phoenix, Arizona, last July, a month marked by an unprecedented heat wave.
The images were generated using data from the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the NOAA-NASA Suomi National Polar-orbiting Partnership (Suomi NPP) satellite.
These maps depict how urban landscapes, including streets, buildings, and airport runways, retained heat even after sunset, contributing to the persistence of high temperatures. This study conducted by researchers at NASA's JPL offers insight into the cumulative effects of continuous daytime heating.
PHOENIX, ARIZONA - JULY 26: In an aerial view, people walk through a section of the 'The Zone', Phoenix's largest homeless encampment, amid the city's worst heat wave on record on July 26, 2023 in Phoenix, Arizona
NASA: Still Hot Even After Sundown in Phoenix
The VIIRS instrument captured data during pre-dawn hours, around 2 a.m., on multiple occasions throughout July. The images notably illustrate how man-made structures, such as roads and buildings, can maintain elevated temperatures, often hovering around 100 degrees Fahrenheit (38 degrees Celsius) for an extended duration following sundown.
From July 1 to July 19, the maps showcased a gradual escalation of temperatures within built surfaces, possibly arising from the compounding effects of the ongoing heat wave and heat accumulation within human-made environments.
Urban surfaces, characterized by their high heat capacity, only had the opportunity to cool significantly after the cycle of the next day's intense heat began.
The core of the analysis centers around Phoenix's Sky Harbor International Airport, a key area for the city's official air temperature measurements.
The data collected at this site revealed that July 2023 experienced a record-breaking heatwave, with temperatures exceeding 110 degrees Fahrenheit (43.3 degrees Celsius) for a record 31 consecutive days. It surpassed the previous record of 18 days, according to NASA.
The maps also illustrate the pronounced temperature differences between urban areas and nearby irrigated surfaces such as agricultural fields, golf courses, and parks. These non-urban regions experienced significantly cooler temperatures during the night.
Notably, the presence of bodies of water, such as the Verde River and other waterways, also contributed to cooler temperatures in their vicinity.
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The Role of Dark Asphalt
The research emphasizes the role of dark asphalt and concrete in urban heat retention. These materials have a high heat capacity, enabling them to store heat absorbed during the day and release it gradually at night, causing the "urban heat islands." This effect causes the densely-built downtown areas to be warmer than rural areas.
"Dark asphalt and concrete have a high heat capacity, so most of the heat they absorb during the day goes into storage below the ground. That heat gets released slowly at night, making air temperatures much warmer during the nighttime in dense urban areas, creating the classic urban heat island effect," Glynn Hulley, the JPL climate scientist behind the series, said in a statement.
The VIIRS instrument, housed on the NOAA-NASA Suomi NPP satellite, is one of five instruments on the spacecraft. The satellite, part of the Joint Polar Satellite System, captured the data utilized to create the maps.
The images were generated from the VNP21IMG Land Surface Temperature product, accessible through NASA's Land, Atmosphere Near-real-time Capability for EOS (LANCE).
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