The most sweltering “hot spots” in the urban centers of Los Angeles and New York have been revealed in Urban Heat Snapshot, a new survey of six cities by global sustainable development consultancy, Arup.

Cities are getting hotter due to climate change, with the number of urban areas exposed to extreme temperatures, 95°F and above, expected to triple by 2050. As global temperatures rise, this survey focuses on how design can influence the urban heat of cities. In combination with the changing climate, the urban heat island (UHI) effect is contributing to dramatically increased urban temperatures. The effect is caused by a lack of natural land cover in cities, where pavement, buildings, and other heat absorbing materials absorb and re-emit heat, causing higher temperatures to be sustained in urban areas as compared to their surrounding areas.

Using artificial intelligence (AI) and satellite images from space, Arup’s Urban Heat Snapshot mapped the temperatures within a 58mi² sample of the urban centers of a diverse range of cities: Cairo, London, Los Angeles, Madrid, Mumbai, and New York. Drawing from rapid analysis conducted using Arup’s in-house digital analytics tool UHeat, the survey reports the urban air temperatures experienced during the hottest days of 2022.

The snapshot is designed to help cities understand how heat is impacting them both between the urban settings and their periphery as well as from one neighborhood to another. This new survey is also one of the few international comparisons of the UHI effect on air temperatures during both day and night-time, which more accurately reflects people’s experience. To continue applying UHeat and the survey findings to real-world scenarios, Arup is keeping in mind the importance of combining digital tools with local factors and knowledge to inform mitigation strategies. Prioritizing the deployment of nature-based solutions can help cities equitably reduce the impact of the UHI effect.

In the United States, the sample area of survey in New York includes Manhattan and a portion of the Bronx, while the Los Angeles sample stretches from Hollywood to Downtown, north of Interstate 10. The results show temperatures in the survey area of Los Angeles spiked a massive 9°F hotter than its surroundings. New York’s urban heat increase in the survey area was slightly less, at 8°F. Overall, Madrid had the most severe temperature increase between urban and adjacent areas of all the cities studied at 15.5°F (see full table of results in the survey).

City design is driving up urban temperatures — with nature often pushed out, streets asphalted, and tall buildings made of steel and glass. The authors are calling on city leaders, urban designers, and planners to better understand how their designs can mitigate urban heat hot spots, particularly for the most vulnerable communities. The use of digital tools like UHeat, when combined with local knowledge of social vulnerability factors for areas experiencing higher urban heat, can be used to prioritize interventions.

Extreme temperatures are proving lethal, with a recent study published by The Lancet suggesting that up to 20,000 deaths a year in North America may be linked to hot temperatures. It is increasingly understood that low-income neighborhoods are also more at risk to heat exposure, due to factors such as a lack of trees, limited access to cooling technologies, cost of energy for cooling, design features, and infrastructure.

The survey found that a lack of vegetation contributed to a higher urban heat, and a difference in vegetation cover additionally contributed to massive temperature differences within cities. New York’s hot spot in the area studied was in Washington Heights, which, with just 3% of its surface covered in vegetation, was found to be 8°F hotter than Ferry Point Park in the Bronx, which has 77% vegetation. Los Angeles’ hot spot in the area studied was Westlake, in Central LA, which, with only 5% vegetation, was found to be 9°F hotter than its coolest spot in Ernest E. Debs Regional Park, which had a massive 96% vegetation cover.

Urban heat affects communities differently, and wealth can play a role in the ability for residents to be able to employ their own cooling strategies, like air conditioning or swimming pools. “Understanding not only the exposure to hot spots but also the vulnerabilities of communities will be key to equitably addressing rising urban temperatures.

Heather Rosenberg

Associate Principal, Arup

While urban heat factors in major American cities are often studied by academics, these models bring this critical information into the hands of engineers and consultants working in the built environment. Arup’s rapid, high-resolution digital approach will allow more informed decision making in design to mitigate urban heat.

Ilana Judah

Americas East Resilience Leader, Arup