A study released by Climate Central confirmed something you likely already intuitively know: cities are hot. In warm weather, cities can actually feel a lot hotter than nearby suburban or rural areas.

What the study by Climate Central, an organization of scientists and journalists who research and relay the effects of climate change, also shows is that some urban areas are much hotter than others. As temperatures rise and summer highs bring more record-breaking heat waves, it turns out not all cities are prepared to handle the extreme heat. And the results can be deadly.

These heat waves are not impacting cities equally. Within the same urban area, researchers have seen temperature differences as high as a whopping 15 to 20 degrees just a few blocks apart. The discrepancy is due to factors like the ratio of asphalt to tree cover in a given neighborhood. These areas are known as urban heat islands, and certain cities are bearing the brunt of their oppressive, potentially fatal temperatures.

According to Climate Central, these 20 cities experience the worst urban heat in the United States:

  1. New Orleans, LA
  2. Newark, NJ
  3. New York City, NY
  4. Houston, TX
  5. San Francisco, CA
  6. Boston, MA
  7. Chicago, IL
  8. Miami, FL
  9. Baltimore, MD
  10. Providence, RI
  11. Sacramento, CA
  12. Salinas, CA
  13. Burlington, VT
  14. Bend, OR
  15. Cleveland, OH
  16. Detroit, MI
  17. Erie, PA
  18. Fresno, CA
  19. Lafayette, LA
  20. McAllen, TX

Anyone who’s visited San Francisco in July may be surprised to see the famously foggy city so high up on the list, among others. Even the director of climate science at Climate Central, Andrew Pershing, acknowledges that the results of the 158-city study were somewhat shocking.

“Anything with ‘urban’ in its name you’re going to be surprised if New York City and Chicago and places like that don’t rate pretty highly,” he told CNN. “But I was certainly surprised to not see places like Phoenix, Las Vegas, Reno on the list.”

According to Pershing, because these cities are used to extreme heat, they have measures in place to mitigate high temperatures, such as alternatives to asphalt. Other cities, and particularly low-income areas within those cities, are less prepared. Communities of color are among the most impacted by urban heat due to lack of tree cover and parks, according to CNN.

“It’s been a real issue in the development of US cities over time — that green spaces tend to be built in the suburbs, or in the places where the rich people live,” Pershing said. “I think it’s important that those be available to everyone.”

Combatting dangerous heat waves and urban heat islands will be a challenge for city planners across the country in the coming years. But as temperatures continue to rise as a consequence of climate change, it’s critical that they be addressed quickly and equitably.