On a summer afternoon, a thermometer in a paved, tree-less section of a city can read ten degrees Fahrenheit higher than one in a leafy neighborhood only a few miles away, even though both areas experience the exact same weather. This gap has a name: the urban heat island effect, and it is not caused by the sun treating different neighborhoods unequally.

The cause is what covers the ground. Asphalt, concrete, and dark rooftops absorb far more of the sun's energy during the day than grass, soil, or tree canopy does, and they release that stored heat slowly overnight. A neighborhood with wide roads, few trees, and closely packed buildings simply holds onto more heat, hour after hour, than one with parks and shaded streets. Multiply that effect across an entire city, and downtown areas can stay measurably hotter than surrounding suburbs even after midnight.

The consequences reach far beyond discomfort. Extreme heat is already one of the deadliest weather hazards in the country, and hotter neighborhoods see higher rates of heat-related illness, particularly among older residents and people without reliable air conditioning. Researchers have also found that the hottest neighborhoods in many American cities are frequently the same ones that received the least investment in parks and street trees for decades, so the heat gap often overlaps closely with existing gaps in income and health.

Cities have started responding directly to the cause rather than only treating the symptoms. Some have painted rooftops white or a reflective color to bounce back more sunlight. Others have expanded tree-planting programs specifically in the hottest neighborhoods, since a single mature tree can cool the air around it by several degrees through shade and the moisture it releases. The goal is not simply a cooler city, but a more evenly cooled one, where a person's neighborhood no longer determines how dangerous a heat wave will be for them.