Researchers Map How Cities Trap Heat, Block by Block

Two streets in the same city, minutes apart, can differ by several degrees on a summer afternoon. Researchers running block-scale sensor networks have now mapped that difference precisely enough to explain it — and to rank the remedies.

The culprits are unsurprising individually: dark pavement, low tree cover, buildings that store the day's heat and release it into the night. What the new maps add is arithmetic. Canopy, it turns out, does the heaviest lifting, but only above a coverage threshold that many hot neighborhoods miss by half.

Reflective roofing helps most where buildings are low and dense. Shade structures beat pavement coatings in pedestrian comfort, even when surface temperatures say otherwise. And nighttime heat — the kind that turns hot spells dangerous — responds to building materials more than to anything planners do at street level.

City agencies are beginning to write the findings into procurement: tree contracts scored by projected canopy rather than saplings planted, and paving tenders that price heat the way they price durability. The maps made the problem legible. Budgets will decide whether legibility matters.