Step off a tree-lined street and onto a wide stretch of dark asphalt, and you can feel the temperature change like flipping a switch. That sensation isn’t your imagination — it’s the “urban heat island” effect, a measurable pattern in which cities run hotter than nearby suburbs and rural areas, often by several degrees.

The reasons are straightforward: buildings and paved surfaces absorb sunlight during the day and release it slowly at night, while a lack of vegetation reduces shade and evapotranspiration — the natural cooling that happens when water evaporates from leaves. Add waste heat from traffic, air conditioners, and industry, and the result is a city that struggles to cool down even after the sun sets.

What makes this more than an uncomfortable summer nuisance is how heat stacks up with other risks. Hotter nights can raise health impacts because the body doesn’t get a break. Heat can intensify smog conditions and strain power grids as air conditioning demand spikes. And the burden is rarely evenly distributed: neighborhoods with fewer trees and more concrete — often historically underinvested areas — can run much hotter than wealthier parts of town.

So what works? Cities have tried everything from reflective “cool roofs” to redesigning streetscapes. The strongest evidence points to a combination approach: add shade where people walk, reduce heat absorption on roofs and pavement, and create nearby refuges — parks, libraries, cooling centers — that are open when it matters.

Street trees are often the most visible solution, but they are not “set it and forget it.” Survival rates can be low if young trees aren’t watered during the first few summers. Species selection matters, too: a tree that thrives today might struggle in hotter future summers. Meanwhile, reflective surfaces can help — but they can also create glare or shift heat to neighbors if not thoughtfully planned.

Shade can drop perceived temperatures dramatically, especially along common walking routes.

One practical rule emerging from city pilots is to focus on “cooling the commute of daily life” — the sidewalks to schools, the bus stops, the routes to grocery stores — rather than only improving headline parks that residents may visit less often. When shade and cooler materials are targeted to where people already spend time outdoors, benefits show up quickly in comfort and, in some cases, energy demand.

In the end, urban heat isn’t a mystery. It’s the predictable outcome of design choices made over decades. The good news is that design can change. The most successful programs tend to be the unglamorous ones: steady tree care, building standards that require better roofs, and community-driven maps that identify the hottest blocks first.