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On a summer afternoon in Houston or Dallas, the temperature downtown can be several degrees higher than in the farmland a few miles outside the city. The difference is often greatest after sunset. Scientists call this pattern the urban heat island effect, and it has become a serious concern for Texas cities, which are among the fastest-growing in the country.

The main cause is what cities are built from. Asphalt roads, dark roofs, and concrete parking lots absorb a large amount of the sun's energy during the day. Like a stone left in the sun, they stay hot for hours and release that heat slowly into the night air. Fields and forests behave differently. Plants release water through their leaves, and as that water evaporates, it cools the air around them, much as sweat cools the skin. When land is paved, that natural cooling is lost.

Cities also produce heat of their own. Cars, factories, and air conditioners all give off heat as they run. This creates a frustrating cycle: hotter nights lead people to run their air conditioners longer, and the air conditioners push even more heat outside.

The effect is more than uncomfortable. When nights stay hot, the human body has less time to recover from the heat of the day, and heat-related illness becomes more likely, especially for older adults, outdoor workers, and people without air conditioning. Higher demand for electricity during heat waves can also strain the power grid.

Cities are testing several solutions. Planting trees along streets provides shade and brings back some of the cooling that plants supply. Cool roofs, which are painted white or made of reflective materials, bounce sunlight away instead of absorbing it. Some cities have also tried lighter-colored pavement on parking lots and playgrounds.

None of these steps can make a city as cool as the countryside. But researchers who measure temperatures block by block have found that shaded, tree-lined streets can be noticeably cooler than nearby streets with no trees. For neighborhoods with few trees, those few degrees can matter.