ACT Aspire Practice Test 2

On a summer afternoon, the center of a large city can be several degrees warmer than the surrounding countryside. Scientists call this effect an urban heat island. Dark surfaces such as asphalt roads and tar roofs absorb sunlight and release the stored heat slowly, keeping neighborhoods warm well into the night. Cities also have fewer trees and less open soil, so less water evaporates to cool the air. The consequences go beyond discomfort. Higher temperatures increase demand for air conditioning, which strains power grids, and prolonged heat poses health risks to older adults and people who work outdoors. Some cities have responded by painting roofs white, which reflects more sunlight, and by planting trees along streets. In one neighborhood study, blocks with dense tree cover measured noticeably cooler on summer afternoons than nearby blocks with little shade.

According to the passage, one reason city neighborhoods stay warm into the night is that