TOEFL Listen to an Academic Talk Practice 001Urban Heat

Today I want to explain why cities can remain warmer than nearby rural areas, a pattern called the urban heat-island effect. The basic cause is not that cities receive more sunlight. It is that urban surfaces handle the same solar energy differently. Consider a grassy field after sunset. During the day, some incoming energy supports evaporation from soil and plants. That process carries heat away. Concrete and asphalt contain little water, so much more of the incoming energy is stored directly in streets, walls, and roofs. These materials release the stored heat slowly after the sun goes down. As a result, the temperature difference between a city and its surroundings is often greatest at night. Building geometry also matters. Tall buildings form what researchers call urban canyons. Heat radiated by one wall may strike another wall instead of escaping toward the open sky. At the same time, narrow streets can reduce airflow. Waste heat from vehicles, air conditioners, and industrial equipment adds another source, although it is usually less important than surface materials and city shape. Researchers do not measure the effect with a single downtown thermometer. They compare many locations at similar times and account for elevation, cloud cover, and wind. Satellite instruments can map surface temperature, while ground sensors measure the air people actually experience. Those two measurements are related, but they are not identical. Cities can reduce heat without rebuilding every neighborhood. Light-colored roofs reflect more sunlight. Trees shade pavement and cool the air through evaporation. Permeable ground surfaces retain water, and carefully placed green spaces can create cooler corridors. No single strategy works everywhere, however. A dry city may have limited water for trees, while a cloudy city may gain less from reflective roofs. Effective planning therefore begins with local measurement and combines several approaches.