What is the urban heat island intensity, and what factors contribute most?

• A. The urban heat island effect is where urban areas are warmer than surrounding rural areas; factors include concrete and asphalt heat capacity, reduced evapotranspiration, waste heat from energy use, and wind obstruction by buildings.
• B. Urban areas are cooler than rural areas due to shade and trees.
• C. The urban heat island effect is caused only by air pollution.
• D. The effect is insignificant and cannot be measured.

Answer: (A)

Urban heat island intensity is the extra warmth that a city experiences compared with surrounding rural areas, especially at night. This happens because urban surfaces like concrete and asphalt soak up and store heat during the day and release it slowly after sunset, keeping the air above warmer. These materials also tend to have lower albedo, so they absorb more solar energy rather than reflecting it. In cities, vegetation is reduced, which weakens evapotranspiration—a natural cooling process from plant water loss—so the air stays warmer. The heat from human activity—cars, buildings, and appliances—adds to the atmospheric heat, pushing temperatures higher. Dense building layouts and tall structures disrupt wind flow, reducing ventilation and trapping heat in the street canyons, which slows cooling. Altogether, these factors produce a measurable and often significant temperature difference between urban and rural areas. The idea that cities are cooler, that the cause is only air pollution, or that the effect can’t be measured doesn’t fit, because the combined effects of materials, vegetation loss, anthropogenic heat, and limited air movement drive the urban heat island.