Air escaping from heated buildings, atmospheric particles absorbing infrared radiation at night, the reduction in wind speed, and the heat from thousands of engines combine to make the city into an island of warmth surrounded by a cool, rural "sea." It is a heat island.
During the day the temperature rises rapidly. On a sunny day in summer it may rise by as much as 31°F (17°C) between dawn and the middle of the afternoon. It falls again at night, but in the early part of the night the city center may be up to 14°F (8°C) warmer than the surrounding countryside. The effect is so strong that urban heat islands are clearly visible in satellite photographs taken in infrared light.
On clear, calm nights the difference between the temperature inside and outside the city sometimes generates country breezes. Warm air rises over the city center, producing a local area of low pressure. Air from outside the city converges on the low-pressure region, entering as a cool, light wind.
A similar breeze occurs when city air becomes trapped beneath an urban dome. This happens when warm air lies above cooler air, producing a temperature inversion above the city. Warm air rises over the city, but cannot penetrate the inversion, because air in the inversion layer is less dense than the air rising from below. Trapped beneath the inversion, the city air spreads to the sides. As it moves, the warm air loses heat by radiating it upward, so the farther it travels from the center the cooler the air becomes. Its density increases as its temperature decreases and the air subsides just outside the city boundary. It then returns to the city center as a low-level Urban dome
breeze. This establishes a cell-like flow of air, with air converging at ground level in the city center and diverging beneath the inversion. As the illustration on page 155 shows, the warm air then forms a dome above the city.
Was this article helpful?