Reading Practice 41

[1] Most people accept the fact that the cumulative impact of human activity on Earth has led to global warming. But many people don’t realize that our activities, and the way we organize ourselves, has a more immediate and direct impact on weather and temperatures, at least in certain locations. More specifically, we see higher temperatures in cities, a phenomenon that scientists have dubbed “urban heat island,” or UHI. And with a gradually accelerating process of urbanization on Earth, UHIs are becoming more intense and more impactful.

 

[2] There are several related causes of urban heat islands. The first and most significant cause is the materials used to construct cities. These materials have different properties than the landscape of rural areas. Firstly, materials such as concrete and asphalt have higher heat capacities and greater thermal conductivity than natural materials. They also have greater radiative and reflective properties than natural materials. This means they absorb heat from the sun during the day, and cool at night much more slowly than the surrounding countryside. Heat is drawn into a cityscape, reflected between buildings, and absorbed by hard surfaces. Moreover, the cooling effect of winds has less impact in urban environments since winds are blocked by the tall buildings.

 

[3] Rural environments, on the other hand, apply the heat they receive into evapotranspiration, which is the uptake and release of water vapor from plants. Evapotranspiration is a cooling process, and so the relative lack of vegetation in cities robs them of another natural way of cooling off. Overall, rural environments have more absorbent surfaces which can trap and transform heat and water, while cities have impervious surfaces over which water runs off.

 

[4] Other causes of UHIs include waste heat and pollution. Waste heat is generated by all the machines and equipment which we use, such as air conditioners, refrigerators, and vehicles, as well as those used in industrial processes. Cooling equipment in particular exacerbates the UHI phenomenon, since the hotter it gets, the more we rely on artificial cooling. Of course, cities also produce significantly higher levels of pollution than the surrounding countryside. This pollution often sits above cities and further increases temperatures, as it may trap heat.

 

[5] The impacts of UHIs are various. One important direct impact relates to human comfort and safety. During periods of intense heat, or heat waves, heat-related illness and fatalities are significantly higher in cities than in rural areas. Problems may range from heat cramps and heat syncope to heat stroke, heat exhaustion, and respiratory distress. These problems are particularly troublesome for the elderly, the ill, and children. And because UHIs raise nighttime temperatures, people are not afforded the same relief that residents of rural areas enjoy during periods of extreme heat.

 

[6] UHIs also have a direct impact on weather and the landscape. Heat rises through convection, and the warm air pushing up from cities leads to increased precipitation not only above the cities, but also downwind. In fact, studies have shown that rural areas downwind of a large city may increase monthly rainfall by 28 to 51%. Downwind areas also experience different levels of erosion than upwind areas, due to the fact that cities block the wind. Thus our cities don’t only impact the landscapes on which they sit, but also landscapes situated around them.

 

[7] The rain that falls in cities and flows over hot pavement and other urban surfaces becomes heated. As this water is drained through sewers and released into nearby environments, ponds, lakes, and rivers may actually see increases in water temperature. Such changes can have deleterious effects on aquatic life, and many cities in the United States have reported loss of fish in surrounding areas after heavy rains.

 

[8] Given these effects, it is unsurprising that municipalities are taking measures to mitigate the temperature differences between urban and rural areas. Since dark colors attract greater sunlight than light colors, there is a push for lighter – or white – building materials. Another common mitigation strategy is increasing the amount of vegetation in urban environments, through the planting of trees and the “greening” of building roofs. Trees and other vegetation not only provide shade, but they increase cooling through evapotranspiration.