Case Study: Heat Islands

Urban Heat Islands

On hot days, you may have noticed that it feels a little cooler in grassy areas and near water than it does on concrete sidewalks or near brick buildings. This phenomenon is called the urban heat island effect, and its results can be measured in local climates.

Scientists have found that natural surfaces such as water, soil, and vegetation actually moderate local temperatures because of their higher moisture content. This is because water has a very high specific heat, which means it can absorb a great deal of heat while showing only a small increase in temperature. Additionally, even dry natural surfaces such as soil or plant matter are cooler than urban surfaces such as pavement and concrete. This is because the natural surfaces absorb heat more effectively. In general, urban surfaces have low specific heat capacities, so their temperatures increase readily as they are warmed by the sun. The net effect is that urban areas tend to be warmer than areas with natural surfaces.

Since populations are concentrated in urban areas, heat islands are a growing concern. Elevated urban temperatures can lead to increased energy demands, air pollution, heat-related illnesses, and even mortality. For a better understanding of heat islands' negative consequences, watch the short video clip titled Urban Heat Island Effect - Heat and Health

Remote Sensing and Heat Islands

Heat islands can be studied using remote sensing techniques. In this aerial view of Sacramento, the left image shows a photographic view and the right image shows the thermal, or infrared (IR) image. In the photograph, the light colored areas are rooftops and roads, while the dark areas are primarily vegetated areas or water. In the IR image, the cooler areas are shown in shades of blue or green, while the hotter areas are orange or red. There is a temperature difference of approximately 50 degrees Fahrenheit (28 degrees C) between the coolest and warmest areas of the image.

While remote sensing allows us to gather temperature data over a large area, the best way to study temperature variations over a small area is by direct measurement. Such measurements will account for heat reflected from vertical surfaces, like building walls, as well as horizontal surfaces, such as parking lots and rooftops. Thus, ground-level temperature measurements provide a more accurate picture of temperature differences.

In this chapter, you will measure and record local temperatures over a variety of different surfaces, then display and analyze your data in a GIS, or geographic information system.