The Urban Heat Island Effect
The air in urban areas can be 2 - 5°C (3.6 - 9°F) warmer than nearby rural areas.This is known as the heat island effect. It’s most noticeable when there is little wind. An urban heat island can increase the magnitude and duration of a heat wave. It can also influence the weather, changing wind patterns, clouds, and precipitation. What makes cities warmer? There are many factors that can influence the urban heat island effect. The modifications to the land surface that are made in urban areas have a large impact on whether a heat island forms. For example, many cities have fewer trees than surrounding rural areas. Trees shade the ground, preventing radiation from the Sun from being absorbed. Without them, the ground surface heats up. Dark rooftops and dark pavement absorb more radiation too. Tall buildings reflect and absorb sunlight. Automobiles, which make heat from their engines and exhaust, also contribute to the heat island effect. Fewer plants in urban settings mean that less evapotranspiration occurs, a process that cools the air. Today, many cities are making an effort to combat the heat island effect. White or reflective materials are being used for roofing and roads. Trees are being planted along city streets. And, in many areas, green roofs - living plants on rooftops – are being installed.
In Part 1 of this activity, students investigate relatively small differences in temperature in their schoolyard. These differences reflect different microclimates. The term microclimate can be used to describe differences in small areas of just a few square meters or much larger areas a few kilometers apart. Factors that contribute to microclimates in a small area like a school yard include the presence or absence of shade (from trees, buildings) and the type of material at the ground surface (dirt, grass, asphalt, concrete). Shaded areas are generally cooler since much solar radiation is unable to be absorbed by the Earth surface. Ground materials like asphalt and concrete absorb solar energy readily and dark paving will typically be warmer than light color paving because dark colors absorb more heat.
Infrared thermometers (IR thermometers) are recommended for Part 1 of this activity. IR thermometers measure temperature by assessing the amount of energy emitted from an object. When sunlight hits the Earth surface, some of that energy is absorbed and some is reflected. The energy that is absorbed heats and is radiated from the surface. Students can alternatively use digital thermometers as long as they measure to tenths of degrees, but it might be more difficult for them to see the patterns emerge, especially if there is any wind.
When unusually hot summer weather lasts for several days, it’s known as a heat wave. Heat waves are a danger to human health – causing heat stroke, heat exhaustion, cramps, and other ailments. Recently, a group of scientists analyzed data about heat waves in Los Angeles, California over the past century (Tamrazian et al., 2008). The data came from records of the Department of Water and Power in downtown Los Angeles and Pierce College, a suburban school. Some of that data forms the basis for Part 2 of this activity. The researchers found that Los Angeles is now experiencing more heat waves and more extreme heat days than it was in the past. The average annual maximum temperature has warmed by 2.8°C (5.0°F). The scientists attributed the rise in heat waves to a combination of increase in the heat island effect and global warming. While Los Angeles is used as the example in this activity, it is not the only location where heat waves have increased. According to the Intergovernmental Panel on Climate Change 4th Assessment Report (2007), the number of heat waves has risen, especially in Europe and Asia, and heat waves are expected to become more common during the 21st Century. The increase in the number and duration of heat waves in urban areas is due to a combination of global warming and the urban heat island effect.
- Heat wave data from: Tamrazian, A., S. LaDouchy, J. Willis, and W.C. Patzert (2008) Heat Waves in Southern California: Are They Becoming More Frequent and Longer Lasting? APCG Yearbook, Vol. 70, pp. 59-69.
- 10-year average temperatures are from Climate of Los Angeles data archived by NOAA National Weather Service - Los Angeles/Oxnard (https://www.wrh.noaa.gov/lox/climate/climate_intro.php)
- Population estimates are from the US Census Bureau. (http://www.census.gov/popest/estimates.php)