This cross section through a typical city shows how temperatures are usually lower at the urban-rural border than in dense downtown areas.
Click on image for full size
Lisa Gardiner / Windows to the Universe, based on a figure from the Lawrence Berkeley National Laboratory
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 urban heat island effect. It’s most noticeable when there is little wind. An urban heat island can increase the temperature and length of a heat wave. And city heat can 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. Changes 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 the Sun’s radiation from being absorbed. Without them, the ground surface heats up. Dark rooftops and pavement absorb more radiation too. 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.
Some people have wondered whether growing cities have caused global warming because of their urban heat islands. There is very strong evidence that this is not the cause of global warming. According to the Intergovernmental Panel on Climate Change, global warming is very unlikely to be affected significantly by growing urban areas.
Today, many cities are making an effort to combat the urban 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.
Shop Windows to the Universe Science Store!Cool It!
is the new card game from the Union of Concerned Scientists that teaches kids about the choices we have when it comes to climate change—and how policy and technology decisions made today will matter. Cool It! is available in our online store
You might also be interested in:
Wind is moving air. Warm air rises, and cool air comes in to take its place. This movement creates different pressures in the atmosphere which creates the winds around the globe. Since the Earth spins,...more
Kingdom Plantae contains almost 300,000 different species of plants. It is not the largest kingdom, but it is a very important one! In the process known as "photosynthesis", plants use the energy of the...more
Earth’s climate is warming. During the 20th Century Earth’s average temperature rose 0.6° Celsius (1.1°F). Scientists are finding that the change in temperature has been causing other aspects of our planet...more
This picture of the Earth surface was taken from high above the planet in the International Space Station. In this view from above, we can see that there are lots of different things that cover the Earth....more
Painting roofs white can cool cities. That’s what scientists discovered in a new study that used a computer model to examine how white roofs affect temperature. "Our research demonstrates that white roofs,...more
Rainbows appear in the sky when there is bright sunlight and rain. Sunlight is known as visible or white light and is actually a mixture of colors. Rainbows result from the refraction and reflection of...more
The Earth travels around the sun one full time per year. During this year, the seasons change depending on the amount of sunlight reaching the surface and the Earth's tilt as it revolves around the sun....more