Rural areas, where most of the land is covered in forests, crops, and grass, tend to be much cooler in temperature than urban areas that are covered in buildings, roads, and sidewalks. This increased temperature in cities is called the “Urban Heat Island” effect or UHI for short. The temperature difference between UHIs and their surrounding areas averages between 1-3°C (2-5°F) but in some regions, it can be as high as 12°C (22°F).
Los Angeles, California starts to heat up as the sun rises over the city.
(©Logoboom/Shutterstock )
(©Logoboom/Shutterstock )
Introduction
Most people live in urban or suburban areas. As much as 80% of the population of the US lives in these areas, and that number is only supposed to go up in the future. Why is this a big deal? Cities have everything humans could want, such as shopping, restaurants, roads, parking, people, and lots of places to live. As nice as it is to have all of these things nearby, it comes at a cost. Scientists have found that urban centers like cities and towns are much warmer than their surrounding areas.
One of the reasons why it is so much hotter is that the asphalt and concrete from all the buildings, roads, parking lots, and sidewalks absorb a lot of heat from the sun and then radiate it back out. Furthermore, trees and other vegetation need to be cleared to make room for all of these urban areas. Unfortunately, fewer plants only increase the problem because they have natural cooling abilities that would help offset the heat absorption.
Heat rising from a paved street, creates a mirage of water on the ground.
(© Brocken Inaglory/Wikipedia)
(© Brocken Inaglory/Wikipedia)
Urban Heat Islands Profile
(© Nasa)
(©Tyrone Turner/National Geographic )
What causes urban heat islands?
Many contributing factors cause cities to be hotter than the surrounding areas. Cities are made up of all kinds of heat-absorbing materials like pavement for roads, cement for sidewalks, shingles for roofs, and bricks and metal for buildings. These materials not only absorb heat during the day increasing the temperatures around and inside the structures, but they also radiate that heat out during the night when the city is supposed to be cooling down. To build all of these urban structures natural surfaces and plants, such as trees, need to be removed. Plants, especially trees, have a cooling effect because they can transpire water vapor into the atmosphere.
Cities tend to have a much lower concentration of trees than any of the areas that surround them. The other main reason that urban heat islands form is human activity. Cities have larger and more concentrated populations than rural areas which means that there are more cars on the road generating more pollutants and greenhouse gases into the air. Cities also have more buildings which need to be heated and cooled regularly. This gives off excess waste heat and helps to increase the temperature.
Big cities like New York often contain miles of buildings with very little greenery.
(©Austin Rogers/Stocksy)
(©Austin Rogers/Stocksy)
Why is the Urban Heat Island effect such a problem?
As cities get hotter, citizens use more energy to cool their individual homes and offices. As a result, they spend more money and produce more greenhouse gases than they did when it was cooler. Trees absorb harmful pollutants and cutting them down results in higher amounts of air pollution such as smog. Without vegetation, the water quality in cities is lowered. Since plants also help to guard against erosion, water runoff from storms and rain carries away topsoil and other pollutants as it runs across the surface of the city. It then deposits this soil and pollutants into rivers and streams. This makes the water unsafe for drinking and also endangers aquatic life. All these factors can jeopardize human health and comfort.
The plants that do live and grow in cities have longer growing seasons because of the heat. A longer growing period may sound like a good thing, but it does not increase plant productivity. The Urban Heat Island Effect reduces the plant’s photosynthetic productivity by 20 days compared to the productivity of plants in rural areas. That is similar to not watering a plant, putting it in the dark or taking away its carbon dioxide supply during its growing season. As scientists have discovered these effects, people that live in cities have realized there is a problem. Many cities are executing plans to fight the urban heat island effect. Read about each of these cities on the map and the different plans they have to fight against the rising temperatures.