The Weather Notebook: Heat Islands

Heat Islands
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Cities are hot. A typical city runs 2 to 5 degrees Fahrenheit warmer than the surrounding countryside. It's called the heat-island effect. Hi, I'm Dave Thurlow for the Mount Washington Observatory and this is The Weather Notebook.

On warm summer days, the air in a city can be 6-8°F hotter than its surrounding areas. Heat Island Group
Cities are warmer than the surrounding countryside for good reason. Buildings and parking lots and roads and more buildings and more roads... soak up the heat of the sun. In the country though, trees and grass and stuff like that intercepts a little of the suns heat to do stuff like evaporate water, photosynthesize, shade rocks and dirt, all which keep the country a little cooler than the city, especially at night. And also city life itself adds to the heat. For instance, all those car engines and factories and air conditioners are pumping out hot air that registers on city thermometers.

Now, does this warm up the earth and add to global warming? Most likely not. But it does make measuring the earth's temperature change a bit of a problem. You see a lot of weather stations have been in the same place for a hundred years. A hundred years ago no city, now - big hot city. So is the planet warming or is it just the cities where the thermometers are. Well that's a good point and lucky for us climate researchers long ago figured in the effect of heat islands into global warming studies by looking mostly at long-term weather stations outside of big cities. This helps to figure as accurately as possible, the earth's temperature changes based on changes in the atmosphere, not at changes in the urban landscape.

If you want to help your own city, let alone the whole earth, stay cool, you can do is plant a tree or make the roof of your home more reflective, so heat bounces back to space instead of storing it here. It's already being done, in cities all over the world.

Today's contributing writer is Bob Henson. Our show is funded by Subaru and the National Science Foundation.