Thursday, April 18, 2019

Mapping Urban Heat Islands

While the governments of the world refuse to take action against climate change you might want to start planning how you are going to cope with the extreme heat of your future summers. Particularly if you live in a large town, city or other large built environment.

It has long been known that certain areas of a town or city can be much warmer than other parts of the same town or city. This 'urban heat island' effect is often most pronounced in the summer and on days with extreme heat. If you know where those heat islands are in your town then you may be able to avoid them on the hottest days of the year.

Last August (2018) NOAA ran a citizen science project in Washington D.C. and in Baltimore in order to discover if these cities had urban heat islands, and if so, where those heat islands were. NOAA mounted thermal sensors on the cars of a number of volunteers. They then asked the volunteers to drive a set route while the monitors recorded temperatures, the time of the temperature recording and the location. NOAA then used this data to create detailed maps of the hottest and coldest places in each city.

In Baltimore on the same day and at the same time some areas of the city experienced temperatures 17 degrees F higher than other areas. In Washington D.C. it was discovered that some areas of the city were 16 degrees warmer than other areas. The detailed maps which NOAA were able to create from the project not only provided proof of the urban heat island effect, it showed where those heat islands were and allowed NOAA to look for common features found in the hottest and coolest locations. In other words it allowed NOAA to explore what causes certain areas in a city to experience more extreme heat than other parts of the same city.

You can view the location of Baltimore's and D.C.'s urban heat islands on NOAA's Detailed maps of urban heat island effects in Washington, DC, and Baltimore. Both these two city maps overlay heat maps of the recorded temperatures in each city on top of an aerial imagery map. A swipe control then allows you to closely examine the common features underneath the hottest and coldest areas in each city.

You don't need to be an environmental scientist to see that the hottest areas in both cities are the areas with the densest built environment and the most roads. This is a result of un-shaded roads and buildings absorbing heat and then radiating it out to their surroundings. The coolest places in both cities are parks or other areas with tree cover. The dark surfaces of roads and built materials, such as bricks and concrete, absorb more heat than grass and vegetation. Which is why the densest built areas tend to be significantly warmer than areas with tree cover or parks.

In order to avoid creating areas that experience a heat island effect city planners can introduce measures which mitigate against the albedo effect of roads and buildings. Roofs can be painted white to reflect heat. Trees can be planted along roadways and parking lots can be replaced with parks (or at least made more green).

You can explore the locations of the urban heat islands in your town and city on Yale University's Global Surface UHI Explorer. This interactive map uses our knowledge of what causes the urban heat island effect to predict where urban heat islands will appear in towns and cities around the world.

You can read more about the algorithm which predicts the urban heat islands on the interactive map in the paper A simplified urban-extent algorithm to characterize surface urban heat islands on a global scale and examine vegetation control on their spatiotemporal variability, published in the International Journal of Applied Earth Observation and Geoinformation.

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