Growing on Urban Heat Islands

Urban heat islands are areas of towns and cities which can become unbearably hot, especially on days with extreme heat. These areas can often become 10-20 degrees warmer than other areas in the very same city. These urban heat islands tend to occur in areas with the densest built environments and with the most roads. 

Urban heat islands are largely a result of un-shaded roads and buildings absorbing heat and then radiating it out to their surroundings. The dark surfaces of roads and built materials, such as bricks and concrete, absorb more heat than grass and vegetation. The coolest places are usually is parks and on streets with lots of tree cover. Which is why the densest built areas tend to be significantly warmer than areas which have lots of tree cover or parks.

One of the best ways to prevent urban heat islands is to provide more tree canopy cover - to create natural shade. Planting more trees in urban environments not only helps to reduce street-level temperatures, they can contribute to a better quality of life and can make neighborhoods more attractive places to live.

Google's new Tree Canopy Lab is an interactive map which visualizes tree canopy cover in the city of Los Angeles. The map is designed to show the current tree canopy coverage in the city and to help identify where new tree planting efforts are most needed. The map allows you to see the level of tree canopy coverage in different neighborhoods in the city alongside demographic data, such as population density. Using the map it is therefore possible to quickly identify where the most people are likely to be living in urban heat islands and where new tree canopy cover is most needed. 

The New York Times has mapped out how racist housing segregation in the United States, dating back to the Home Owners' Loan Corporation's Redlining maps, is a major contributory factor to the location of urban heat islands in towns and cities across the country. 

In How Decades of Racist Housing Policy Left Neighborhoods Sweltering the New York Times has created a story map which shows how across the United States neighborhoods which were redlined are usually the hottest parts of towns. Conversely neighborhoods which weren't redlined tend to be the coolest areas. The reason for this is that redlined neighborhoods have largely remained areas of deprivation and tend to have fewer trees and a  more densely built urban environment. Non-redlined neighborhoods have remained mostly more affluent and are therefore more likely to have lots of parks and lots of tree cover.

In August 2018 NOAA ran a citizen science project in Washington D.C. and in Baltimore to discover which part of those cities were effected by urban heat islands. You can see the results of this study on NOAA's Detailed maps of urban heat island effects in Washington, DC, and Baltimore.

The detailed maps produced by NOAA show that the hottest areas in both cities are the areas with the densest built environment and the most roads. The coolest places in both cities are in parks or in other areas with lots of tree cover. 

Guess who lives in areas with parks and trees and guess who lives in densely built areas with lots of bricks and concrete. That's right the hottest areas in cities tend to be the areas where the poorest residents live, while the richest residents can afford to live in the coolest areas with lots of parks and trees. You can observe this pattern in cities across the USA using NPR's interactive heat island maps.

In As Rising Heat Bakes U.S. Cities, The Poor Often Feel It Most NPR has published an interactive tool which allows you to view heat island maps of US cities side-by-side with a map of income levels. Using this comparative tool you can directly see if there is any correlation between high surface temperatures in a city and the level of income. When you select a city the tool even tells you directly if there is a strong, moderate or weak link between surface temperatures in the city and where different income groups live.