Heat Island Defined: How cities act like giant ovens in the summertime

Weather Glossary

In the heat of summer where the days are warmest, highly populated areas such as Rochester become prone to trapping in warmer temperatures than the surrounding areas. Cities essentially become ovens in the summertime due to an effect known as the urban heat island effect.

What is a heat island?

Heat islands are urbanized areas that are able to trap and retain the surrounding heat better than other outlying areas that include abundant plants, forests, lakes and other bodies of water. They literally become their own island of heat, hence the name “heat island.” 

How does this happen?

This happens because structures often found in cities that consist of abundant roads, large buildings, and dark asphalt have a higher tendency to absorb and keep the heat in due to a phenomenon known as specific heat. This means that certain materials have a higher heat capacity, or tendency to heat up and stay hot longer than others. In other words, it takes less energy to heat up these substances than say, water or wood. Darker colors also have a higher tendency to absorb heat than lighter colors due to how easily different wavelengths of light pass through objects.

Image courtesy of epa.gov

Notice how the surface temperature during the day varies significantly more than the air temperature during the day. This just goes to show how much of an impact residential areas can have on the surroundings. Also take note of how the surface and air temperature during the day become much less different over a body of water such as a pond. This shows how bodies of water help to mitigate the effects of a heat island.

Heat islands form due to many factors:

  • Areas with limited greenery (plants, trees, grass)
  • Structures built with high heat absorbing material like concrete, asphalt, and metal (specific heat)
  • Dense areas of tall buildings that prevent and block wind flow 
  • Added heat and pollution from human machinery such as A/C, cars and other vehicles 
  • Weather Patterns, landscapes and geography 

Hard and dry surfaces that are found within cities provide significantly less shade and moisture than areas with abundant vegetation. Natural landscapes are able to provide more shade and moisture from the transpiration of plant leaves, and evaporation of surface water. You lose a lot of that natural cooling effect the farther you go from these vegetated areas. 

Heat islands make the days and nights feel hotter, and often contain the warmest temperatures out of their outlying areas during the warmest months. They’re determined by comparing the measured temperatures between the city and its relative surroundings. 

For example, check out this screenshot of current temperatures on a typical, warm afternoon in August.

Notice how the bigger cities of Buffalo, Rochester, and Syracuse are slightly warmer than Sodus, which is a less populated town right next to the influence of Lake Ontario. 

Temperatures in Dansville and Penn Yan are within much more rural areas in comparison, and are also a bit higher in elevation than Rochester is. Since Rochester is a highly populated city containing lots of human activity, buildings and infrastructure, it’s very common for this area to see the influence in warmer temperatures.

As noted above, weather and geography play a decent role in affecting heat islands. Having calm and quiet weather conditions can make the effects of heat islands worse by maximizing the amount of heat they receive, and reducing the amount of heat they can lose. This prevents radiational cooling from being as effective closer to the urban areas, which keeps cities much warmer overnight while the surrounding areas get much cooler. Having mountains nearby can also prevent winds from reaching a location, and therefore keeping the warm air close. Heat islands can make heat wave events feel even worse as it’s much more difficult to escape the heat outside, while still inside a trapping “island” of heat.

According to Climate Central, climate change is known to be having a worse impact on heat islands as summers in cities have been getting hotter, faster since 1970.

Learn more about heat islands and ways to mitigate their effects here.

Copyright 2021 Nexstar Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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