- Urban heat islands are metropolitan areas that are hotter than their outlying regions, with the impacts felt most during summer months. About 85% of the U.S. population lives in metropolitan areas. Neighborhoods in a highly-developed city can experience peak temperatures that are 15°F to 20°F hotter than nearby areas with more trees and less pavement.
- Climate Central, an independent organization of scientists and journalists researching and reporting changing climate and its impact, created an index to evaluate the intensity of urban heat islands and applied it to 159 cities across the U.S. The cities with the five most intense urban heat islands are New Orleans, Newark, N.J., New York City, Houston, and San Francisco.
- Heat islands are heavily influenced by albedo, which measures whether a surface reflects sunlight or absorbs and retains the sun’s heat. Other factors include the amount of impermeable surface, lack of greenery and trees, building height, and heat created by human activities.
- Extreme urban heat is a public health threat, especially for individuals and communities that are more vulnerable due to health, social, economic, or other reasons. But there are a number of short-term and long-term solutions to adapt to an increasingly warmer future, as well as to mitigate some of the urban heat stress.
What are the critical components in heat islands?
In addition to the albedo measures of heat retention and absorption, other factors include the amount of impermeable surface, such as buildings, driveways, sidewalks, roads, and parking lots. Hard, dry surfaces provide less shade and moisture than natural landscapes and contribute to higher temperatures. Other components include a lack of greenery and trees, the dimensions and heights of buildings, and heat created by human activities like running engines and air conditioners.
What are the impacts?
Extreme urban heat is a public health threat. It amplifies air pollution and creates dangerous conditions for people working outside or living in buildings without air conditioning. Discriminatory housing practices like redlining along with other socioeconomic factors mean that communities of color are often in areas with fewer trees and parks and thus are exposed to higher urban heat.
Where does Austin stand?
The city of Austin has an “intensity score” of 6 degrees, meaning on average the downtown city of Austin is 6 degrees warmer than the surrounding areas. This is actually not too bad in comparison to other cities across the country. We are not even on the top 20 list with greatest urban heat island intensities. This is in part due to the still relatively low amount of skyscrapers we have (close to 170). For reference NYC has over 6,400 high rises. More buildings = more heat retained at the surface.
What are the solutions?
There are a number of short-term and long-term solutions to adapt to an increasingly warmer future, as well as to mitigate some of the urban heat stress. Short-term solutions are mostly about getting people out of the heat and ensuring their health and safety. But there are also ways to reduce urban heat island effects such as:
- Planting trees, particularly along paved streets.
- A green roof, or rooftop garden, is a vegetative layer grown on a rooftop and can provide shade and lower temperatures of the roof surface and surrounding air.
- Cool roofs are made of highly reflective and emissive materials that remain cooler than traditional materials, and help to reduce energy use.
- Cool pavements, or whitewashing roads and sidewalks, is more complicated than roofs. In cities with urban canyons, the sunlight may not even reach the street level long enough to make a significant difference. In places like Los Angeles, a cool pavement study showed that heat was reflected off the white surface, but onto pedestrians and made people feel hotter.