Boulder Urban Heat Mapping

As climate change increases temperatures across the globe, including here in Boulder, the most vulnerable community members are facing a greater heat burden. Extreme heat  kills more Americans than any other weather event , however, not everyone’s risk is the same. Denser urban areas typically have a lot of dark surfaces like buildings, parking lots, roads, and other infrastructure that absorb and hold heat, consequently making urban areas much hotter than their surrounding rural and suburban areas. This localized increase in heat is known as the “urban heat island effect.” Zooming in further, heat is not distributed evenly within these urban heat islands. Oftentimes, the effect disproportionately affects low-income communities and communities of color. Some groups of people are also more vulnerable to the adverse effects of extreme heat such as the elderly, infants, outdoor workers, people with preexisting conditions, and people who don’t have access to cooling in their home. To understand how heat is distributed across the City of Boulder, the  Cool Boulder  campaign partnered with the  National Oceanic and Atmospheric Administration  (NOAA) and  CAPA Strategies  to find out exactly which places in Boulder were the hottest by participating in their  Urban Heat Mapping Campaign .

Many cities are exploring more creative ways to mitigate this heat such as implementing natural climate solutions, which include tree planting, greening of neighborhoods, and conserving natural areas that absorb carbon and stormwater. Trees and other vegetation have a cooling effect by providing shade and releasing moisture through a process called evapotranspiration, making them an excellent tool in cooling down our communities. 

Once the hottest areas of Boulder are identified and mapped through this data collection project, the city and its local partner organizations can work with communities to utilize natural climate solutions that will provide cooling benefits in our city. Urban heat maps of Boulder were created using temperature and humidity data captured using sensors provided by CAPA Strategies. Sensors were attached to the cars of volunteers, who drove 7 predetermined routes at 3 different times of day. These routes were designed by CAPA Strategies based on points of interest that were identified by looking at some of the most socio-economically and environmentally vulnerable parts of Boulder. Volunteers members of the Boulder Heat Watch Team drove these 7 routes on one of the hottest days of the year, July 22nd, 2022.

Below you can see a map of Boulder and the 7 routes that were driven by Boulder community scientists. We could not have done this project without the 42 community members who drove and navigated these routes to collect the data. One volunteer, Corinne, said she was proud to be involved because issues like climate change "can feel overwhelming in figuring out how to help" and "this project felt more connected to broader community issues than perhaps changing a personal behavior at home." Similarly, another volunteer, Albert, said that he was, "drawn to the idea of cities mitigating climate change - things we can do on the smaller level to address a large scale issue."

Zoom in and out on the map to get a closer look at each of the routes that the Boulder Heat Watch Team drove for collecting heat data.

This first heat map below shows the data that was collected along those routes during the morning shift (6am-7am). Note the range in temperature from 61°F to 78°F, almost a 17°F difference! This was the greatest range in temperature that NOAA collected across all cities this year and likely occurred because of Boulder's unique topography at the base of the foothills of the Rocky Mountains, at an elevation of 5,430 feet. Elevation is a major factor that influences temperature, as temperature tends to drop as elevation rises.

The next set of maps are area-wide maps that cover the whole city. As data was only collected along the routes, experts at CAPA Strategies used a combination of predictive models and satellite data to fill in the gaps beyond traversed routes. Please examine the maps in this context, particularly for areas further away from the above routes.

The first area-wide map below shows the temperature for the 6am-7am driving shift and compares it to tree canopy data. Because trees provide shade and release moisture into the air via a process called evapotranspiration, they provide great cooling benefits!

 ^{Click and drag the arrows to compare the two datasets (temperature data on the left, tree canopy data on the right).} 

It's clear that trees play a huge role in keeping our neighborhoods cool and this was not lost on our volunteers. One volunteer, Bishoy, grew up in Cairo, Egypt, where they experience extreme temperatures in the desert environment. He recalled going down to the Nile River with his friends to cool down as it was one of the only places in the city that had trees. Corinne, who earlier discussed responding to climate change, said that when she and her husband were looking to buy a home in Boulder that they were sensitive to and aware that wealthier neighborhoods typically have streets with huge mature trees on all sides. She added that when she runs errands along 28th street she "always noticed as soon as [she] goes to Valmont and 29th it’s so clear that it gets industrial and there are no trees." See if you can find this intersection on the above map to see if the heat data aligns with Corinne's experience.

Another volunteer, Kristine, was surprised at "how much of a difference there was in the core downtown area in the morning - how it didn’t cool down overnight", which is a result of all of the impervious and dark surfaces in the area. Impervious surfaces are often composed of materials like concrete and asphalt, that are known to absorb and hold more heat, so we would expect areas that have a high concentration of impervious surfaces to be hotter.

The map below shows the temperatures for the 3pm-4pm driving shift and compares it to a map showing the percentage of impervious surface coverage in the area.

Driving volunteer, Bishoy, said he wasn't surprised that downtown Boulder was so hot because of all the buildings and roads there, but he did not expect the contrast to be so stark. Albert noted that he feels much hotter temperatures when he goes from his house to run errands in a shopping center with large parking lots, which are exclusively made of impervious surfaces. Think of some shopping areas you frequent and find them on the above map and see how hot they are compared to some more shaded areas you know of.

An important aspect of heat is that some community members face a greater burden, so we are going to compare our heat data to a social characteristic of Boulder. This map shows the temperature for the 6pm-7pm driving shift and compares it to the number of affordable housing units.

Let's look at our afternoon temperature data again, which shows the hottest time of day, when people face most extreme temperatures. This time we are going to compare it to the number of affordable housing units to see if our more socially vulnerable neighbors are facing a disproportionate exposure to heat.

As you can see by swiping between the two maps, the hottest areas of Boulder (that aren't strictly commercial areas) seem to be found in places with more affordable housing residents. A volunteer who collected data with her son, Kristine, expressed concern about heat as an equity issue and "worries about disproportionate impacts of heat on low-income communities." As we can see by examining these two maps, Kristine's concerns appear to be valid. Kristine grew up in the Southwest U.S. and was very aware of urban heat island issues in her hometown. As Kristine and her son left a parking lot and drove westbound on Valmont Road, they entered a neighborhood with lots of mature trees. They watched the temperature readings fall on her car, demonstrating the cooling benefits of trees and the heat that impervious surfaces release into our cities. This cooling should not be limited to just those who can afford it.

You may be thinking, that this is an overwhelming issue that we need to address. How are we going to cool down the city and how can I help? That's where the Cool Boulder campaign comes in! This joint campaign between the City of Boulder and local partner organizations aims to use natural climate solutions like tree planting, pollinator gardens, and landscapes that absorb carbon, moisture, and energy, which will mitigate effects of climate change such as extreme heat. Hopefully this information has shown you how much of an effect the built environment and urban green space has on the temperatures of our neighborhoods.

We have the ability to cool down the hottest and the most vulnerable parts of Boulder with tree planting. However, this is not something the Cool Boulder campaign can do alone. We need more community scientists to help us implement these solutions, as you are the expert of your community and have insights into what solutions might work best for you and your neighbors. If you want to get involved, follow the link to the Cool Boulder website by clicking on the button below.