We hit the streets to document Houston’s heat island effect. Here’s what we found.

Houstonians don’t need anyone to tell them our summers are hot. Even people who don’t live here know it. 

But did you know that where you live or work in Harris Country impacts exactly how hot it will get on a sweltering August day? 

That’s because the things in our built environment – buildings, asphalt, and concrete – absorb more heat than our natural environment – like grass and trees. This creates an urban heat island – higher temperatures in cities compared to suburbs and rural areas. Even within the urban heat island, there can be hotter areas, and beyond the discomfort of sweating through your clothes, it can have significant health impacts. 

Measuring these differences is the first step in understanding them and developing solutions to ease the heat for families who live and work in Houston’s hottest areas. That’s the mission of the H3AT campaign, a partnership between HARC, the City of Houston, Harris County, and several other local and national groups. 

We launched the campaign in 2020 with over 80 volunteers who drove across 320 square miles around Harris County with temperature and humidity sensors on their cars. We expanded the effort in August 2024, with over 150 volunteers to include 40 additional routes and 700 square miles. Over the campaign, volunteers drove a total of 3,446 miles, longer than the length of the US. We just finished analyzing the data, which you can explore at www.h3at.org. 

Here are the biggest takeaways: 

• We conducted the measurements on Saturday, August 10, 2024, when Houston’s high temperature was forecasted to be 91 degrees. Hot, but not blistering by Houston standards. 

• We conducted three rounds of measurements – in the early morning, midday, and early evening. 

• Across all the areas we measured, we found a range of morning, afternoon, and evening temperatures. 

• A 14-degree difference in the high temp of the hottest area and the high temp of the “coolest” area.  

• The “coolest” areas – or those with the lowest low temperatures in the morning and the lowest highs in the afternoon were in less-densely built areas, like the suburbs or areas of Harris County with large trees, open green spaces and less pavement.  

• The hottest areas were the opposite – dense development, lots of asphalt or concrete, and little to no tree cover or open green spaces. 

This may seem completely obvious and intuitive. If you live in one of the hottest areas on the map, you probably expected to see that in the data. But seeing the variations on a map helps community members in these hot spots show their leaders what they already know. The discrepancies are unmistakable. More importantly, the data provides the insight needed to prioritize solutions. 

Which neighborhoods would most benefit from tree planting or artificial shade? In which areas are residents most susceptible to heat-caused illnesses? What can we learn from the planning decisions of “cooler” areas that can be replicated in the hotter ones? 

With all our work at HARC, our goal is to apply science to not only identify our greatest challenges, but to apply science to develop solutions to overcome them. For our neighbors who suffer the most during our brutal summers, we hope the H3AT campaign can do both. 

Learn more about the Houston Harris Heat Action Team (H3AT) by visiting www.h3at.org  

Funding for this project was provided by the Inflation Reduction Act and the USDA Forest Service, Urban and Community Forestry Program.

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