Heat Happens at the Scale of a Sidewalk

Thermal impacts of 5-year tree establishment at solar noon / Keenan Gibbons/SmithGroup

By Keenan Gibbons

The heat island is not just an urban condition. It is a material condition.

This changes how we think about responsibility. Because materials, unlike climate, are something we choose. We tend to talk about heat in averages - daily highs, regional trends, climate projections. But that's not how heat is experienced.

Heat happens step by step. On a sidewalk. At a bus stop. Standing next to a building. A street with no shade. When you measure heat at that scale, everything changes, but you can't solve what you can't see.

Resolutions of infrared satellite sensors at 30m x 30m vs drone at 1.5cm x 1.5cm / Keenan Gibbons/SmithGroup

Most heat analysis has relied on satellite data. It's useful, but it's coarse. Each pixel represents about 100 feet by 100 feet (30m x 30m). That means an entire block - trees, sidewalks, asphalt, shade - is averaged into a single value. In 2018, I began conducting novel research that used drone-based thermal imaging to capture heat at less than 0.6 inches (1.5 cm) resolution.

At that scale, the "average" disappears. What you see instead is a landscape of extremes:

• A concrete sidewalk reading 110°F (43°C)
• A shaded patch just a few feet away closer to 89°F (32°C)
• Black tar in direct sun pushing well above 150°F (66°C)
• Exposed vegetated surfaces near the air temperature of 86°F (30°C)

Same block. Same time. Totally different conditions.

Building on my work with SmithGroup and colleagues at the University of Michigan, a forthcoming study we developed uses AI trained on years of my past drone thermal scans to infer fine-grain heat patterns from satellite data. This approach allows us to identify the effects of sidewalk-scale materials at city-wide scales.

The takeaway is simple: heat is not evenly distributed. It's controlled by what's under your feet and above your head.

Why am I recording surface temperatures exceeding 160°F (71°C) on relatively mild summer days in Michigan, a northernmost U.S. state? The surface materials are the problem.

The Urban Heat Island Effect Material Palette, detailing average spot temperature readings of typical urban materials / Keenan Gibbons/SmithGroup

One of the clearest findings from my research is that materials have the greatest heat impact. Dark, dense surfaces - especially black tar and asphalt - absorb solar radiation and store it. Then they re-radiate that heat back into the environment. That's what pedestrians feel.

In contrast, surfaces light in color, vegetation and tree canopy consistently measure near the air temperature because they block solar radiation. Nature-based solutions also benefit from evapotranspiration, which is the process by which water evaporates from surfaces and is released by plants into the air, helping to cool the environment.

Heat isn't just spatial though - it also changes through time.

Diurnal variation of the urban heat is land effect, showing surface temperatures nearly 20°F (11°C) warmer after sunset than before sunrise / Keenan Gibbons/SmithGroup

Using drone scans across different times of day, we can see how exposure shifts. A sidewalk that's comfortable at 9am can become extremely hot by 1pm, depending on orientation and shade.

Drone scans to monitor project pre- and post-construction year over year show how nature-based solutions and alternative material colors perform over time.

Across every dataset I've worked with, the biggest thermal difference is between sun and shade, planted and unplanted. As materials explain heat, shade explains relief. In direct sun, Midwest surface temperatures routinely exceed 120-150°F (49-66°C). Step into shade and temperatures drop 20°F+ (11°C+) immediately.

That shift isn't subtle. Shade fundamentally changes the thermal experience. It is the difference between tolerable and dangerous.

At the high resolution of a drone, this becomes unmistakable. Thermal imagery shows sharp transitions where shade begins. A few feet can separate extreme heat exposure from relative comfort. These aren't gradients, but boundaries defined by whether solar radiation is absorbed by a surface or not.

This is where the idea of "green space" starts to break down when you look at it thermally.

Infographic of a single-family detached dwelling at solar noon / Keenan Gibbons/SmithGroup

Mowed lawns, empty planters, or unmaintained and neglected landscape areas can tell a different story from the lush green havens they are intended to characterize. Soil and mulch routinely measure 20°F to 40°F (11-22°C) above air temperature when left unplanted and exposed. Mowed grass lawns typically measure 20°F (11°C) or more than the air temperature or nature-based alternatives like a habitat-supporting, low maintenance, and drought-tolerant wild grass prairie.

In these instances, spaces labeled as "green" exhibit the thermal behavior of a conventional hardscape, underscoring the importance of landscape design and maintenance that ensure green spaces are actually green with drought-tolerant native plantings and densely-spaced shade trees.

The biggest misconception about heat is that it's a big, systemic problem that requires big, systemic solutions. It is systemic but also granular. The same tools that reveal the problem also point to block by block solutions:

• Adding shade results in an immediate drop in heat exposure
• Changing pavement mixtures to better reflect the sun results in reduced stored heat
• Adjusting landscape materials and color leads to a shift in solar absorption and reflection that improves thermal comfort and reduces adverse effects on people
• Planting trees strategically and closer together helps us maximize positive impact where it matters and create urban forests

Grand Parklet project for City of Detroit Department of Public Works / SmithGroup

These aren't theoretical strategies. They're design decisions landscape architects already make. Every design decision - sidewalk, plaza, rooftop, roadway, or park - is also a thermal decision that affects communities.

The encouraging part is that this knowledge can empower communities to take action. Given heat happens at the scale of a sidewalk, that's where we can start to solve it. Instead of saying, "This neighborhood is hot," we can say, "This sidewalk is exposed at peak hours" or "This material is creating excess heat" or "This gap in the tree canopy is creating a hotspot."

Once we can say that, we can fix it.

Keenan Gibbons, ASLA, PLA, LEED GA, is ASLA's Climate & Biodiversity Action Fellow for 2026-2027. He is principal and director of landscape architecture at SmithGroup and lecturer at the University of Michigan.