🌳 Tree Canopy & Heat Risk

About Tree Canopy & Heat Risk Analysis

This tool measures the urban heat vulnerability of transit station environments by analyzing two complementary land surface characteristics: tree canopy cover and impervious surface coverage. Together, these two variables are the primary structural drivers of the urban heat island effect — the phenomenon by which developed areas are measurably warmer than surrounding natural and rural land.

What We Measure

The analysis characterizes each station's ½-mile walkshed along two dimensions drawn from the USDA Forest Service National Land Cover Database (NLCD), 2021 edition:

• Tree Canopy Cover (TCC) — The percentage of land within the walkshed covered by tree canopy, measured at 30-meter resolution. Tree canopy provides cooling through two mechanisms: shading, which blocks incoming solar radiation from reaching paved and built surfaces, and evapotranspiration, through which trees release water vapor that lowers ambient air temperature. Research consistently finds that each 10% increase in urban tree canopy reduces local summer surface temperatures by 1–2°C.
• Impervious Surface Coverage — The percentage of land covered by pavement, rooftops, and other hard surfaces that shed water rather than absorb it. Impervious surfaces absorb and re-radiate solar heat throughout the day and into the evening, are the primary driver of urban heat islands, and eliminate the cooling effect of evaporation from soil and vegetation. The NLCD measures imperviousness at sub-pixel precision — each 30-meter cell carries a value from 0 to 100 representing the fraction of that cell that is impervious.

How the Analysis Works

For each station, the tool requests two 64×64 pixel GeoTIFF tiles from the MRLC GeoServer covering the ½-mile bounding box, applies a circular walkshed mask, and returns the mean pixel value within the half-mile circle as the station's score. This walkshed-mean approach correctly reflects the environment that riders experience traveling to and from the station — not just the concrete platform underfoot.

The scores are then combined to place each station in one of four heat exposure quadrants based on threshold values calibrated to the NLCD distribution for developed urban land nationally:

• Cool Green Corridor (TCC ≥ 35%, Impervious < 40%) — Well-canopied walksheds with limited pavement. Riders experience relatively cool conditions, with shade and permeable surfaces moderating heat.
• Mixed / Partial Cover (TCC ≥ 35%, Impervious ≥ 40%) — Tree canopy provides meaningful cooling but significant impervious surfaces still absorb and re-radiate heat. Conditions are moderate — better than unshaded urban environments but not optimal.
• Bare / Open Land (TCC < 35%, Impervious < 40%) — Limited canopy but also limited paving — typically low-density suburban areas, parks, or undeveloped fringe land. Moderate heat risk from sun exposure without the compounding effect of heat-absorbing pavement.
• Urban Heat Island (TCC < 35%, Impervious ≥ 40%) — The most heat-vulnerable condition: minimal shade combined with extensive pavement and built surfaces. Stations in this quadrant are most at risk during extreme heat events, with riders exposed to compounding heat stresses.

Why This Matters for Transit

Transit riders are disproportionately exposed to urban heat. Unlike automobile commuters who move from air-conditioned buildings to climate-controlled vehicles, transit users spend meaningful time outdoors — walking to stations, waiting on platforms, and navigating transfers. This exposure is concentrated precisely in the environments this tool measures: the half-mile walkshed around each station.

• Health Risk — Heat-related illness is the leading cause of weather-related mortality in the United States, and urban heat islands amplify risk in ways that fall unevenly across income, race, and age. Lower-income transit-dependent riders, who are less likely to have access to air conditioning at home, face compounded vulnerability when their transit environment is also a heat island.
• Environmental Justice — Tree canopy coverage shows a well-documented inverse correlation with lower-income neighborhoods in U.S. cities — a pattern this tool helps make visible at the station level across all 5,100+ stations in the Atlas.
• Infrastructure Planning — Stations identified as Urban Heat Islands are candidates for targeted greening interventions: street tree planting, shade structures, cool pavement treatments, and green stormwater infrastructure that simultaneously reduces runoff and heat.

Important Notes

Data Source: Both layers are drawn from the 2021 NLCD, produced by the USDA Forest Service in partnership with the Multi-Resolution Land Characteristics (MRLC) Consortium using Landsat and Sentinel-2 satellite imagery. Both carry an accuracy of approximately ±10 percentage points at the pixel level, with better accuracy in dense urban areas.

What This Does Not Capture: This analysis does not measure actual air or surface temperature, the cooling effect of water bodies, building height and canyon geometry, waste heat from vehicles and HVAC systems, or the quality and availability of shade structures and other designed cooling infrastructure. A station near a river or large park may be cooler than its canopy score alone suggests; a station in a dense canyon of tall buildings may be hotter.