Linking Multi-Scalar Site Design to Personal Heat Exposure in Schools

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Abstract
Outdoor heat mitigation strategies such as trees, shade structures, and reflective pavement are widely deployed in cities, but the cooling benefits attributed to them depend on two analytical choices that are rarely examined together: the spatial scale at which performance is evaluated and the temperature metric used to assess it. Most assessments rely on a single scale or single metric, and how these choices jointly shape conclusions about intervention performance remains poorly understood. This question is consequential in schoolyards, where heat-vulnerable children spend time outdoors and major cooling investments are underway. We address this gap using high-resolution microclimate modeling, validated against field measurements, to evaluate four interventions (clustered trees, dispersed trees, artificial shade structure, and reflective pavement) across four nested functional spatial scales reflecting how children use schoolyard space and three temperature metrics at an elementary school in Los Angeles, California. Both dimensions reordered intervention rankings. Across spatial scales, the best-performing intervention for mean radiant temperature (MRT) shifted from dispersed trees at the whole-school scale (-1.2 °C) to clustered trees at the point scale (-24.5 °C), while reflective pavement increased MRT at every scale (up to +3.0 °C). At the schoolyard scale, reflective pavement ranked first for reducing surface temperature (-1.5 °C) but last for MRT. Air temperature responses were negligible (≤0.2 °C). These results demonstrate that conclusions about which interventions cool most effectively depend jointly on spatial scale and temperature metric, and that human-centered evaluation requires aligning both with how heat is experienced.

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