Highlights • The 2020 Tokyo Olympics is expected to present heat and humidity challenges. • Spatially-explicit meteorological data provide a baseline of expected conditions. • Spectator thermal comfort varies significantly along the course. • Shade provided by trees and buildings is closely related to thermal comfort. • Enhanced preparedness efforts are needed to limit discomfort and heat illness. The 2020 Olympic Games marathon will be run through the streets of Tokyo on the mornings of August 2nd and 9th, a time of year that is typically hot, sunny, and humid. Few studies have assessed the potential impact of extreme heat along the marathon course to understand the multiple factors (e.g., radiation, wind flow) affecting human thermal comfort (TC) as influenced by urban design and vegetation. The current research establishes a baseline of microclimate conditions and scenarios to estimate the projected TC along the marathon route for spectators. Mobile microclimate data (air and surface temperatures, solar radiation, humidity, wind speed) were collected along the marathon course over 15 periods in the summer of 2016 and aligned with sky view factors (SVF). Human energy budget modeling was applied to provide spatially-explicit heat budget and TC information along the route. Conditions are expected to create the most discomfort along open, sun-exposed locations, with ~50% of the area along the second half of the course resulting in `Hot` (budget > 200 W m-2) or `Very hot` (budget > 295 W m-2) conditions. The heat strain index frequently rises above 80% in these locations, with high humidity and low wind flow exacerbating discomfort. Buildings and trees producing a low SVF over roads and sidewalks protect spectators from the morning radiant heat, but such locations should be balanced with wind flow to optimize comfort. The modeling and spatial information can aid in preparing for and mitigating heat stress during the Olympics. Potential solutions can be implemented in collaboration with local organizers and government. This `research through design` strategy can aid in preparing for and mitigating heat illness during the Olympics. Knowledge gained can be extended to other areas of Tokyo to reduce urban heat, and further provide targeted guidance for effective environmental cooling techniques for human health.
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