Many MindedTen minutes under a tree: measuring heat relief for people sleeping outside
University of Utah researchers followed eight people experiencing unsheltered homelessness in Salt Lake City from July 29 to Aug. 3, 2024. Participants carried GPS-enabled phones and portable heat sensors, which produced nearly 10,000 heat-index readings. The paper, published in the Journal of Urban Health, is led by Jeff Rose, associate professor of Parks, Recreation, & Tourism.
Participants spent nearly 20% of daytime hours at moderate heat risk (a heat index around 91 °F / 33 °C). High and very high or extreme conditions made up less than 4% of the time. Nights were the harder stretch: on average 69% of nighttime hours sat above the 68 °F (20 °C) hot-night threshold, and 16% reached 77 °F (25 °C) or higher. Time under tree canopy between noon and 6 p.m. was linked to heat-index drops of about 1.7 to 3.2 °F. The cooling grew with time in shade and tapered off after about 15 minutes, and 10 to 15 minutes was linked to a significantly lower heat index.
This matters for sustenance and security because shade is a cheap, public form of protection from heat. Some shade visits lasted under 10 minutes, which suggests access barriers limit the benefit. An Oct. 5 report combining three University of Utah studies found that shade, drinking water, bathrooms and rest places matter as much as temperature. It also found that businesses act as informal cooling refuges, though access varies. The report calls for a statewide "Code Red" policy with longer access to public cooling spaces such as libraries and senior centers, plus outreach to high-risk people.
Caveats: eight people over about six days is a small sample. The interview studies in the October report, one with 24 unsheltered people and one with 30 business managers and staff, are preliminary. The authors say trees cannot replace secure housing or indoor cooling in extreme heat. Our source text was cut off before the full tree-planting recommendations.
Source: Phys.org