We used the System for Observing Play and Recreation in Communities to examine the association between variations in natural elements (ie, season, weather, and temperature) and adults’ use of a rail-trail in South Carolina (2006-2009). Most (62%) of the 4,468 trail users observed were walkers; 38% were observed participating in vigorous physical activity. Adults were most often observed using the trail during the summer (32%), sunny weather (78%), and moderate temperatures (54%). When promoting trail use among adults, natural elements should be considered.