In the United States, approximately 600,000 ha of pine trees are fertilized with urea each year, with NH3 volatilization losses ranging from <1% to >50% depending on environmental conditions. Previous work showed that the timing of rainfall after urea application plays a significant role in controlling NH3 loss, but the effect of other environmental variables is not well understood. We conducted 10 29-d studies under different environmental conditions during 2 yr to identify important variables controlling NH3 loss from urea applied to loblolly pine (Pinus taeda L.) at 200 kg N ha–1. Ammonia loss was measured with dynamic chambers that adjusted the rate of air flow through the system based on wind speed at 1 cm above the soil surface. Regression analysis indicated that a variable related to the initial water content of the forest floor and a variable related to the relative humidity (RH) during the study explained 85 to 94% of the observed variability in NH3 loss. Relatively high initial water content followed by consistently high RH led to large NH3 losses. In contrast, low initial water contents resulted in slow rates of NH3 loss, which increased when elevated RH led to an increase in the water content of the forest floor. These results indicate that RH can play a significant role in NH3 loss by accelerating urea dissolution and by increasing or decreasing the water content of the forest floor, which in turn can affect the rate of urea hydrolysis.