In managing smoke from wildland biomass fires, much effort has been placed on lofted trajectories that may influence human health, regional haze, scenic vistas, and effects on incoming radiation. It has been found, however, that neutrally-buoyant smoke from the smoldering phase of a fire causes significant impact to nearby homeowners and highway visibility. Even with relatively low concentrations, the smoldering smoke that settles into valleys and basins at night creates such a nuisance that prescribed burning programs are significantly affected, and in some cases even terminated. To help understand the patterns of smoldering smoke in boreal forests and to build tools that can help manage such smoke, we designed a series of observations during the Frostfire experimental burn. We launched a tethered balloon from a site near the outflow of the basin. The balloon carried sensors that measured wind speed and direction, dry bulb and wet bulb temperature, carbon monoxide, and passive particle concentration. Launches began around midnight and continued every 1/2 to 2 hours until about 8 am for 5 nights following ignition. In addition to the balloon- mounted instrumentation, a ground-based weather station and carbon monoxide sensor operated continuously at the balloon site. Also, two vacuum-powered nephalometers continuously recorded particle concentrations; one from the balloon site and the other about 2 Ian down valley in the marsh. The observations showed a strong diurnal cycle of about 12 hours duration in which a vertical temperature inversion each night caused particles and gases to concentrate near the ground while weak surface winds transported the trapped smoke down valley. Also, smoke tended to concentrate in the warm sector of the atmosphere, near the top of the inversion, especially at times when burning occurred at places above the inversion. The layered concentrations of smoke and lack of vertical mixing affect transport patterns and resulting impacts on local visibility and health. Because the strength, duration, and consistency of the nighttime inversion is comparable to locations at lower latitudes, algorithms to predict the structure of nighttime inversions may be globally consistent. We used the Frostfire observation data to show that reasonable approximations of nighttime smoke distributions can be made from a simple algorithm of topographic features.