The free-convection temperature field within a buoyant plume rising from a heated horizontal wire was explored experimentally. Laminar flow measurements were facilitated by use of an isolating enclosure. The laminar plume exhibited a slow, regular swaying motion. The quantitative dependence of the laminar temperature field on the heating rate and on the elevation above the wire, as deduced from the experimental data, was found to be in excellent accord with the analytical predictions for a plume induced by a line heat source. Measured temperature profiles at a range of elevations and heating rates also agreed well with laminar theory. Turbulent bursts occurred at higher elevations above the wire and at higher heating rates, thus signalling the onset of transition. The frequency of the bursts increased with increases in elevation and heating rate. The onset of transition, as indicated by the initial appearance of the turbulent bursts, occurred at a modified Grashof number of 5 × 108. Essentially fully-turbulent conditions prevailed when the modified Grashof number was 5 × 109.