The paper considers the initial stage of formation of convective plumes emerging from finite size heat sources. Experimental studies were carried out to investigate the evolution of a boundary layer adjacent to localized heat sources of various shapes - circular, square and triangular. Rhodamine was used to visualize the flow structure, and the temperature field was recorded by an infrared camera. Two scenarios (convective and conductive) of thermal plume organization were distinguished. In the conductive case, heat boundary layer take a dome-like shape, that leads to single convective torch formation. In the convective case, there is a boundary layer instability near the heater edges. It leads to separation of the thermal plume. The critical Rayleigh number separating these regimes was determined. (C) 2016 Elsevier Ltd. All rights reserved.