This paper describes the three-dimensional (3D) vortex formation process behind a heated cylinder at low Reynolds numbers. Both experimental and numerical techniques are used, including an electrochemical tin-precipitation visualization method, a two-dimensional (2D) high resolution particle velocimetry technique, and a 3D spectral element method. The wake flow is simultaneously investigated in two perpendicular planes using a "dual-plane" configuration. It appears that for Reynolds number around 100 and Richardson number larger than 1.0 thermal plumes occur in the far wake. Correspondingly distinct counterrotating vortices, with a spanwise wavelength of two cylinder diameters, are shown in the near wake. Furthermore, a difference in the flow motion for "in-plume" and "out-of-plume" positions is observed. The vortex shedding process for the "out-of-plume" positions is quite similar to the one for an unheated cylinder, which is a 2D flow. At in-plume positions, it is observed that the flow moves upward from the lower to the upper half of the wake. From the calculated temperature field, a region of unstable thermal stratification in gravity direction is observed at the in-plume positions. This indicates that the upward motion at the in-plume positions is induced by buoyancy when the temperature gradient is large enough.