The opposite is true at the top of the boundary layer, but we rarely fly that high. There are, as always, complications. Sometimes we fly in the surface layer and sometimes in the lower part of the boundary layer. These plumes converge near the top of the surface layer to form thermals. The surface layer to boundary layer transition is not sharp, so we often find ourselves flying in either well-organized thermals or disorganized plumes, or some of both.
Thermals evolve over time, are influenced by terrain, and are shaped by and move with the wind.
Boundary layer thermals form and dissipate with time scales of 10-30 minutes, surface layer plumes faster. If the wind is light, thermals may stay attached to the hot spot. If not, thermals may form repeatedly over the hot spot and drift downwind. Thermals drift with the average wind over their height, so they may travel at a higher speed and in a somewhat different direction than the surface wind. Thermals also tilt if the wind is stronger at higher altitude, the usual case.