From observations made from the ground and from aircraft it is considered that clear air thermals are continuing plumes. They form above the forced convection region near the surface; initially their temperature excess is about 1C, their size 200 m, and upward velocity about 1 m sec−1. The thermals rise through a neutral to slightly stable environment of thermally quiet, slowly descending air; however, the air both inside and outside the thermal is strongly turbulent. The thermal initially accelerates upwards and probably decreases in size. Air is continually mixing into and out of the thermal, and at levels above about 100 m the size remains sensibly constant. The whole field increases in temperature, partly as a result of the detrainment of warm air from the thermals and partly from downward motion of their stable environment.