Satellite observations and ERA-Interim reanalysis data have been used to investigate the seasonal transition of atmospheric heating source (AHS) in the Northern Hemisphere from winter to summer and its possible causes in the present study. Results show that the earliest formation of summertime diabatic heating occurs over South China in early April, which is closely associated with the development of rainy season to the South of Yangtze River. The sensible heating is dominant in the AHS during February-March when the major component of total rainfall is the large-scale precipitation. After early April, convective precipitation becomes more important, corresponding to increasing importance of condensation heating in the AHS. Diagnostic analysis suggests that seasonal changes in the thermal condition over the southern Tibetan Plateau (TP) are critical for this seasonal transition of the rainfall character and the AHS. During February-March, the sensible heating over the TP is becoming stronger with the enhancement of the solar radiation. The westerlies strengthen the warm advection in the middle troposphere over South China, leading to stronger ascending motions and large-scale precipitation that increase the soil moisture. This is favorable for the subsequent rapid development of local convective precipitation. Further enhancement of surface sensible heating over the TP can make low-level moisture to converge to the north by its "air pumping" effect during April-May, which intensifies the low level southerly winds. As a result, abundant water vapor are transported from the South China Sea and western Pacific to the South of the Yangtze River, where convective precipitation strengthens and the resultant condensation heating becomes dominant in the AHS.