Thermal excitation as the cause of semidiurnal atmospheric oscillations is discussed, special emphasis being put upon the formation of node around 30 km. It is theoretically shown that if somewhere in the mesosphere above around 30 km is thermally excited only one node is formed around 30 km. The height of the node is dependent very little upon the temperature of the atmosphere which is assumed isothermal. No node is formed by tropospheric thermal excitations. Provided that the existence of 30 km node is true, the place of thermal excitation responsible for the semidiurnal oscillation seems to be in the mesosphere