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The mean jet stream over the Indian sub-continent and Burma has been computed from all available radiosonde data and is found to exist at about 27°N at an altitude of 12 km. No secondary' equatorial jet' as postulated by Chaudhury (1950) is found. A study of the mean distribution of thermal gradients and lapse rates in the troposphere has been made both for the Indian cross section and the Hess' cross section for North America. The thermodynamic properties of the jet stream typified by the cross sections have been compared. It is found that the gradient of the lapse rate concentrates into a maximum in the vicinity of the jet stream. The occurrence of the two types of jet streams is discussed from thermodynamic considerations.