Abstract
The height of 10∼12 km has been selected as an appropriate level for drifting of balloon bombs, for this level is the well-defined isopycnic level with maximum westerly currents. During the winter of 1944∼1945,9,000 balloons were sent out to the United States from Japan, for the winter season is characterized by heavy baroclinicy in middle latitudes and hence the greatest strength of the westerlies is found. Two or three days will be enough to send balloons through the upper troposphere if the conditions are favourable.
