DISCUSSION 103 
physical mechanism explaining how thunder- 
storms form and move in the manner I de- 
scribed. 
Dr. Hitschfeld—In the other study to which I 
have already referred, Report MW29, we at- 
tempt to re-interpret Dennis’ (J. Met. 11, 157- 
162, 1954) observations of seeding trails as 
plumes. Such plumes can travel considerable dis- 
tances and thus can disseminate nuclei widely. 
It is possible that this might account for Mr. 
Namias observation that a favored direction of 
storm propagation 1s that of the high-level winds. 
