Nov., 1921 THUNDERSTORMS 35 



observers, about 130 co-operative observers and the six regular 

 Weather Bureau stations in the State. We also received some 

 assistance from the telephone and telegraph companies in the 

 State and even dealers in lightning rods. 



3. Forms and Instruction. 



Each observer was then supplied with full instructions and 

 a suitable card on which to make his report of each storm. 

 This form called for the exact date and time of the storm, the 

 exact location of the observer, time first, loudest and last 

 thunder was heard, direction the storm moved, time rain 

 began and ended, time hail began and ended, direction of wind 

 before and after the storm, etc. The weakness of the plan was, 

 of course, in the fact that it was dependent upon voluntary 

 service and as was to have been expected, some observers 

 failed us at the critical moment, so that we were not always 

 sure we had the complete history of each storm. However, we 

 assembled quite a mass of thunderstorm data and these have 

 been charted and otherwise prepared for publication. 



3. A Resume. 



Among the many interesting facts brought out in the 

 special study of thunderstorms in Ohio during the year 1917, 

 may be mentioned, briefly, the following : 



(a) Thunderstorms in Ohio are incident to the passage of 

 those cyclonic areas (see M. W. R., Supplement No. 1) that 

 move directly over or just north of the State, and to the 

 approach of those that move just south of the State. The first 

 group includes the Alberta, the North Pacific, the Rocky 

 Mountain and probably the Central and Colorado types; the 

 second group includes the South Pacific and the Texas types, 

 especially those that follow a northeasterly course. 



The passage of the Alberta type, especially in the late 

 winter or early spring months, will cause thunderstorms in 

 Ohio when the wind-shift line, or "squall line," is pronounced, 

 and extends in a north-south, or a northeast-southwest direc- 

 tion. These thunderstorms will set in slightly in advance of this 

 line and will continue until it has passed. See weather map 

 of January 31, 1917, 7 A. M. The passage of the North Pacific, 

 the Northern Rocky Mountain, the Central, and the Colorado 



