210 Proceedings of the Ohio Academy of Science 
types, will cause thunderstorms in Ohio only when an area of 
high pressure prevails over the eastern Lake Region or New 
England. See weather map of April 17, 1917, 7 A. M. As these 
types are usually followed by a high pressure area from the 
northwest of more or less intensity and therefore move with 
considerable rapidity, the thunderstorms incident thereto are 
apt to be of short duration and are seldom of a violent 
character. 
But to the approach of the South Pacific and Texas types is 
to be attributed by far the greater portion of the thunderstorms 
in Ohio. These types prevail from early May into late October. 
As a rule thunderstorms will set in over the western part of 
the State when the center of the “‘low’’ reaches Missouri or 
southern Illinois and will probably become general over the 
State. These cyclonic types often bring thunderstorms of a 
very violent nature. When the “‘low’’ passes over the north- 
western corner of the State, thus forcing the isotherms far 
northward of their normal position, and is followed by a “‘high”’ 
of moderate intensity, hailstorms are likely to occur with the 
shift of the wind—passage of the squall line—and subsequent 
increase in pressure. See weather maps of March 10 and 11, 
1917. The position of the Atlantic high does not seem to have 
any material effect on the rain-producing characteristics of these 
‘‘lows.’’ When the path of these cyclonic types suddenly 
curves to the north and passes into the Lake Region from 
northern Indiana or I]linois, thunderstorms are likely to occur 
in Ohio both on the approach and the passage of these areas. 
Normally, however, their passage just over or just south of 
the State is followed by brisk westerly winds, clearing weather 
and falling temperature. 
The East Gulf and South Atlantic types gave rise to no 
thunderstorms in Ohio during the year 1917. 
(b) The data seem to show certain centers of maximum 
activity and storm-frequency. The southwestern part of the 
State is certainly the most favorable portion for the develop- 
ment of the tornado as all tornadoes of consequence in the his- 
tory of the Bureau have occurred in that section. 
(c) Thunderstorms were reported on 169 days, midnight 
to midnight. Of these 169 days, thunderstesms occurred in the 
forenoon only on 22 days, on the afternoon only on 80 days, on 
