. Compendium of meteorology. Meteorology. Ol40 > X 34 DAYS — MEAN 140 ^^ ^-_JAN^_^,-°'^,,-<-rr];f^^^7;^^ '' 120=-^^ .^''^ ^^ 14 DAYS< 100 140 innlV^ Fig. 7.—Diurnal variation of potential gradient on the oceans, Carnegie Cruises IV, V, and VI, 1915-21 and Cruise VII, 1928-29. concerning the electrical constitution of storms, but changes of types (6) and (c) are of minor interest as geophysical phenomena, especially when thej^ depend upon the activities of man. The discovery of the universal diurnal variation of potential gradient was delayed because most of the at- mosph
. Compendium of meteorology. Meteorology. Ol40 > X 34 DAYS — MEAN 140 ^^ ^-_JAN^_^,-°'^,,-<-rr];f^^^7;^^ '' 120=-^^ .^''^ ^^ 14 DAYS< 100 140 innlV^ Fig. 7.—Diurnal variation of potential gradient on the oceans, Carnegie Cruises IV, V, and VI, 1915-21 and Cruise VII, 1928-29. concerning the electrical constitution of storms, but changes of types (6) and (c) are of minor interest as geophysical phenomena, especially when thej^ depend upon the activities of man. The discovery of the universal diurnal variation of potential gradient was delayed because most of the at- mospheric electric observations, prior to about the year 1915, were made near cities or at other places where field changes of types (a), (6), and (c) are prevalent. This feature, first noted by Mauchly in 1921 [6], has great importance for the subject. His analysis of the data for potential gradient over the oceans, obtained on Cruises IV, V, and VI of the magnetic survey yacht, Carnegie, showed that the diurnal variation of the gra- dient at sea proceeds according to universal time, not according to local time. This is illustrated by the results which are exhibited in Fig. 7. Average values of potential gradient, in volts per meter, are there plotted as ordinates against the hours of the Greenwich day, counting from midnight to midnight. The combined results for Cruises IV, V, and VI are showTi separately from those for Cruise VII. The latter, although generally of greater amplitude, vary in nearly the same manner as the former. If at- tention is fixed on either of the two lowest graphs, which represent hourly averages for the year, it is to be seen that the gradient is smallest about four hours after Greenwich midnight and greatest at about 19''. That this feature varies somewhat throughout the year is sho\vn by the other graphs which represent different quarters of the year. When these same data are aver- aged for hours of the local day, no significant diurnal variation is
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Keywords: ., bookcentury1900, bookcollectionbiodivers, booksubjectmeteorology
