The study of thunder storms has received very elaborate and extensive consideration. M. Ciro Ferari in Italy finds that almost invariably the storms come from directions between north and northwest, the tendency in northern Italy being directly from the west, and in the more southern sections from the northwest. The velocities of storm movements are much greater from the west than from the east, considerably more so in the centre and south of Italy than in the north; and in the months, largest in July.
The velocity of propagation increases with greater velocities of the winds accompanying the storms, with also greater attendant electrical intensity. The front line of propagation while more often curved, is sometimes straight and sometimes zigzag, and appears to undergo a series of successive transformations, more or less affected by the topographical nature of the country passed over.
Ferari thinks their principal cause is to be found in high temperatures coincident with high vapor pressures. Thunder storms, he considers, are essentially local phenomena, superposed on the general atmospheric phenomena. A principal general cause of thunder storms in Italy is the existence of a deep depression in northwest Europe, with a secondary depression in Italy dependent on the first. This secondary feeble area remains for several days over upper Italy, and nearly always is followed by thunder storms. Minimum relative humidity precedes, and maximum follows a storm, while the vapor pressure conditions are exactly reversed. Ferari notes, as one matter of interest, the passage of fully developed thunder storms from France into Italy over mountains 4,000 metres (13,000 feet) in elevation.
Dr. Meyer, at Gottingen, has investigated the annual periodicity of thunder storms, while Carl Prohaska has made a statistical study of similar storms in the German and Austrian Alps. The latter writer thinks they are most likely to occur when the barometer is beginning to rise after a fall, thus resembling heavy down-pours of rain.
In connection with Schmucher's theory on the origin of thunder storm electricity, Dr. Less has been able to satisfactorily answer in the affirmative an important point in the theory, as to whether the vertical decrement of temperature is especially rapid. Less finds evidences of very rapid decrement of temperature during thunder storms, as shown by the examination of records of 120 stations for ten years.
Mohn and Hildebrandsson have also published a work on the thunder storms of the Scandinavian peninsula. The rise in the barometer at the beginning of rain, they agree with Mascart in attributing largely to the formation of vapor and the evaporation of moisture from rain falling through relatively dry air.
A. Croffins has discussed thunder storms at Hamberg from observations for ten years. He believes that all such storms are due to the mechanical interaction of at least two barometric depressions.
As a matter of interest bearing on the much discussed phenomena of globular lightning, an incident is recounted by F. Roth, where a man feeding a horse was struck by lightning and lost consciousness. The man states that he felt no shock, but was suddenly enveloped in light and that a ball of fire the size of his fist, traveled along the horse's neck. This points to the fact that "ball" lightning is probably a physiological phenomenon.
In view of the recent extended interest in the question as to whether the climate of the United States is permanently changing, it should be remarked that this question has lately been under consideration with regard to Europe. Messrs. Ferrel, Richter, Lang, Bruchen and others conclude, from an examination of all available data, that there is no permanent climatic change in Europe. In connection with this discussion in Europe, long series of vintage records, going back to the year 1400, have been used. Apart from the ocean borders, extensive simultaneous climatic changes occur over extended areas, which changes—as might be expected—are more accentuated in the interior of the continents. These changes involve barometric pressure, rainfall and temperature, which all recur to that indefinite and complex phenomenon—the variation in the amount of heat received by the earth. The idea is advanced that these oscillations have somewhat the semblance of cycles, the period of which is thirty-six years. It may easily be questioned, however, in view of the fragmentary and heterogeneous character of the data on which this assumption is based, whether the error in the observations is not greater than the range of variation. Blanford, in one of his discussions, has pointed out that the temperature or rainfall data in India can be so arranged as to give a cycle with a period of almost any number of years, but, unfortunately, the possible error of observation is greater in value than the variations.
As to the United States, it is pertinent to remark that the Signal Office is in possession of temperature observations in Philadelphia, covering a continuous period of one hundred and thirty-two years. The mean annual temperature for the past ten years is exactly the same as for the entire period.