REPORT ON ATMOSPHERIC CIRCULATION. 33 



summer months, thunder is quite unknown ; and places such as Coimbra and Lisbon, 

 where the summer rainfall is small and its occurrence rare, thunderstorms become 

 less frequent, and the hours of their occurrence become later than before and after the 

 dry season. Further, when during a particular season an anticyclone, with its great 

 descending current in the centre, remains over a region, as happens in the centre of the 

 old Continent during winter, thunder is equally unknown. 



In this connection much interest is attached to the thunderstorms of Mauritius, 

 arising from its isolated position in a vast ocean, and its relations to the great move- 

 ments of the atmosphere in that part of the globe. In this island there are two 

 maxima in the diurnal curve, the larger of the two occurring from noon to 4 p.m., and 

 the smaller from 3 to 6 A.M., these being the times of the two barometric minima, or 

 the times of maximum occurrence from the Challenger observations over the open sea 

 and inland at Oxford ; and two times of minimum occurrence, from 9 p.m. to 1 a.m. 

 and from 8 to 10 A.M., these being near the times of the barometric maxima. Another 

 important fact, as regards the thunderstorms of Mauritius, is, that during twelve years 

 none were recorded in June and July, one in August, one in September, and three in 

 October. Observations show that the annual period of thunderstorms is the seven 

 months from near the end of October to the middle of May, or during the time of the 

 greatest rainfall, while practically none occur during the other five months. In these 

 five months rain, however, continues to fall, amounting to an average of about two 

 inches each month. Thus, during these months, there is in the atmosphere the aqueous 

 vapour, and these being relatively dry months, there are also the conditions of ascend- 

 ing currents. There is, however, wanting another element essential to the electrical 

 manifestations of the thunderstorms during the relatively dry season of Mauritius. 

 Now during the months when thunderstorms are of no infrequent occurrence, the high 

 atmospheric pressure of Asia repeatedly advances, as Dr. Meldrum has pointed out, 

 southward towards Mauritius, so that frequently the belt of variable winds and calms, 

 between the two trades, stretches in a slanting direction from Madagascar to Ceylon. 

 While this distribution of pressure occurs with more or less frequency, the conditions of 

 a descending cold current of large volume are provided, and thunderstorms are fre- 

 quent ; and it is under analogous conditions afforded by the cyclones and anticyclones 

 of north-western Europe, that nearly all the winter thunderstorms in the west of 

 Scotland occur. But from June to September there is an unbroken increase of pressure 

 from Central Asia southwards to beyond Mauritius, thus placing it within this high 

 pressure area and in the heart of the south-east trades, and while this continues the 

 conditions favourable for the development of the thunderstorms are wanting. 



It has been shown that over the open sea thunderstorms are essentially nocturnal 

 phenomena. As regards thunderstorms over the land surfaces of the globe, the 

 disturbance of atmospheric equilibrium, resulting in ascending and descending currents, 



(PHYS. CHEM. CHALL. EXP. FART V. 18S9.) ^ 



