Feb., 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



33 



Svin 



Our 

 QLiid "Weather." 



By William J. S. Lockyer, M.A., Ph.D. 



(continued.) 



In confining this study, therefore, to pressure, the first 

 step is to see whether the pressure does change from year 

 to year, and then, if it does, to see if the curves which 

 indicate this change are similar to those which represent the 

 variation in the number of the solar prominences. Taking 

 the Indian region, for reasons previously given, the reader 

 will notice that the Bombay curve in fig. 7 does exhibit 

 short period waves which agree for many years with those 

 on the solar prominence curve ; it is important to note, 

 however, that the main eleven-year variation of the 

 prominences is not so conspicuous as the 

 shorter-period changes. The apparent se- 

 condary nature of the former and the pro- 

 nounced character of the latter is a con- 

 spicuous feature of pressure curves nearly 

 all over the world. It will be gathered, 

 therefore, that greater attention must be given 

 to this short-period barometric change. 



Since the rise in the prominence curve 

 denotes greater solar activity, and this is 

 coincident with an excess of atmospheric 

 pressure over the Indian area, and since this 

 latter means that a greater amount of air than 

 usual is piled over India, some part of the 

 world should be experiencing the reverse 

 conditions ; in other words, there should be a 

 large area on which a deficiency of atmos- 

 pheric pressure exists simultaneously. Now 

 this is exactly what happens, only one has to 

 go to the other side of the world to find the 

 locality. In such a region, then, the curve 

 representing the pressure variation should be 

 the reverse of that of India, that is, when 

 there is excess pressure in one year in India 

 there should be in the same year a deficiency 

 in the other. If the reader will glance at the 

 accompanying diagram (fig. 7) he will see the 

 curve of the pressure variation at the observatory 

 at Cordoba in South America, a locality in nearly the 

 antipodal part of the world to India. This curve is 

 nearly the exact opposite in every detail, and if one 

 be reversed and compared with the other their similarity 

 can be more easily observed. 



1870 18800 1890 1900 C 



PR0MINENCES5O- 



The fact that when theic ;„ ..;. l....^.,, amount of air 

 over the Indian area in some years, and a corresponding 

 deficiency over the Cordoba region during the same 

 years, or a deficiency over India when there is an excess 

 over Cordoba, makes one immediately intiuire, What 

 occurs at other places on the earth's surface ? Such an 

 investigation has led to some most interesting conclu- 

 sions. Australia, for instance, Hke Arabia, Ceylon, East 

 Indies, Straits Settlements, East Africa, Mauritius, Sic, 

 behaves like India. On the other hand. South America, 

 the southern parts of the United States, and Honolulu, 

 resemble the Cordoba type of pressure variation. Thus 

 we have the world divided into two portions which 

 behave in opposite ways as regards these barometric 

 changes. As was to be expected, those regions neigh- 

 bouring the limits of these two large areas are somewhat 

 indeterminate, and sometimes favour the one and some- 

 times the other. The accompanying map (fig. 8) will 



Fig. 8.-Ma| 

 India ( + 

 other, 

 portions. 



.—Curves to illustrate the wavy nature of the eleven. year 

 prominence change and its relation to the atmospheric pressure 

 variations that simultaneously occur in India and S. America. 



I illustrating the positions of the two large pressure area types, namely 

 and +'?) and S. America l— and — V), which behave inversely to each 

 The ntutrat tine approximately divides the earth into two equal 



convey at a glance this pressure distribution, the Indian 

 and Cordoba regions being indicated by a(-f)and(— ) 

 respectively ; the indeterminate areas are shown by 

 a (+ ?). The latter can be easily seen by following the 

 track of the neutral line which approximately divides the 

 eastern and western hemispheres. 



The detection of this pressure variation may turn out 

 to be an important clue to the close connection between 

 the meteorological behaviour of regions which are widely 

 separated. Thus to mention one of many incidences, Sir 

 John Eliot has recently pointed out that the drought in 

 the Indian region during the years 1895-1902 was a 

 more or less general meteorological feature of the 

 whole area, including Abyssinia, East and South Africa, 

 Persia, Baluchistan, Afghanistan, probably Tibet, and the 

 greater part or whole of Australia. Since these areas all 

 lie within the Indian type of pressure variation above de- 

 scribed, their meteorological connection seems undoubted. 



Many people are more familiar with rainfall variations 

 than they are with those of pressure, so that the impor- 

 tance of the latter can best be shown by indicating how- 

 rainfall is affected by pressure. As a general rule low- 

 pressure means increase of rain, but this is not always 

 the case. The main point to be considered in this con- 

 nection is the nature of region, that is, whether it is land 

 or water over which the air current has passed, before it 

 reaches the area in question. Thus what may be a rain- 



