CHAPTER III 



TROPICAL CLIMATOLOGY 



Preliminary remarks — Climate — Tropical climates — Temperature and 

 humidity — Pressure — Winds — Electrical conditions — Sun's rays- — 

 Moon's rays — Acclimatization — References. 



PRELIMINARY REMARKS. 



We have delineated the tropics by Supan's lines, as this appears 

 to us to be the most suitable method of bounding warm climates, 

 and we have noted the characters of the races of mankind which 

 occupy the lands so defined. These, we may incidentally mention, 

 amount to a little less than one-half of the surface of the earth, 

 and now we must consider what is meant by the terms ' climate ' 

 and ' tropical climates,' and then turn our attention to the effects 

 of these climates on mankind. 



CLIMATE. 



Climate may be defined as the combined effects of the sun, the 

 atmosphere, and the earth upon living objects at any one place 

 on the earth's surface. The factors which produce a climate are 

 therefore threefold — viz., the sun, the atmosphere, and the earth; 

 and the living object with which we are concerned being man, we 

 must study the effects of these factors upon him, but we restrict our 

 attention to mankind as seen on that portion of the earth's surface 

 called the tropics. 



TROPICAL CLIMATES. 



In the previous chapter we have noted Supan's simple division 

 of warm climates by isotherms, but there is a better subdivision 

 given by Ward, and based upon wind systems and their control 

 over rainfall (vide Fig. 7). 



Around the Equator lies the region of equatorial calms, called 

 the doldrums (Fig. 7), while north and south of this region are the 

 trade winds (Fig. 6). These zones of calms and winds are not 

 stationary; on the contrary, they move so as to reach a maximum 

 extension northwards of some five to eight degrees of latitude in 

 July and a maximum extension southwards of some three to four 

 degrees in January. 



These movements are dependent upon the changes in the inclina- 

 tion of the axis of the earth towards the sun, but are not coincident 

 with these changes. 



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