178 ATMOSPHERIC PHENOMENA. 



face. The storm rises in the plains two hours after 

 the sun passes through the meridian, and therefore 

 shortly after the period of the maximum of the di- 

 urnal heat in the tropics. In the inland districts it 

 is exceedingly rare to hear thunder at night or in the 

 morning, nocturnal thunder-storms being peculiar to 

 certain valleys of rivers which have a particular 

 climate." 



It maybe interesting to present a very brief state- 

 ment of Humboldt's explanation of these phenome- 

 na : — The season of rains and thunder in the northern 

 equinoctial zone coincides with the passage of the 

 sun through the zenith of the place, the cessation 

 of the breezes or north-east winds, and the frequency 

 of calms, and furious currents of the atmosphere 

 from the south-east and south-west, accompanied 

 with a cloudy sky. While the breeze from the north- 

 east blows-, it prevents the atmosphere from being 

 saturated with moisture. The hot and loaded air of 

 the torrid zone rises and flows off again towards the 

 poles, while inferior currents from these last, bring- 

 ing drier and colder strata, take the place of the 

 ascending columns. In this manner the humidity, 

 being prevented from accumulating, passes off to- 

 wards the temperate and colder regions, so that the 

 sky is always clear. When the sun, entering the 

 northern signs, rises towards the zenith, the breeze 

 from the north-east softens, and at length ceases ; 

 this being the season at which the difference of tem- 

 perature between the tropics and the contiguous 

 zone is least. The column of air resting on the 

 equinoctial zone becomes replete with vapours, be- 

 cause it is no longer renewed by the current from 

 the pole ; clouds form in this atmosphere, saturated 

 and cooled by the effects of radiation and the dilata- 

 tion of the ascending air, which increases its capacity 

 for heat in proportion as it is rarefied. Electricity 

 accumulates in the higher regions in consequence 

 of the formation of the vesicular vapours, the pre*. 



