ATMOSPHERIC I'HENOMENA. 179 



nocturnal thunder-storms being peculiar to certain val- chap, xvi 



leys of rivers which have a particular climate." 



It may be interesting to present a very brief state- Explanation 

 ment of Humboldt's explanation of these phenomena : °\^^'^ 

 — 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 tlie north-east blows, it prevents 

 the atmosphere from being saturated with moisture. 

 Tlie hot and loaded air of the torrid zone rises and flows 

 off again towards the poles, while inferior currents from 

 these last, bringing drier and colder strata, take the 

 place of the ascending columns. In tliis manner the 

 humidity, being prevented from accumulating, passes 

 off towards 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 temperature 

 between the troj)ics and tlie contiguous zone is least. 



The column of air resting on the equinoctial zone ,r , 



° , . . , Vapour ana 



becomes replete with vapours, because it is no longer clouds. 



renewed by the current from the pole ; clouds form in 



this atmosphere, saturated and cooled by the effects oi 



radiation and the dilatation of the ascending air, which 



increases its capacity for heat in proportion as it is rari- 



lied. Electricity accumulates in the higher regions in 



consequence of the formation of the vesicular vapours, 



the precipitation of which is constant during the day, 



but generally ceases at night. The showers are more 



violent, and accompanied with electrical explosions, 



shortly after the maximum of the diurnal heat. These 



phenomena continue until the sun enters the southern 



signs, when the polar current is re-established, because 



the difference between the heat of the equiuoctial and 



temperate regions is daily increasing. The air of the 



