124 THE BEASON WHY. 



" In the morning ye say, it will be foul weather to-day, for the sky is red and 

 lowering." MATT. xvi. 



atmospheric conditions, in which refractions and reflections are mul- 

 tiplied by the different densities of atmospheric layers. They chiefly 

 occur in hot countries, where, from the varying effects of heat, the 

 conditions of atmospheric refraction and reflection frequently prevail 

 in their highest degree. 



528. Why do we have twilight mornings and evenings ? 



Because the coming and the departing rays of the sun are 

 refracted and reflected by the upper portions of the atmosphere. 

 (See Fig. 13.) 



529. How long before the sun appears above the horizon 

 does the reflection of his light reach us? 



The time varies with the refracting and reflecting power of the 

 atmosphere, from twenty minutes to sixty minutes. But the sun's 

 position is usually eighteen degrees below the horizon when twilight 

 begins or ends. 



530. Why is the sky Hue ? 



The white light of the sun falls upon the earth without change ; 

 it is then reflected back by the earth, and as it passes through the 

 atmosphere portions of it are again returned to us, and this double 

 reflection produces a polarised condition of light which imparts to 

 vision the sensation of a delicate blue. (See 549.) 



531. Why do the clouds appear white ? 



Because they reflect back to us the solar beam unchanged. 



532. Why does the slcy appear red at sunset ? 



Because the light vapours of the air, which are condensed as the 

 sun sets, refract the rays of light, and produce red rays. The 

 refraction which produces red requires only a moderate degree of 

 density. 



533. Why do the clouds sometimes appear yellow ? 



Because there is a larger amount of vapour in the air, which 

 produces a different degree of refraction, resulting in yellow. 



534. Why does a yellow sunset foretell wet weather ? 



