106 



KNOWLEDGE 



[May, 1902. 



they are gi.'ueralh- jiieces carried off by the winds from 

 the maiu bodies of clouds. 



Occasionally two forms of cloud are present at the same 

 time. This is ordinarily taken as a case of Cumulus and 

 Stratus, and has become known as Cumulo-stratus ; but, 

 if observed in the zenith, it may readily be noted that the 

 two forms of cloud are distinct, and they had bettor be 

 dealt with separately. The appearance of Cumulo-stratus 

 is an effect of persjiectivc. (Pig. l~ and Plate IV,) 



Clouds float at varying altitudes, according to the lati- 

 tude and elevation of the ground; the vertical temperature 

 and adiabatic gradients determining the level at which the 

 vapour becomes visible as cloud. It is desirable in all 

 cloud observations, that note should be made of the ap- 

 proximate relative altitudes of clouds and of their velocity 

 of motion. This is particularly desirable when dealing 

 with the stratiform clouds, whether as ordinary Cirro- 

 cumulus or as very high Cirro-macula. 



The rate at which they move may be noted from the 

 following table : — 



0. — No perceptible motion. 

 1. — Just moving. 

 2. — Moving at moderate speed. 

 3. — Moving quickly. 



The beautiful colouring of clouds results from the 

 breaking up of light beams in passing through them or 

 along their edges. This phenomenon is caused by diffrac- 

 tion, and to it is due our lovely sunrises and sunsets. 

 When the sun is high in the heavens, the light is white, 

 but as the orb nears the horizon, and its rays pass through 



Fio. 9.— Higli Stratus, Mackerel Sky. 



thicker layers of atmosphere, the smaller light waves get 

 gradually cut off, until the sun sinks as a red ball below 

 the horizon. The largest waves of light produce the red 

 rays and the after glow which are so beautiful. Sunrise 

 and sunset effects are matters of much interest, but are of 

 too complicated a nature to be fully gone into here ; we must, 

 however, notice them briefly, because of their importance in 

 weather forecasts. Soft sunset colours indicate tine settled 

 wcathei' ; fiery brilliant hues denote change to stormy or 

 wet weather. 



Other colour effects in clouds are due to phenomena, 

 known as halos and corona;. Halos appear as rings round 

 the sun and moon ; they are caused by the shining of the 

 orb through very high Stratus or Cirrus clouds, and have 

 a diameter of 42" (Fig. 11). Sometimes shades of colour. 



Fig. 10.— Cirrus. 



resembling those of a rainbow, are visible — red appears 

 on the inside and blue ou the outside. These rings of 

 colour are due to the reflection and refraction of light 

 passing through the fine ice crystals of which high Stratus 

 or Cirrus clouds are composed. Occasionally a compli- 

 cated series of beautifully coloured rings is noticeable, 

 producing the effect represented iu the accompanying 

 picture (Fig. 13). Generally speaking, these rings are 

 due to the thinness of the high cloud through which 

 the light is passing. Still more curious arrangements of 

 halos sometimes occur. 



Corouse are broader rings seen quite close to the sun or 

 moon, and are due to the shining of light through the 

 edges of loose Cumulus or Stratus clouds. They have red 

 on the outside and blue ou the inside of the ring ; the 

 colours are, generally, easily distinguishable. The more 

 brilliant hues occasionally seen, as has been said, in the 

 vicinity of the sun and moon, would appear to be incom- 

 plete sections of circles intermediate in size between 



Fig. 11.— Halo. 



coroufE and halos. An interested observer will be well 

 repaid if he chooses to study more closely the many 

 curious ojitical phenomena connected with clouds, but it 



