784 The Outline of Science 



minute droplets of water or crystals of ice, that have condensed 

 out of the air because the latter has become too cool to retain 

 them in the form of water-vapour. Our skies are seldom without 

 a cloud ; we watch them with ever fresh wonderment and curiosity. 

 The forms of clouds are endless and ever-changing, but there are 

 certain characteristics which allow them to be grouped roughly 

 into several classes. There are the thin feathery lines and bands 

 of cirrus, commonly called "mares' tails," floating nearly five 

 miles high ; these are formed of ice-crystals and give rise to many 

 beautiful sights in the sky, such as the wide "rings" round sun and 

 moon, called halos. Somewhat lower are the "speckled," 

 "mackerel," arid "dappled" skies of cirro-cumulus and alto- 

 cumulus, the most beautiful of all clouds; we see a wonderful 

 arrangement of orderly and serried ranks. Between broad, 

 straight, parallel bands of snowy-white cloud we have a multi- 

 tudinous sea of little wavy rippling cloudlets. These often show 

 close "rings" of rainbow colours (corona?) round sun and moon, 

 and they are generally seen during a spell of fine weather. In 

 place of these types there is sometimes seen the "watery" sky, 

 known as alto-stratus, a greasy-grey sky with a patch of pallid 

 light where the sun or moon may be dimly showing; this sky 

 usually follows after the appearance of mare's tail cirrus in front 

 of a depression and is practically always succeeded by rain 



Thunderstorm and Hail 



Lower still, about a mile high, we see the great rolling masses 

 of clouds heaped up in the sky which are called the cumulus, the 

 ordinary domed "woolpack" clouds of the summer sky; they are 

 the dark clouds with the silver lining ; the cloud is due to moisture 

 carried upwards by ascending air-currents which have been 

 warmed by contact with the ground. Here we have also the 

 heavy grey ragged pall of nimbus the rain-cloud. The gigantic 

 cumulo-nimbus or "thunder-cloud" may sometimes grow till it 

 becomes three miles deep from summit to base! It is simply a 



