vin Atmospheric PJienomena 115 



gives rise to halos, wide faintly-coloured rings which appear 

 to surround the Sun or Moon. The name Mare's tail is 

 sometimes applied to it. Little rounded tufts which often 

 cover the whole sky in summer and are familiarly called 

 mackerel scales belong to a class of cloud which floats 

 about 3 miles above the Earth's surface, and may be looked 

 upon as half-way between cirrus and the next type ; they 

 are termed cirro-cumulus. Cumulus is the cloud type 

 which comprises the great white billowy clouds common in 

 summer. They are usually flat on their under surface, and 

 rise above into rounded forms often of wonderful beauty. 

 The base of cumulus cloud is usually about ^ of a mile 

 above the Earth's surface, while the summits may rise as 

 high as 2 or 3 miles. These clouds are formed by the 

 condensation of vapour in ascending currents of air, and 

 each mass of cumulus has been likened to a grandly carved 

 capital topping the invisible column of rising heated air. 

 The lowest clouds are sheets of fog floating within half a 

 mile of the Earth's surface, and being so low they are 

 usually seen edgeways when at any distance, and so appear 

 as long layers parallel to the horizon. This arrangement 

 gave rise to their name of Stratus, A cloud, presenting a 

 dark gray or black colour and a ragged stormy appearance, 

 from which rain usually falls is called Nimbus, or simply 

 rain-cloud. It forms at the elevation of about a mile, and 

 is described as a mixture of cumulus and stratus. The 

 upper clouds act as floats, by the study of which much has 

 been learned as to the movements of the upper atmosphere. 

 The lower clouds are of great value as heat curtains, pre- 

 venting the Sun's heat from being excessive by day, and 

 almost entirely Checking the loss of heat by radiation from 

 the Earth at night. 



169. Rain. Sometimes the temperature of air remark- 

 ably free from dust falls below the dew-point, and a large 

 quantity of water-vapour must condense, while there are 

 very few solid motes to act as nuclei. Each mote conse- 

 quently gets a very heavy coating of water, and drops are 

 formed which are too large to be much checked by friction 

 of the air as they fall. Thus a shower of rain may fall 



