60 THE BOOK OF NATURE STUDY 



are those called cirrus, which are very lofty, varying from five to 

 nearly ten miles in height, and forming the " mare's tails" of 

 country people. They are popularly supposed to foretell wind, 

 but this is rather true of the modification called cirro-stratus. 

 Scattered cirrus clouds after stormy weather indicate generally the 

 probability of a return to calmer conditions, while the appearance 

 of cirrus after settled weather may indicate a break. Notes should 

 be taken of the appearance of these clouds at different times, if 

 only to show how difficult it is to lay down any certain predic- 

 tions of weather from the clouds. They will probably be noticed 

 to be very frequent in spring, especially in windy weather. The 

 origin of such cirrus clouds has been explained on the supposition 

 that the difference between the lower air, which is being warmed 

 by the sun, and the cold upper air is so great that air currents are 

 produced which extend upwards to a great height. The result is 

 that the ascending moist air is carried upwards to a great height 

 also before it reaches a still region where condensation can take 

 place. In general, cirrus clouds appear to be the final product of 

 cooling by ascent of moisture-laden air. 



Cumulus, stratus, and cirrus are the three main types of clouds. 

 A considerable number of other types have been recognised, but 

 their practical significance, at any rate in teaching, is not very 

 obvious, and there is not as yet entire agreement as to the names 

 which should be given to the different types. There are occasions, 

 however, especially in connection with thunderstorms, when it is 

 of interest to be able to give names to the different types of cloud 

 seen. The following are the terms most usually employed. Cirro- 

 cumulus is the name given to clouds consisting of dense roundish 

 masses forming what is commonly called a " mackerel" sky. Cirro- 

 cumulus clouds occur in the middle layer of the atmosphere, as 

 compared with the lofty cirrus and the relatively low cumulus. 

 In winter the appearance of such a mackerel sky often denotes the 

 approach of a thaw ; this is a point which may very well be studied. 

 In warm summer weather cirro-cumulus clouds sometimes appear 

 after rain, and then mean heat. If, on the other hand, the cloudlets 

 are thick and show a tendency to aggregation in hot summer 

 weather, a thunderstorm is likely to follow. 



Cirro-stratus clouds occur at considerable altitudes (average 



