THE SKY 61 



29,500 feet), and take the form of thin, horizontal streamers, 

 with bent or undulated edges. Such cloud sheets may sometimes 

 be seen in summer forming above the upward limit of the ordinary 

 cumulus clouds. They are also generally associated with thunder- 

 storms, and almost always occur on the advancing side of cyclonic 

 storms. Thus at all seasons the appearance of much cirro-stratus 

 may be said to presage rain and probably wind. The mingling 

 of cirro-stratus clouds with cumulus produces the condition 

 known as cumulo-stratus (or strato-cumulus) . Such cloud masses 

 often form heavy banks, and then a part of the cloud descends as 

 rain, forming the so-called nimbus. Cumulo-stratus clouds often 

 accompany windy weather with some rain ; the appearance of 

 masses of these clouds indicates a change in the weather. Some 

 other cloud modifications are recognised by meteorologists, but 

 it seems only necessary to speak of nimbus, a somewhat indefinite 

 name, given to any form of cloud from which rain is descending. 

 Thus a cumulo-stratus cloud from which rain is descending or is 

 about to descend is called cumulo-nimbus. In its most char- 

 acteristic form nimbus accompanies thunderstorms, and is then 

 seen as a dense black or very dark curtain, topped by cumulus, 

 or some modification of this form, which is in its turn crowned 

 by some cirrus modification. 



THUNDERSTORMS. As a general rule it is unlikely that it will 

 be found possible to make with young children detailed observa- 

 tions on thunderstorms, sufficient detachment of mind being 

 rarely available. With minor and distant storms some observa- 

 tions can, however, be readily made. It should be noted that as a 

 general rule such storms come in the hottest part of the year and 

 of the day ; they are generally also commoner in hot summers 

 than in cold ones. When severe they are frequently associated 

 with showers of hail. This should be noted, for many people do 

 not clearly realise that hail is more frequent as a concomitant of 

 a summer thunderstorm than in winter, 



The first sign of the approaching storm is often a layer of 

 cirro-stratus cloud, followed by heavy masses of storm clouds of 

 the cumulus type, often called cumulo-nimbus, from the dense 

 rain which pours from their lower edge. Before the rain falls 



