620 



THE NATURE BOOK 









THE GROWTH AND MOVEMENT OF A 

 OBSERVED AND DRAWN BY PROF. 

 YORK, JULY, 1887. 



the storm from its birth to its fully- 

 fledged state. 



In these islands thunderstorms are due 

 to excessive heating of the air at the 

 earth's surface, which causes up-currents 

 of air to rise into the higher regions of 

 the atmosphere. It is only natural that 

 these storms occur most frequently in 

 our summer montlis, for then the sun is 

 abo\-e the horizon, higher in the 



longer 



tinned ascent of the 

 moist warm air makes 

 it in time assume enor- 

 mous proportions. To 

 watch such a cloud 

 grow in size is of the 

 greatest interest and 

 beauty, for it usually 

 begins to form in a 

 cloudless sky, and its 

 extreme whiteness, with 

 a background of blue, 

 renders it a striking 

 object. As its dimen- 

 sions increase, fold upon 

 fold tower one upon 

 the other on the upper 

 surface. Such an em- 

 bryo thunder-cloud is 

 seen in the illustration 

 on page 6ig, which is 

 from a photograph I 

 secured at South Ken- 

 sington in the spring of 

 iqo6. This cloud, be- 

 fore it passed out of 

 sight, grew to considerable dimensions, 

 the billowy masses rolling upwards tier 

 upon tier, giving it a magnificent 

 appearance. 



In the initial stages of formation a 

 thimder-cloud has a flat under surface and 

 a billowy top. It, however, undergoes con- 

 siderable changes before it can be stated 

 to be properly de\'eloped. Perhaps the 

 best idea of these transformations will be 



THUNDER-CLOUD, AS 

 W. M. DAVIS, AT NEW 



sky at midday, and therefore under the gathered from a very interesting series of 



drawings which were made by a distin- 

 guished American meteorologist. Professor 

 William M. Da\-is, and here reproduced. 

 These drawings show the increasing size 

 and change of form of the thunder-cloud 

 which passed over New York on July 2, 



best conditions to heat the earth's surface 

 to an excessive extent. 



The rush of warm and moist air into 

 more elevated and cooler regions gives 

 rise to cloud, for there the air is cooled 

 and the water vapour is condensed. 

 Such a cloud is 

 termed a " cu- 

 mulus " cloud, ^_ „^ _^ _ 



because it has 

 aheaped-up 

 appearance hke 

 a tuft of wool. 

 N o w, such a 

 cloud forms the 

 embryo of a 

 thunderstorm. It 

 is at first smaU, 

 but the con- 



f 



Tij-' 



PROF. KOPPEN'S DIAGRAM OF A VERTICAL SECTION OF A THUNDERSTORM. 



