CONCERNING THUNDERSTORMS 



621 



1887, and they illustrate at tlie same imiicates. lurther. both the high and the 

 time the progressi\'e movement of the low clouds, including the very ele\atrd 

 cloud over the land. cirro-stratus cloud, which, as pre- 



Looking at the first of the four sketches, viously stated, considerably precedes 

 the cloud will be obser\-ed to be o\'er the front of the storm. It will also be 

 positions a and b, and to be an ordinary noticed, by observing the small arrows 

 cumulus cloud at eleven o'clock. By (which blow with the wind) near the 

 1 1. 15 it had grown considerably, both front of the storm, that as the whole 

 vertically and laterally, and it will be storm moves from left to right, the (jb- 

 noticed not only to have travelled to c ser\^er at a. who is stationary, will think 

 but to ha\'e 

 extended its 

 uppermost front 

 in the direction 

 of travel. At 

 11.40 its base 

 had decreased in 

 height and its 

 uppermost front 

 and back por- 

 tions assumed a 

 mushroom r 

 overhanging ap- 

 pearance. The 

 cloud was then 

 over the posi- 

 t i o ns c, D, 

 and E. 



In the last 

 drawing, made 

 at 12.45. or 

 more than one 

 hour later, it 

 had nearly 

 passed over e, 

 and assumed 



the t>T3ical form of a well developed 

 thunder-cloud, namely, a form something 

 like an anvil. 



When one considers the great mass of 

 cloud involved, the large area over 

 which it extends, and the comparatively 

 small altitude of the under surface, it 

 is not surprising that considerable dark- 

 ness prevails on the surface over which 

 the storm travels. 



The study of a thunder-cloud, with its 

 attendant winds, would not be com- 

 plete without a brief reference to a 

 very instructive diagram (on the oppo- 

 site page) which sums up the main 

 facts above described. We owe this dia- 

 gram to the celebrated German meteor- 

 ologist. Professor Koppen. This vertical The accompanying illustration shows 

 section of a storm displays the anvil- the profile view of the front portion of a 

 shaped form of the whole mass. It thunderstorm as it moved from left to 



79 



THE FRONT PORTION OF A THUNDHK-CLOL D MOVING FROM 



LEFT TO RIGHT. 

 Photographed by Dr. Lockyer at Sandown, Isle of Wight, in Aug., 1906. 



that the storm is coming up against the 

 wind. As a matter of fact, the observer 

 is only in a large current of air which 

 flows in towards the base of the storm- 

 cloud, and must therefore blow in the 

 opposite direction to that in which the 

 storm is tra\'elling. When the storm has 

 moved on a little, and the observer is at 

 position B in relation to it. he will then 

 feel the full blast of the storm, the wind 

 here being in the same direction as that 

 in which the storm is traveUing. 



These apparently opposing wind cur- 

 rents which exist near the front of the 

 storm thus clearly explain the very 

 comnum statement that a thunderstorm 

 comes u]") against the wind. 



