988 Marvels of the Universe 
are not so dependent on solar heat as the other kind, frequently take place at night and come along 
with gales of wind. They are perhaps more dangerous than the heat thunderstorms, since though 
they are not so violent, yet, as the clouds from which they come are at a lower level, the lightning 
discharges more frequently strike the ground. 
A theory as to their occurrence attributes them to the ascent of warm, moist air by convection. 
The cloud is often isolated and continuous in “ summer storms,” its height varying from one thousand 
feet to the level of the cirrus clouds (say, thirty thousand feet at times). Spreading out in all direc- 
tions in a sheet, it often presents the appearance known by the name of “ anvil shape.’’ Cyclonic 
thunderstorms are considered to be due to the cooling of the upper air, producing similar effects to 
the heating of the warmer air, in disturbing electrical equilibrium. 
There is both an annual and a diurnal 
periodicity in the frequency of thunderstorms. 
More storms occur in summer than in winter, 
the summer storms most frequently in the 
early afternoon at the time of the day when 
the temperature is the highest. The winter 
‘cyclonic ”’ storms occur at all hours, frequently 
during the night, more common on the western 
coast (Atlantic side) than on the other, be- 
cause, owing to the proximity of the sea, 
great contrasts of temperature at different 
heights in the atmosphere less frequently arise. 
That storms are, however, associated with 
sudden changes of temperature is evident from 
the fact that hail so often falls during their 
occurrence, and that the thunderclouds are 
mainly of the cumulus type, whose presence 
indicates the existence of a colder and drier 
layer of air above that in which the clouds 
form, and, lastly, changes of wind occasioning 
a rapid fall of the thermometer also take place. 
Dr. Scott (Meteorology) remarks that changes 
of temperature form an essential condition for 
the generation of a thunderstorm. “To a 
Photo by] [L. 2. A. certain extent we may regard fogs and thunder- 
LIGHTNING. 
What is commonly known as forked lightning is here 
shown. It would be described more correctly as branched 1S Characteristic of conditions of stability in 
Pstatin the stratification of the atmosphere; the 
thunderstorm of marked instability”? (Shaw). The peculiarity which the weather seems to 
display in taking on “moods” for special types of occurrence is also remarked by this 
storms as inverse phenomena. The surface fog 
author. 
If a thunderstorm occurs in any part of the country on one day it is quite common for 
other parts of the country to be visited by similar phenomena on the next day or the day after, 
though the other weather conditions do not specially point to this. Thus, though thunderstorms 
are peculiarly local phenomena, yet, after a spell, few places seem to have been exempt, the storm 
bursting out here to-day and to-morrow elsewhere, the next day further off, and so on. The thunder- 
storms of Iceland all occur in the winter, though it was at one time thought that they were quite 
unknown in Arctic regions. In Abyssinia, over a period of observations during which two 
thousand storms were registered, nearly all these happened in the afternoon hours, only 
