AND ON THE ELECTRIC FIELD OF THUNDERSTORMS. 
107 
now be equal to F 0 H, we have for the energy dissipated, ^QV = |-QF () H, about 
10 11 joules. 
The rate at which electrical energy would be going to waste in a storm in which 
one such discharge occurred in every 10 seconds would amount to 10 16 ergs per second 
or 1,000,000 kilowatts. It is of interest to compare this with the total power 
which would be available if it were possible to catch the rainfall of a thunder-shower 
before it fell and utilise the water power thus stored. The rate of rainfall in a severe 
thunderstorm may reach values approaching 10 cm. per hour. The water power 
available if it were possible to catch the rain at a height of 1 km. would amount to 
3 x 10 15 ergs per sq. kilometre per second. Thus a rainfall of the above amount over 
an area of about 3 sq. km. if intercepted at a height of 1 km. would furnish sufficient 
power to produce the required electrical energy. The total power available for the 
production of lightning flashes may obviously greatly exceed the above estimate 
based on the rainfall. 
XVIII. Interpretation of “ Recovery" Curves. 
In a typical record of the changes of the vertical electric force due to a distant 
thunderstorm each vertical portion of the trace—representing the sudden change 
produced by a discharge—-is followed by a characteristic “recovery” curve. This 
may be interpreted as follows :—The charge in the head or base of the thunder¬ 
cloud—-or in both—is suddenly destroyed by the passage of a lightning flash. The 
field at once begins to be re-established at a rate represented by the initial steepness 
of the curve immediately after the discharge. But as the charge increases, its field 
tends to diminish the rate of increase of the charge in two ways: (l) by hindering 
the separation of oppositely charged rain-drops and cloud particles; and (2) by 
producing an ionization current which tends to neutralise the charge and increases 
with the increasing intensity of the field. Unless the field previously reaches the 
sparking limit, a steady condition will finally Be approached when the two opposing 
processes, which tend respectively to increase and diminish the field, balance one 
another. 
The initial rate of increase of the field immediately after the passage of a distant 
discharge is thus an important quantity. It is proportional to the rate at which a 
charge destroyed by the flash is regenerated by the action of the thunder-cloud, i.e., 
it is proportional to the vertical electric current which is carried through the thunder¬ 
cloud by the convection of charged masses. If the distances and height of the charge 
destroyed are known, the vertical electric current may at once be deduced from the 
initial rate of increase of the field. If this information is not available the ratio of 
the current to the quantity which passed in the previous discharge can always be 
obtained from the record. 
Q 2 
