Free electricity is always in the air. During clear weather it is
generally positive; during cloudy weather it is negative. This
electricity is carried in the air by the moisture. As dry air is a
non-conductor of electricity, in fair weather the electrified particles
of air are insulated and therefore acquire very little intensity. The
clouds having been formed and being filled with moisture, form an
excellent conductor of electricity, which acquires considerable
intensity. It is a well-known physical law that two bodies having
opposite electricities attract each other, and those having like charges
repel each other. From this, two clouds having opposite charges rush
together and produce the phenomena, called lightning, which is
accompanied by an explosion called thunder. Often we see several flashes
of lightning and then hear several thunder crashes, which is caused by
only one section of a cloud discharging its electricity at a time.
As a cloud attracts the opposite charge of electricity from the surface
of the earth beneath it by inductive influence, often we see a discharge
of electricity from the cloud to the earth, the charge usually being
received by such objects as hills, trees, church spires, high buildings,
etc. Bodies containing large quantities of moisture are susceptible to
strokes of lightning, as the moisture causes them to become good
conductors of electricity. Also trees on the outer edge of a forest are
more liable to be struck than those farther in.
There are several forms of lightning, such as zigzag, ball, sheet, and
heat lightning.
Zigzag lightning, as the name implies, follows an irregular course,
producing a long zigzag line of light, sometimes ten miles in length,
and is caused by the air producing a field of resistance to the path of
electricity, causing it to seek a path of less resistance.
Ball lightning appears like a large ball of fire, usually accompanied by
a terrific explosion. This is the result of the bodies being charged
with electricity of great intensity, and travels in a straight path, as
it has enough strength to oppose any resistance placed in its path.
Heat lightning is usually seen on warm evenings, especially during the
summer, and very often unaccompanied by thunder, due to the great
distance of the lightning clouds from where we are located, thus
diminishing the intensity of the thunder. The electricity of the clouds
escape in flashes so feeble as to produce no audible sound.
Sheet lightning is a diffused glare of light sometimes illuminating only
the edges of a cloud, and again spreading over its entire surface.
Ordinary flashes of lightning last but the minutest part of a second.
Thunder is the re-entrance of air into an empty space. The vacuum is
created by the lightning in its passage through the air. The violence of
thunder varies according to the intensity of the electrical flashes.
Because of the fact that light is transmitted almost instantaneously,
while sound travels at a speed of eleven hundred feet per second, the
sound will not reach the ear for some few seconds after the flash of
lightning. Average space of time between a flash and a report is about
twelve seconds. The longest interval is seventy-two seconds and the
shortest one second. Prolonged peals of thunder are, in some cases, due
to the effect of echoes. These peals are especially noticeable in
mountainous countries. The echoes are also produced by the reflection of
sound from the clouds.
Thunder storms are distributed over certain sections of the globe,
occurring most frequently in the equatorial regions and diminishing as
we approach the polar regions. Within the tropics, where there are trade
winds, thunder storms are rare. Thunder storms are common in warm
climates because evaporation supplies electricity in great abundance,
and thus precipitation of the air is brought about.
[Illustration:
Fig. 11
]