tremendous energy, bright light and thunder. Occasionally, where a thunderstorm grows
over a tall Earth grounded object, such as a radio antenna, an upward leader may
propagate from the object toward the cloud. This "ground-to-cloud" flash generally
transfers a net positive charge to Earth and is characterized by upward pointing branches.
The initial breakdown and propagation are similar for intra-cloud lightning, but the
discharge generally occurs between regions of opposite charge. Without the benefit of air
conducting Earth, intra-cloud lightning does not produce a return-stroke-like feature.
Rather, it is characterized by slower propagating "recoil streamers" and "K" changes.
Nevertheless, tremendous energy, bright light, and thunder are still produced by intra-
cloud lightning.
Electricity is defined as the flow of electrons between points
of high potential (concentration) to low potential. The volume or actual
number of electrons that flow is the current and is measured in amperes (I).
Voltage, as measured in volts (V), is the electromotive force that drives
electrons across the potential difference. Resistance describes the hindrance
to the flow of electrons and is measured in ohms (R). Ohm's law describes
the relationship: I = V / R ...current is directly proportional to voltage and
inversely related to resistance. The transformation of electrical energy into
heat is what causes some of the harmful effects and is described through
Joule's law:
Heat = k(I) x R x T
Heat generated is directly proportional to the amount of current, tissue
resistance and the duration of contact.
Epidemiology:
NOAA data of the estimated number of lightning deaths from
1940-1973 is larger than the number caused by tornadoes, floods and
hurricanes. Since 1973, lightning is reported to have killed an average of 67
people a year, second only to 146 flash-flood/flood-related deaths. In 2003,
44 deaths were reported. Tornadoes rank third and hurricanes fourth with 17
deaths per year. Lightning is consistently one of the top three causes of
deaths/year with flash floods, winter weather and temperature extremes
exchanging places from year to year. The number of people injured by
lightning averages more than 2.5 times the number of lightning-related
deaths. Recent data from the Nat. Weather Service show that lightning
casualty stats are underreported by at least 28% and 42% for injuries. Hence,
lightning kills about 100+ people [US] a year and injuries 400 more; the
actual number of injuries is probably much larger.
Risk times and places:
a.
92% of casualties occurred between May and Sept, peak in July.
