Localized Storms - Natural Hazards - page 90

can be monitored by satellites, and located to a
high degree of accuracy. A network of automatic
direction-finding, wide-band VHF , magnetic receivers
has been established, across North America and other
continents, to locate and measure the direction,
polarity, intensity, and stroke patterns of lightning.
In the United States, 100 stations in the National
Lightning Detection Network measure the exact time
and direction of electromagnetic energy bursts
produced by lightning. Lightning can also be located
by measuring the difference in the time of arrival of
the electromagnetic wave between two sensors. The
latter method uses radio noise produced by lightning in
the 7-16 kHz very low frequency (VLF) spectrum. In
Europe, a network of six very low frequency receivers
has been established in conjunction with global
positioning systems and signal processing algorithms
to locate lightning flashes globally. This network, in
real time, can detect lightning within severe storms
resulting in flash flooding. This can be verified using
the Lightning Imaging Sensor (LIS) on the Tropical
Rainfall Measuring Mission (TRMM) satellite.
Lightning is a localized, repetitive hazard. Because
the lightning discharge interacts with the best con-
ductors on the ground, tall objects such as radio and
television towers are continually struck. When light-
ning strikes a building or house, fire is likely as the
lightning grounds itself. However, this effect, while
noticeable, is not severe. In the United States, about
$US25 million in damage a year is attributable to
lightning. Insurance companies consider this a low
risk. However, because most of the damage can be
avoided by the use of properly grounded lightning rods
on buildings, insurance companies in North America
either will not insure, or will charge higher rates for,
unprotected buildings. Lightning, besides igniting
forest fires, is also a severe hazard to trees and other
vegetation. In North America, tree mortality in some
areas can reach 0.7-1.0 per cent per year because of
lightning strikes, and up to 70 per cent of tree damage
is directly attributable to lightning. Scorching is even
more widespread. A single lightning bolt can affect an
area 0.1-10 hectares in size, inducing physiological
trauma in plants and triggering die-offs in crops and
stands of trees. Lightning can also be beneficial to
vegetation, in that oxygen and nitrogen are combined
and then dissolved in rain to form a nitrogen fertilizer.
The greatest threat from lightning is the fact that the
high voltage kills people. Lightning enters a person

through the body's orifices, flowing along blood vessels
and the cerebral spinal fluid pathways. The current
builds up in the brain and then discharges along the
skin, leading to a dramatic drop in charge inside
the body. This causes failure of the cardiac and
respiratory systems linked to the brainstem. Victims of
lightning die from suffocation because breathing stops.
About 40 per cent more people die in the United
States and Australia from lightning than from any other
meteorological hazard. This incidence is decreasing
over time despite a growth in population. Decreased
fatalities reflect increased urbanization where protec-
tion is provided by tall buildings, an increase in safety
education, and the improved availability of medical
services. Despite the misconception that standing
under a tree during a lightning storm is playing with
death, statistics indicate that the number of people
who die from lightning-induced charges inside houses
or barns is equal to the number of those who die
outside standing under trees. This statistic may merely
reflect the fact that most people know about the risks
of standing under trees during a storm, but not about
other hazardous activities. In Australia, Telecom had to
mount a major campaign in 1983 warning subscribers
about the risk of using telephones during thunder-
storms. Risky behavior also includes standing near
electrical wiring or metal piping (including toilets)
inside homes, or standing at the edges of forests, near
mountains or on beaches during thunderstorms. In the
latter cases, an observer may be the first tall object
available for lightning discharge for some kilometers.

Hail
(Eagleman, 1983; Yeo et al., 1999)
The formation of hail depends upon the strength of
updrafts, which in turn depends upon the amount
of surface heating. The likelihood of hail increases with
more intense heating at the Earth's surface and cooler
temperatures aloft. The surface heating produces the
updrafts, while the cooling ensures hail formation.
Almost all hailstorms form along a cold front
shepherded aloft by the jet stream. The jet stream
provides the mechanism for creating updrafts. Wind
shear, which represents a large change in wind speed
over a short altitude, also facilitates hail formation.
The degree of uplift in a thunderstorm can be assessed
by the height to which the storm grows. In colder
climates, however, this height is less than in more
temperate or subtropical latitudes.