Geoscience Reference
In-Depth Information
Localized Storms
CHAPTER 4
INTRODUCTION
THUNDERSTORMS,
LIGHTNING AND HAIL
The previous chapter was mainly concerned with the
effects of strong winds generated by secondary
features of general air circulation. These winds were
associated with the development of low-pressure cells
spanning areas of 10 000-100 000 km
-2
. While tropical
cyclones and extra-tropical depressions produce some
of the strongest winds and highest amounts of precipi-
tation over the widest areas, they are by no means the
only source of high winds or heavy precipitation. These
values can be matched by thunderstorms, which cover
no more than 500 km
2
in area and rarely travel more
than 100-200 km before dissipating. They can produce
high-magnitude, short-period rainfalls leading to flash
flooding. Thunderstorms are also associated with a
wide range of climatic phenomena such as lightning,
hail and
tornadoes
that bring death and destruction.
Tornadoes generate the highest wind speeds and can
produce localized wind damage just as severe as that
produced by a tropical cyclone. This chapter will
examine first the development and structure of
thunderstorms resulting in lightning and hail. This is
followed by a description of tornadoes and the major
disasters associated with them. The chapter concludes
with a discussion of warning and response to the
tornado threat, an aspect that has been responsible
for decreasing death tolls throughout the twentieth
century.
Thun
derstorms
(Whipple, 1982; Eagleman, 1983)
Thunderstorms are a common feature of the Earth's
environment. There are about 1800-2000 storms
per hour or 44 000 per day. In tropical regions, they
occur daily in the wet season. However, in these
regions thunderstorms may not represent a hazard
because they do not intensify. As thunderstorms repre-
sent localized areas of instability, their intensity is
dependent upon factors that increase this instability.
On a world scale, instability is usually defined by the
rate at which the base of the atmosphere is heated by
incoming solar radiation, especially where evaporation
at the ground and condensation in the atmosphere
both occur. In this case, the saturated
adiabatic lapse
rate
prevails. Under these conditions, large quantities
of heat energy (2400 joules gm
-1
of liquid water that
condenses) are released into the atmosphere. This
process causes convective instability, which terminates
only when the source of moisture is removed.
On a localized scale, the degree of instability is
also dependent upon topography and atmospheric
conditions such as convergence. If air is forced over a
hill, then this may be the impetus required to initiate
convective instability. Convergence of air masses by
