Geography Reference
In-Depth Information
4
Extreme weather events
Rory Walsh
and uncertainties associated with recent and
predicted future changes in extreme event
frequency.
INTRODUCTION
Extreme weather events and the weather-related
events they may induce, such as landslides, floods
and storm surges, form an important part of what
have been termed 'natural hazards'. They have a
major influence not only on the physical
landscapes and human societies directly affected
by them but also on the wider community
through their impact on the insurance industry
and the costs of emergency aid or relief. It is
increasingly being recognised that their
distribution in time and space is dynamic rather
than static and significant changes in the frequency
of extremes such as heavy daily rainfalls, droughts,
extreme heat and cold and tropical cyclones are
envisaged in 'global warming' predictions for the
next century (IPCC 1996; United Kingdom
Climate Change Impacts Review Group 1996;
Hulme and Viner 1998). This chapter reviews the
roles that geographers have played in examining
the climatology of extreme events, their spatial and
temporal distribution (including past, current and
future changes), their impacts on natural systems
and human activities, and the design and
effectiveness of strategies aimed at reducing their
adverse effects. With particular reference to
geographical research on tropical cyclones, it
highlights the problems that stem from their
inherent rarity, which reduces the sample size
upon which to base conclusions and advice, the
mismatch between the spatial scale at which one
can offer reliable advice and the spatial scale most
useful for planning purposes, and the implications
EXTREME WEATHER EVENTS: THE
PROBLEMS THEY PRESENT AND THE
ROLES PLAYED BY GEOGRAPHERS
Extreme weather events can be divided into
absolute
and
relative
event types (Figure 4.1).
Absolute extreme events are considered extreme
simply because of their magnitude and nature.
Thus tropical cyclones, tornadoes, hailstorms and
climate-related events such as avalanches,
landslides and floods are considered extreme
events, even in areas where they are frequent.
Relative extreme events, on the other hand, are
considered extreme on the basis of their rarity at
the location concerned; they are distinguished
from normal events on the basis of their
probability of occurrence (Smith 1997) and its
inverse, the return period or recurrence interval,
which is the average time interval between events
of a specified magnitude. If one views events as
being statistically distributed around a long-term
mean value, then most events lie relatively close to
the mean within what Smith terms the socio-
economic band of tolerance and can be
interpreted as resources, as they constitute the
near-normal events on which human activities in
that climate are based. Weather extremes such as
unusual cold, warmth, drought or wetness are
measured relative to (and hence are specific to) the
