Select Glossary of Terms - Natural Hazards - page 294

Critical Incidence Stress Syndrome. The condition
affecting rescuers who perform superhuman, selfless
feats of heroism or who are involved with death during
a disaster. Such people often suffer severe guilt,
become reclusive, and generally broken in spirit for
months or years after the event.
Crust. The outermost shell of the Earth, 20-70 km thick
under continents, but only 5 km thick under the
oceans. It consists of lighter, silica-rich rocks, which
flow on denser liquid rock making up the mantle.
Cryogenic. Any process involving the freezing of water
within soil or rock.
Cumulonimbus. A dense cloud consisting of a lower dark
layer that is potentially rain-bearing, and a multiple
white fluffy crown reaching the top of the troposphere.
Such cloud forms by thermal convection and is
characteristic of thunderstorms.
Cycles, cyclic or cyclicity. Recurring at regular intervals
over time: for instance, sunspot numbers, which recur
at 10-11 year intervals.
Cyclogenesis. The process forming an extra-tropical,
mid-latitude cell or depression of low air pressure.

Dust Veil Index. A subjectively defined index that
attempts to measure the relative amounts of dust
injected into the stratosphere by volcanic eruptions.
The index is reference to a value of 1000 for the
eruption of Krakatau in 1883.

E
East-Coast Lows. Very intense storms that develop in mid-
latitudes on the seaward, eastern sides of continents.
Such lows develop over warm ocean water in the lee of
mountains and are dominated by intense convection,
storm waves, and heavy rainfall. Events where wind
velocities reach in excess of 100 km hr -1 within a few

hours are called 'bombs'.
Easterly Wave. In the tropics, pressure tends to increase
parallel to the equator with easterly winds blowing in
straight lines over large distances. However, such
airflow tends to wobble, naturally or because of some
outside forcing. As a result, isobars become wavy and
the easterly winds develop a slight degree of rotation
that can eventually intensify over warm bodies of water
into tropical cyclones.
El Niño. The name given to the summer warming of
waters off the coastline of Peru at Christmas time
(from Spanish, meaning 'The Child').
Elastic. A property of any solid that deforms under a load
and then tends to return to its original shape after the
load is released.
Electric Resistivity. The degree to which a solid body can
conduct an electric current.
Electrosphere. The zone of positively charged air at
50 km elevation, the strength of which determines the
frequency of thunderstorms.
Electrostatic. The generation of electrical charges on
stationary particles.
Endemic. A term used to describe a disease that exists
naturally within a specific region because of its
adaptation to climate and the presence of vectors and
reservoirs. If the vectors and reservoir species return
or controls are relaxed, then the disease can easily
become re-established. See also Vectors, Reservoirs.
Energy. The capacity of any system to do work. Energy
can be described as 'kinetic', which is the energy any
system possesses by virtue of its motion; as 'potential',
which is the energy any system possesses because of its
position; and 'thermal', which is the energy any system
possesses because of its heat.
ENSO. Abbreviation for the words 'El Niño-Southern
Oscillation', referring to times when waters off the

D
Debris Avalanche. A flow of debris of different particle
sizes that takes on catastrophic proportions. Volumes
of 50-100 million cubic metres traveling at velocities of
300-400 km hr -1 have been measured.
Debris Flow. Any flow of sediment moving at any velocity
downslope in which a range of particle sizes is involved.
Deflation. The removal by wind, over a long time period,
of fine soil particles from the ground surface.
Degassing. The sudden or slow release into the
atmosphere of gas dissolved in melted rock.
Degradation. The slow loss from a soil of humus,
minerals, and fine particles essential to plant growth; or
the contamination of a soil by salts or heavy minerals
that are toxic to plant growth.
Desertification. The process whereby semi-arid land
capable of supporting enough plant life for grazing
or extensive agriculture becomes infertile because of
increased aridity, degradation, or overuse.
Dilation. Rock in the Earth's crust will tend to crack and
expand in volume when subjected to forces capable of
fracturing it.
Dust Bowl. Part of the American southern Great Plains,
centered on Oklahoma, which experienced extensive
dust storms during the drought years of 1935-1936 at
the height of the Great Depression.