Select Glossary of Terms - Natural Hazards - page 293

followed, nine years, later by another drought before
the alternating cycle is re-established. This is bistable
phasing.
Blizzard. Any wind event with velocities exceeding
60 km hr -1 , with temperatures below -6 °C, and often
with snow being blown within tens of metres of ground
level.
Blocking High. A high-pressure system whose normal
eastward movement stalls, blocking the passage of sub-
sequent low- or high-pressure systems and deflecting
them towards the pole or equator. Mobile polar highs
are high-pressure cells that have either run out of
momentum in their movement away from the poles or
have not been displaced by another high.
Blocks. A type of lava that consists of angular, boulder-
sized pieces of solid rock blown out of an erupting
volcano.
Bombs. (1) See East-Coast Lows. (2) Boulder-sized
pieces of liquid lava blown out of an erupting volcano.
Boreal. (1) Pertaining to the northern hemisphere. (2)
Northern hemisphere forest, consisting mainly of
conifers and birch.
Bruun Rule. A relationship stating that a sandy beach
retreats under rising sea level by moving sand from
the shoreline and spreading it offshore to the same
thickness that sea level has risen.

small evergreen oaks This vegetation is found in the
Mediterranean climate of the United States south-west.
Chernozem. A very black soil rich in humus and carbon-
ates forming under cool, temperate, semi-arid climates.
Clustering. The tendency for events of a similar
magnitude to be located together in space or time.
Coherence. (1) Of soils: the binding together of soil
particles due to chemical bonding, cementation or
tension in thin films of water adhering to the surface
of particles. (2) Of time series: the tendency for a
phenomenon to be mirrored at another location over
time. For instance, wet periods in eastern Australia
often occur simultaneously with a heavy monsoon in
India.
Cohesion. The capacity of clay soil particles to stick or
adhere together because of physical or chemical forces
independent of inter-particle friction.
Colloids. Fine clay-sized particles 1-10 microns in size,
usually formed in fluid suspensions.
Conflagration. A particularly intense fire with a heat
output of 60 000-250 000 kW m -1 along the fire front.
Continental Shelf. That part of a continental plate
submerged under the ocean, tapering off seaward at an
average slope of less than 0.1 degrees, and terminating
at a depth of 100-150 m in a steep drop to the ocean
bottom.
Convection. The transfer of heat upward in a fluid or air
mass by the movement of heated particles of air or
water.
Convective Instability. A parcel of air continuing to rise
when lifted if its temperature is always greater than
the surrounding air - a process that is guaranteed if
condensation occurs releasing heat into the parcel.
Coriolis Force. The apparent force caused by the Earth's
rotation, through which a moving body on the
Earth's surface deflects to the right in the northern
hemisphere and to the left in the southern hemi-
sphere.
Correlation. The statistical measure of the strength of a
relationship between two variables.
Cracking Clays. Soils consisting of clays that have a high
ability to expand when wetted, and to shrink and,
hence, crack when dried.
Creep. The imperceptibly slow movement of surface soil
material downslope under the effect of gravity. See also
Tectonic Creep.
Cretaceous. The geological time period 135-65 million
years BP, which was characterized by sudden, large
extinctions of many plant and animal species.

C
Caldera. The round depression formed at the summit of a
volcano caused by the collapse of the underlying magma
chamber that has been emptied by an eruption.
Capillary Cohesion. The binding together of soil
particles by the tension that exists between water
molecules in a thin film around particles.
Catastrophists. A group of people, little challenged
before 1760, who believed that the Earth's surface was
shaped by cataclysmic events that were acts of God.
Cation. An ion or group of ions having a positive charge.
Cavitation. A process, highly corrosive to solids, whereby
water velocities over a rigid surface are so high that the
vapor pressure of water is exceeded and bubbles begin
to form.
Cementation. The process whereby the spaces or voids
between particles of sediment become filled with
chemical precipitates such as iron oxides and calcium
carbonate, which effectively weld the particles
together into rock.
Chaparral. The name given to dense, often thorny,
thickets of low brushy vegetation, frequently including