GLOSSARY - Physical Geography - page 157

downwards through the soil collecting aluminium and iron. The soil
becomes very acidic as a result.
Polar front The boundary at which cold air from the polar cell and warm
tropical air from the Ferrel cell meet, causing air to rise.
Precautionary principle A decision-making approach which believes
that lack of scientific evidence for warnings about future threats of
serious damage should not be used as an excuse to avoid action in
order to prevent damage from happening; action should be taken as
early as possible.
Precipitation The condensation of water vapour to form water drop-
lets in the atmosphere, which are then deposited on the Earth's surface
in a liquid (e.g. rain) or solid form (e.g. snow, hail).
Pressure-melting point The melting point of solids such as ice is not
constant, but varies according to pressure. The melting point of ice
becomes lower with increasing pressure, meaning that under thicker
ice there is likely to be water.
Protalus ramparts A linear arrangement of coarse sediment at the base
of a periglacial slope, created by frost shattering of slope material which
slides down over the snow pack and settles below it.
Quaternary The last 2.4 million years to the present day, characterised
by the expansion and contraction of ice sheets in predictable cycles.
Reef (coral) Underwater landform constructed from the remains of
corals and typically consisting of diverse ecosystems.
Refraction A process caused by a reduction in velocity as a wave enters
shallow water, resulting in the wave front changing direction and
'bending' as it reaches the shore.
Regelation When ice meets an obstacle, such as a rock, pressure increases
on the upstream side of the rock. This lowers the pressure melting point,
resulting in melting of the ice on the upstream side of the rock, which
then flows around the obstacle and refreezes on the downstream side due
to the pressure melting point being higher. This allows ice to flow
around obstacles.
Regolith The layer of soil overlying the bedrock which contains
unconsolidated weathered parent material which provides the raw
material for soil development.
Riffles The accumulation of coarse sediment which forms bar deposits
across a river, which tend to be spaced between five and seven times
the channel width apart.
River regime The variability of river flow over time, typically charac-
terised over a year.
Roche mountonées Smaller versions of stoss-and-lee forms.