Glossary - Fundamentals of the Physical Environment - page 902

too restrictive and may be misleading, so a definition which emphasizes the
predominance of cryospheric processes - without the need for glaciers - is preferred.
Perihelion The point on Earth's orbit when it is closest to the sun.
Permafrost The enduring and continuous presence of freezing temperatures at and below
the land surface in which all soil and ground water is frozen except for a thin surface
active layer of summer melting.
Permafrost soil processes Soil processes occurring in frozen ground ( permafrost ).
Permeability The capacity of Earth materials to circulate and transmit fluids (water,
solutions, air, etc.) through their pores and fractures and measured as the fluid volume
passing through a unit cross-section area.
P - form 'Plastically sculptured' sinuous bedrock grooves of uncertain origin but believed
to be formed by high-pressure subglacial meltwater rather than glacier abrasion.
pH The measure of acidity or alkalinity of a substance, measured by the number of
hydrogen ions per litre, on a logarithmic scale where neutrality = 7·0; acid and alkaline
substances have a pH of less than 7·0 and more than 7·0 respectively.
Photic zone The thin, surface layer of a water body penetrated by sunlight.
Photosynthesis The synthesis of organic compounds from water and carbon dioxide,
using energy absorbed by chlorophyll from the radiant energy of the sun.
Phreatic water Water in the saturated zone of an aquifer .
Phreatophytes Plants which survive aridity by developing deep root systems to exploit
soil water reserves.
Phytoplankton The plant (primary producer) form of marine micro-organisms which
form the base of the marine food web.
Piedmont fan A fan-shaped lobe of alluvium or other debris accumulated at the break of
slope along a mountain front.
Piedmont glacier A glacier which fans out across the unconfined surface of a piedmont
zone as it leaves the confined channel of an outlet or valley glacier.
Piedmont lobe The lobate terminal zone of a piedmont glacier .
Piezometric surface An imaginary surface defined by the level to which water rises in a
well and representing the static 'head' of water.
Pingo A large ice-cored mound elevated by hydrostatic pressure and segregated ice
growth on a flat permafrost land surface which is waterlogged in summer.
Pioneer community The first set of plant species to colonize a newly available site
which was previously unvegetated.
Pipe A narrow water conduit in soil formed through the connection of macropores or the
removal of swelling clays .
Placer A terrestrial or shallow marine deposit of heavy minerals in a body of clastic
sediments, sourced by erosion, transport and gravity deposition from a parent mineral
body.
Plagioclimax A vegetational state where burning or grazing modifies the natural state of
the vegetation.
Planar discontinuity A plane surface (such as a fold, fracture, fault, thrust, joint,
lamination, etc.) in rock mass or other Earth materials at which the continuous or
homogeneous properties of material on either side of the plane - providing its intact
strength - are momentarily interrupted or lost;