GLOSSARY - Physical Geography - page 160

Thermohaline circulation Large-scale deep ocean circulation involv-
ing vertical and lateral movements of large parcels of water, driven by
gradients of water density which results from variations in water tem-
perature and salinity.
Thermokarst A collection of irregularly spaced thaw lakes and depres-
sions as a result of the melting of segregated ice.
Throughflow The movement of water draining through the soil in a
downslope direction.
Tidal current The flow of water that is produced by the rise and fall of
the tides, which is most pronounced in river mouths, estuaries and
where flow is squeezed through inlets.
Trade winds Strong winds that flow from east to west towards the
equator, between 30° north and south. They are deflected to the west
due to the Coriolis effect.
Tragedy of the commons A theory that explains how communal
resources can be degraded due to the selfish nature of individuals who
use more than their fair share. It is used as an analogy for the unsustain-
able use of finite ecological resources.
Transpiration The evaporation of water through the pores of plant
leaves which is released into the atmosphere.
Trophic levels Groupings of organisms within a food chain (see defini-
tion above). Primary producers (photosynthesising plants which form
organic material from the Sun's energy, carbon dioxide and water) are
at the lowest trophic level and these are fed upon by creatures at the
second trophic level and so on.
Troposphere The lower layer of the atmosphere that extends between
6 and 15 kilometres in altitude above the Earth's surface.
Tsunami An energetic sea wave triggered by an earthquake, landslide
or meteor impact in the ocean, which can become very large once it
reaches shallow water, and cause devastation in coastal zones.
Water table The upper limit of the saturated zone of the soil or rock.
Watershed This term is often used in two ways. In many parts of the
world it refers to the area of land that drains into one point in a river
or lake. However, in other parts of the world the term can also mean
the divide between an area that drains one way and an area that drains
another.
Whaleback forms Smoothed rock outcrops formed by the sliding of a
glacier, streamlined in the direction of glacier flow, with a smoothed
steep side facing upstream and a tapered end facing downstream.
Yardangs Hills that have been smoothed and streamlined by erosion
from dust carried by the wind.