coast of Peru become abnormally warm and the
easterly trade winds blowing in the tropics across
the Pacific Ocean weaken or reverse.
Epicenter. The point at the surface immediately above the
location of a sudden movement in the Earth's crust
generating an earthquake.
Episodic. The tendency for natural events to group into
time periods characterized by common or loosely
connected features. Weak cyclicity or regularity may
be implied in this term.
Equinox. The time occurring twice yearly, around
21 March and 21 September, when the apparent
motion of the sun crosses the Earth's equator. This
results in all parts of the Earth's surface receiving equal
amounts of daylight.
Eucalyptus. A genus of tree, which is mainly evergreen, is
rich in leaf oil, flammable, drought-resistant, and
capable of growing in nutrient-deficient soil.
Eustatic. Worldwide sea level change.
Evaporite. A sediment deposit that has formed from the
residue left after the evaporation of salt water.
Evapotranspiration. The amount of moisture lost by
plants by evaporation to the atmosphere, mainly
through leaf openings or stomata. This amount is always
greater than that lost from equivalent, unvegetated
Expansive Soils. Soils consisting of clay particles - mainly
montmorillonite - that absorb water when wetted and
thus expand in volume.
Extra-Tropical Low or Depression. A mid-latitude, low-
pressure cell with inwardly and upwardly spiraling
winds. Unlike tropical cyclones, the cell can develop
over land as well as water, usually in relation to the
polar front and with a core of cold air.
Eye. The center of a tropical cyclone (or tornado), where
air is descending, surrounded by a wall structure created
by a sharp decline in pressure and characterized by
intensely inwardly spiraling winds.
catches its own sound waves and rebroadcasts them
louder. This is positive feedback.
Fetch. The length of water over which wind blows to
generate waves. The longer the fetch, the bigger the
Firestorm. Under intense burning, air can undergo rapid
uplift and draw in surrounding air along the ground to
replace it. This movement can generate destructive
winds at ground level in excess of 100 km hr -1 .
Flood. A large discharge of water flowing down - if not
outside - any watercourse in a relatively short period
compared to normal flows.
Flood Lava. Lava flows occurring over a very large area.
Floodplain. Flattish land adjacent to any watercourse
built up by sediment deposition from water flowing
outside the channel during a flood.
Föhn Wind. Air rising over a mountain and undergoing
condensation releases latent heat of evaporation and,
thus, cools adiabatically at a slow rate of about 0.5°C
per 100 m. When this air then descends downslope on
the leeward side, it tends to warm at the faster, dry
adiabatic lapse rate of 1°C per 100 m. The resulting
wind is much warmer and drier than on the windward
Foreshocks. The small seismic shocks preceding a major
earthquake or volcanic event.
Freezing Rain. Light rain falling through a layer of cold
air or upon objects with a temperature below freezing
will tend to freeze or adhere to those objects. Over
time the accumulated weight breaks power lines and
Friction. The force due to the resistance of one particle
sliding over another because of a weight pressing
irregularities on the surface of both particles together.
Fumarole. A hole or vent from which volcanic fumes or
General Air Circulation. Equatorial regions have an
excess of heat energy relative to the poles because
incoming solar radiation exceeds outgoing long wave
radiation. To maintain a heat balance over the Earth's
surface, this excess heat flows mainly in air currents
from the equator to the pole.
Geochemical. Involving chemical reactions on, or inside,
Geo-Electric Activity. Processes involving the transmission
or induction of electrical currents along the surface of,
or through, the Earth.
Fault. A planar zone where one part of the Earth's crust
moves relative to another. A fault is categorized
according to the direction of movement in the hori-
zontal or vertical plane (see Figure 9.8). Transcurrent
faults are fault lines approximately at right angles to
major lines of spreading crust in the oceans.
Feedback. The situation where one process reinforces
(positive) or cancels (negative) the effect of another. For
instance, a microphone placed close to an amplifier