Geomagnetic Activity. Fluctuations in the Earth's
magnetic field over space and time. The strength of
this field and its variation can be changed by solar
Geophysical. Dealing with the structure, composition,
and development of the Earth, including the atmos-
phere and oceans.
Geosyncline. An area of sediment accumulation so great
that the Earth's crust is deformed downward.
Gilgai. The tendency for soil, affected by alternating
wetting and drying, to overturn near the surface. The
process can result in local relief up to 2 m.
Glacier Burst. The sudden release of huge volumes of
water melted by volcanism under a glacier and held in
place by the weight of ice until the glacier eventually
Graupel. Aggregated ice or hail coated in supercooled
Greenhouse Effect. The process whereby certain gases in
the atmosphere, such as water vapor, carbon dioxide
and methane, transmit incoming short wave solar
radiation, but absorb outgoing long wave radiation,
thus raising surface air temperature.
Gumbel Distribution. The plot of the magnitude of an
event on a logarithmic scale against the average length
of time it takes for that event to occur again.
Gyres. Large rotating cells of ocean water that often com-
pletely occupy an ocean basin. The Gulf Stream forms
the western arm of one such gyre in the north Atlantic.
Hectare. Standard international unit measuring area.
1 hectare = 1000 m 2 = 2.471 acres.
Hectopascal. Standard international unit, abbreviated as
hPa, measuring air pressure: 1 hPa = 1 millibar.
Hindcasting. The procedure whereby wave characteristics
can be calculated from the wind direction, strength and
duration derived from weather charts.
Holocene. The most recent part of the Quaternary,
beginning 15 000 years BP, when the last major
Howling Terrors. An Australian term for small, destructive,
tropical cyclones with an eye diameter of less than 20 km
- also called 'kooinar' by Aborigines.
Hydrostatic. Describing pressure in groundwater exerted
by the weight of water at levels higher than a particular
location on a slope.
Hydrothermal. Describes the processes associated with
heated, or hot melted, rock rich in water.
Ice-Dammed Lakes. Lakes formed by glaciers or icesheets
completely infilling a drainage course or flowing against
higher topography, thus blocking the natural escape of
Ignimbrites. Very hot ash can be blasted laterally across
the landscape. When deposited, the ash particles fuse
together to produce a hard, welded rock .
Illite. A clay mineral slightly more weathered than
montmorillonite, containing hydroxyl radicals and
having less iron and silica. Chemical formula
[KAl 2 (OH) 2 (AlSi 3 (O,OH) 10 )].
Interfluves. The hill slopes between streams that flow in
the same direction.
Intertropical Convergence. Air in the tropics rises
because of heating beneath the seasonal position of
the sun. Winds flow from the north and south, and
converge on this zone of heating, effectively producing
a barrier to the exchange of air between hemispheres.
Intra-Plate. Locations lying near the center of a plate or,
at least, away from tectonic activity associated with the
Inverted Barometer Effect. The height of sea level
inversely relates to the pressure of the atmosphere
above, such that a decrease of 1 hectopascal (hPa) in
air pressure results in a rise in sea level locally of 1 cm.
Island Arc. A series of islands located on the continental
plate-side of a deep ocean trench, under which gas-
rich crust is being subducted to produce andesitic
Haboob. The sand or dust storm resulting from a Har-
mattan wind. The term is used in Arabia, North Africa
and India. See Harmattan.
Hadley Cell. Heated air at the equator rises and moves
poleward in the upper atmosphere. It cools through
the loss of long wave radiation and begins to sink at
about 20-30° north and south of the equator, where-
upon part of the air returns to the equator. This forms
a large, semi-permanent vertical cell on each side of
the equator. Postulated by George Hadley in 1735.
Hale Sunspot Cycle. Sunspot numbers peak every
10-11 years. Every alternate peak is larger, forming
a 22-year cycle, a phenomenon first noted by the
American astronomer George Hale. The Hale cycle is,
in fact, the full magnetic sunspot cycle.
Harmattan. A dry, dusty trade wind caused by mobile
polar highs moving from the east over North Africa.
These are the source of dust storms.