Glossary - Global Environmental Issues - page 192

at each of these points allows powerful computers
to provide simulations of current and future
climates.
ground control The use of observation and
measurement at the earth's surface to verify
information provided by remote sensing from
satellites or aircraft.
groundwater The water which accumulates in the pore
spaces and cracks in rocks beneath the earth's surface.
It originates as precipitation which percolates down
into sub-surface aquifers . The upper limit of
groundwater saturation is the water-table.
growing season The period of the year when mean daily
temperatures exceed the temperature at which plant
growth takes place. Since different plants mature at
different rates, the length of the growing season will
determine the mix of natural vegetation and the types
of crop that will grow in a particular area.
Gulf Stream A warm ocean current, originating in the
eastern Caribbean, which flows north along the
eastern seaboard of the United States before
swinging north-eastwards into the Atlantic Ocean
where it becomes the weaker and cooler North
Atlantic Drift.

that make up polar stratospheric clouds, leading
to the release of chlorine which attacks the ozone
layer. Similar reactions have been identified on the
surface of stratosphere sulphate particles such as
those released during the eruption of Mount
Pinatubo in 1991.
humidity A measure of the amount of water vapour
in the atmosphere. It may be expressed as specific
humidity—the ratio of the weight of water vapour
in the air to the combined weight of the water
vapour and the air—or as relative humidity—the
amount of water vapour in the air compared to
the amount of water vapour the air can hold at
that temperature.
humus Partially decomposed organic matter which is
an essential component of fertile soil.
hydrocarbons Organic compounds composed of
hydrogen and carbon bound together in chains or
rings. The largest sources of hydrocarbons are fossil
fuels such as petroleum and natural gas.
hydrochlorofluorocarbons (HCFCs) A widely-used
substitute for chlorofluorocarbons (CFCs) . Being
less stable than CFCs, hydrochlorofluorocarbons
begin to break down in the troposphere before they
can diffuse into the stratosphere and destroy the
ozone layer. They are about 95 per cent less
destructive than CFCs.
hydrogen oxides A group of naturally occurring
compounds, derived from water vapour, methane
and molecular hydrogen, which can destroy ozone
through catalytic chain reactions (see catalyst ).
They include atomic hydrogen, the hydroxyl radical
and the perhydroxyl radical, referred to collectively
as odd hydrogens.
hydrologic cycle A complex group of processes by
which water in its various forms is circulated
through the earth/atmosphere system. It is powered
by solar radiation which provides the energy to
maintain the flow by way of such processes as
evaporation, transpiration, precipitation and
runoff. Short- and long-term storage of water in
lakes, oceans, ice sheets and the groundwater
reservoir is also part of the cycle.
hydroxyl radical (See hydrogen oxides .)

H
Hadley cells Convection cells which form in the
tropical atmosphere north and south of the equator.
Named after George Hadley who, in the eighteenth
century, developed the classic model of the general
circulation of the atmosphere based on a simple
convective circulation .
halons Synthetic organic compounds containing
bromine. Commonly used in fire extinguishers, they
are more effective in destroying the ozone layer than
chlorofluorocarbons. (See also

bromofluorocarbons .)
Harmattan A hot, dry, dusty wind which blows out
of the Sahara Desert over the Sahel and much of
West Africa during the northern hemisphere winter.
It brings continental tropical air southwards which
contributes to the seasonal drought characteristic
of the area.
heavy metals Metals such as mercury, lead, tin and
cadmium which may be converted into a soluble
organic form or concentrated by hydrological or
biological processes so that they become hazardous
to natural ecosystems and human health.
heterogeneous chemical reactions Chemical reactions
which take place on the surface of the ice particles

I
Ice Ages Periods in the geological history of the earth
when glaciers and ice sheets covered large areas of
the earth's surface. The Ice Ages occurred in series,
separated by periods of temperate conditions called
interglacials. The most recent series began some