Habitat – particular environment in which any one species of plant or animal is able to live.

Hadean – an eon of geologic time lasting from 4600m to 3800m years ago.

Hadley Cell – that portion of the tricellular model of air circulation where air rises at the equator due to convection, spreads in the upper troposphere and then sinks over the tropics before returning to the equator.

Hail– balls of ice a few millimeters in diameter which fall as precipitation. The balls form when water in clouds is lifted high enough to freeze. The droplets may fall a little and accumulate more water before further uplift cause more freezing and thus they grow. Eventually, their mass is enough to allow them to fall through the uplift. Ground temperatures must be low to prevent melting before they reach the surface.

Halophyte -a plant that is able to tolerate environments with a high level of salt. Found usually in coastal areas or arid areas where potential evapotranspiration exceeds precipitation for large parts of the year.

Halosere - plant succession in a saline environment e.g. estuary, salt marsh.

Hamada -flat, exposed bedrock in an arid area.

Hamlet -a small settlement with a purely residential function. Usually fewer than 100 inhabitants and no services except perhaps a post box.

Hard-engineering – solutions to problems of resource management involving building of structures e.g. sea walls (mitigation).

Harris-Ullman -model of urban land-use based on multiple nuclei i.e. more than one centre. Used to explain land-use in larger urban areas such as conurbations where there are many locales exhibiting features of CBDs as smaller settlements have been enveloped by the growth of the main city or have coalesced into it with their own growth.

Hanging valley -a small glacial valley entering a larger one someway up the side of the latter. Created by the differential in erosion between the different sized glaciers that formed the respective valleys.

Hardpan -a thin crust of material within a soil as a result of the illuviation of iron and/or aluminium from layers above or the precipitation of calcium carbonates which cement sands together.

Hazard -an event or condition which threatens people and property. May be natural e.g. earthquake, blizzard, or human e.g. industrial waste, HIV, or a combination e.g. drought, famine.

HDI -see human development index.

Headland - at the coast, land utting out-further into the sea and separating bays.  see also discordant coast.

Headward erosion - the lengthening of a young valley or gully by water erosion at the head of its valley.

Headwaters -the uppermost portion of a river course close to the source.

Heat budget -see energy budget.

Heat equator -the tilt of the earth means that during an orbit cycle the line of latitude receiving the most concentrated insolation igrates-between the tropics. On 21^st June, the sun is most directly above the Tropic of Cancer and, on 22^nd December, most directly above the Tropic of Capricorn.

Heat island -the warm air found around and above an urban area, distinct from the air temperature above the surrounding rural land.

Heathland- an environment of grass and shrubs which develops in nutrient deficient parts of temperate areas. In Europe, they are often the result of forest clearances but are now deliberately maintained for rough grazing and shooting estates.

Heavy industry -those secondary industries using bulk raw materials for both energy (e.g. coal) and production (e.g. ores). They often manufacture products for other secondary industries.

Helical (or helicoidal) flow -the spiraling flow of water in meandering channels caused by the deflection of flow as water hits the bending banks.

Hemisphere -lit. half a sphere. In geography, the division of the earth either into north and south by the equator, or into east and west by any one line of longitude.

HEP -see hydro-electric power.

Herbicide - a chemical used to control/prevent weed growth in arable farming.

Herbivore - any organism which feeds exclusively on plants ( producers).

Heritage - lit. anything that is passed one from one generation to the next. Commonly applied to the buildings and rural landscape of a nation and especially in relation to those that are considered worth keeping for the future.

Hierarchy - organisation by class of importance, status or authority. Usually a negative correlation between class and number of members i.e. the higher the class the lower the number of members of that class.

High-order good/service -one which is high in value and long-lasting and therefore generally bought infrequently e.g. car, accountant.

High tide - the highest point to which the sea rises against the land in its daily vertical movement.

High-yield variety -grain crop which has been selectively bred to produce seeds which produce far more than otherwise.

Hinterland -(also periphery) the area surrounding a core settlement from which the settlement gains resources.

Histogram - a bar chart where the vertical (y) axis is frequency and the horizontal (x) axis is the classes/values of data that have been measured for frequency.

HIV -see human immunodeficiency virus.

Hi-tech industry -those industries that have developed since the late 20^th century in areas such as electronics, IT, pharmaceuticals etc. They have a high component of research and development and are at the forefront of technological innovation.

Hjulstrّm curve -a graph showing critical stream velocities for the erosion, transport and deposition of different sizes of load.

Holocene - the current geological period, currently about 10,000 years long.

Honeypot- a place special interest, especially with reference to tourist visitors. Honeypots may be developed deliberately as a way to concentrate tourists into manageable areas and reduce pressures on surrounding fragile environments.

Horizon - two uses:

• the line where the visible surface appears to 'meet' the sky.

• in a soil, a distinct layer having unique characteristics.

Horizontal integration -the merging of firms at the same stage of production.

Horizontal transfer - transfer or transmission of energy from lower latitudes to higher latitudes. Two kinds: in the air by the movement of air (wind). Highly complex and not fully understood. Transfers take place between the cells of general circulation (see tricellular model) but the exact mechanisms are not clear. in the ocean by the existence of warm and cold currents. Warm currents move energy in surface waters from low to high latitudes, returning as cold currents at depth.

Horst -a raised block of land bordered by fault lines.

Horticulture-see market gardening.

Hot spots -a localized area of extreme heat. On earth, they refer to places where an upward plume of magma comes close to the surface in a shield area such as in Hawaii. The term may also be used to refer to areas of the surface of the sun giving off unusually large amounts of energy.

Housing association-in the UK, non-profit organizations offering a third 'way' outside of the traditional housing provision of the private sector and local government. Originally set up to provide rental accommodation they now also offer a way for people to become owners. They still focus on helping lower income groups find appropriate accommodation.

Hoyt, Homer-architect of the sector model of urban land-use.

Human Development Index -an attempt to make 'fairer' but still statistical measurements of development that became popular at the UN in the 1990s. Assigned value between 0 and 1 according to ranking position for each of the factors of purchasing power of income, literacy rate and longevity. Theoretical maximum of 3 was index 1. Criticized for measuring ranking rather than absolute idea of development, plus probably too biased in favour of wealth-dependent factors.

Human immunodeficiency virus -a virus which forms DNA while replicating its RNA leading to the development of Aids.

Humidity -the water vapour content of the atmosphere. Two main measures are absolute humidity and relative humidity. Humidity is controlled by air temperature (the higher the temperature the more water vapour can be held) and water availability for evapotranspiration.

Humus -the decomposed organic element of the soil. Is identified both as an independent horizon in some soils, or in the clay-humus complex. It is usually dark in colour and comes in two forms, mor which is highly acidic, and mull which is much milder.

Hurricane-term for a tropical cyclone forming in the Atlantic or Caribbean. They usually pose a hazard to the Caribbean islands and south-eastern USA.

Hybrid-a plant with certain characteristics achieved by cross building different plants for their particular qualities.

Hydration-a process of chemical weathering. Some minerals take up water and form weaker compounds e.g. anhydrite + water = gypsum which more readily dissolves. The expansion can also put physical stress on the rock.

Hydraulic action -lit. due to the action of water. In geography, a process of erosion whereby flowing water moves unconsolidated material due to forces of drag.

Hydraulic radius -stream efficiency measured by dividing cross-sectional area of a stream by the wetted perimeter. A higher ratio indicates greater efficiency as area will be greater compared to wetted perimeter and it is the latter which reduces efficiency through friction.

Hydro-electric power (HEP) -electricity produced through the use of moving water to turn turbines and drive generators. On a small scale HEP stations can be placed to take advantage of natural situations such as waterfalls, but usually they are built on a large scale by damming suitable valleys to create a reservoir. The water in the reservoir thus gains potential gravitational energy. When allowed to fall through the dam to the turbines this energy is converted to kinetic energy which drives the turbines. Require huge initial capital outlay but then produce highly efficient and cheap electricity. Also criticized for flooding farming land and villages thus displacing people and changing the environment and the river regime downstream of the dam. Nevertheless, a non-polluting and renewable form of energy. Provides a small percentage of world energy but highly important in some countries having ideal geological, meteorological and political conditions e.g. Norway and Sweden.

Hydrograph -a graph which combines a bar chart of precipitation in a particular event with a line graph of discharge for a particular river channel in the catchment area of that event and thus the reaction of the channel to the precipitation.

Hydrological cycle -the circulation of water around the world between stores by various transfers. At a simple level: water evaporates from the ocean, moves with the wind, condenses into clouds, precipitates and returns to the ocean via a river.

Hydrolysis-process of chemical weathering whereby a chemical reaction takes place between ions in a mineral and either the H⁺ or OH^- ions in the water.

Hydrophyte -any plant which has adapted to moist environments.

Hydrosere -a freshwater environment in which primary plant succession takes place.

Hydrosphere -the earth water habitats: oceans & seas, lakes & rivers, ice and underground water.

Hygroscopic -a substance that attracts water.

Hygroscopic water -that water in the soil which exists as a microscopic level as a layer around particle of clay. It is held by electrical attraction and is unable to leave the soil.

Hypersaline -lit. with high salt content. Hypersaline lakes such as the Dead Sea have a much higher salt content than 'normal' sea water.

Hypothesis -the proposition of a relationship between variables which can then be tested and either rejected or accepted.

HYV-see high yielding variety.