Na.ppe Nappe refers to a large horizontal recumbent fold in rock strata formed through a combination of folding and thrusting. Nappes are formed- during mountain building. They were first described in the Alps.
Natural Region Natural region refers to a region on the earth's surface that displays a distinctive and uniform set of physical characteristics, such as climate, relief or structure.

Nebular Hypothesis Nebular hypothesis suggests that the matter which forms the sun and the planets originated as disc-shaped cloud of gas or nebula:. This cloud of gas eventually contracted into discrete bodies. The hypoth­esis was first proposed by Marquis de Laplace in 1796.

Nekton Nektons are the adively swimming marine organ­isms near the ocean surface. They have the ability to move against tides and currents.

Nephanalysis The analysis of cloud particle depicted on weather~satellite photographs is called nephanalysis.

Neritic Zone Neritic zone refers to that part of the ocean which includes both the estuarine zone and the conti­, nental shelf. It covers over 10 per cent of the total ocean floor and lies below the littoral zone. The neritic deposits are of shallow-water origin.

Nevados Nevados or nevadus is a Katahatic wind, characterised by its extreme coldness, blowing from the Andean peaks down into the valleys of Ecuador.

Nimbostratus Nimbostratus ,is the name given to a thick grey layer cloud that blots out the sun. The base of the cloud is low and fairly uniform. It is most commonly associated with the warm front of a depression.

North Atlantic Drift The north-eastward extension of the Gulf stream is named Atlantic drift flows as water.

Nor'wester Nor'wester is a squall occurring in North Ine during hot season. It also refers to a hot, dry wi] similar to the fohn, that descends from the Southe Alps to the plains.

Oasis The term oasis refers to a fertile patch in an arid ar, at which the water table reaches or is very close to ti ground surface. The water reaches the surface naturall e.g., at an artesian well, or is raised by deep-well borin

Ocean The mass of water occupying all of the earth surface not occupied by land, but excluding all lakes ar inland seas is called an ocean. More specifically, an ocea is the mass of water which fills the ocean basins an excludes marginal seas, such as Mediterranean, Caril bean and Baltic.

Ocean Currents Ocean currents refer to the faster-movin flows of water in the oceans. They are different froi drifts that move slowly.

Okta Okta is a meteorological term that is used to expre! that portion of the sky which is cloud-covered. It i measured in eighths of the total, i.e., 8 oktas are equc to total cloud cover.

Open-cast Mining The extraction of a mineral from jw: below the ground surface by removing the overburder i.e., surface layers, is called open-cast mining. It i synonymous with strip mining.

Orogeny The term orogeny refers to a major period 0 fold-mountain formation, during which the process 0 orogenesis occurs. Orogenesis is the process by which mountains are formed. It includes folding, faulting anI thrusting.

Orographic Rainfall The orographic rainfall is defined a: the precipitation that is caused by moisture-laden ail being forced to rise over a relief barrier.

Orography Orography is a branch of geography that i: concerned with the description of the relief of mountaim and mountain ranges.

Outcrop Outcrop refers to the part of a rock formation thai occurs at the surface of the earth.

Outwash Plain (Sandur) An area of fluvioglacial material deposited by meltwater streams emerging from ice­sheets and glacier is called an outwash plain or sandur. An outwash plain consists of sand, gravel and clay that has been sorted into layers by the water.

Oxbow Lake An oxbow lake is the small crescent-shaped lake in a floodplain that originally formed part of a river meander. It is formed by the erosion of a meandering river and is variously referred to as bayou, billabong or mortlake.

Oxisol Oxisol refers to a soil type characterised by loamy or clayey texture and low base exchange capacity. There is an extreme degree of weathering of oxisol minerals to form free oxides and kaolin. The oxisols occur over vast areas of the humid tropics.

Ozone Ozone is the triatomic form of oxygen (03), It is