Some Terms

Radiation The transfer of energy emitted by two bodi (not in contact) through a medium by means of ele tromagnetic waves of different wavelengths is callE radiation. In meteorology, radiation includes both SOliradiation (a process by which solar energy reaches ~ earth) and terrestrial radiation (reradiation of the sole energy by the earth).

Radioarbon Dating Radio-carbon dating refers to th method of determining the age of an organic materij by measuring the proportion of the carbon-14 isotop contained within its carbon content. This method ha been used to date organisms up to 70,000 years old, b\: accuracy diminishes beyond 30,000-40,000 years.

Rainbow Rainbow is defined as an arc of light, displayinl the colours of the spectrum that is observed when ray of sunlight pass through raindrops. It is caused bJ refraction and internal reflection of sunlight. It is onl) visible when the (i) sun is at an elevation of 42° or less (ii) observer is standing with the sun behind, and (iii. rain is in front of the observer.

Rainfall The total amount of precipitation that falls over a given period is called rainfall. Rainfalls may be orographic, convectional or cyclonic.

Rain Gauge An instrument designed to measure rainfall is called rain gauge.

Rain Shadow Rain shadow is the term used to describe the area on the leeward side of a'topographic barrier, such as mountains, where the amount of rainfall is less than that on the windward side of the barrier. The reason for low precipitation in the leeward side is that the descending air has less moisture and it is also warmed by its descent.

Rapids Rapids are a stretch of swift-flowing water where river bed suddenly becomes steeper. They are formed at the points where rivers cross an outcrop of more resistant rock.

Reef A reef is defined as a line of rocks in the tidal zone of a coast, submerged at high water but partly uncov­ered at low water. It can be composed of any type of rock.

RegimelRegimen The term regime or regimen refers to the organisation or pattern of a natural system. It may also be defined as the characteristic movements of a stream/river as it attempts to remain adjusted to its channel. A river is said to be in regime or regimen if its channel has achieved a stable form.

Regolith Regolith refers to a layer of unconsolidated weathered material or waste material lying above bedrock and below the ground surface. It comprises all types of rock waste together with superficial deposits of loess, alluvium, etc..

Regur (Black Cotton Soil) A Hindustani word, regur is the name given to a dark-coloured, fertile, loamy soil in India. Regular soil, also called black cotton soil, is developed on the basaltic lava of the Deccan.

Rejuvenation Rejuvenation is the state of a river when its erosional activity or capacity is revived. Such a change of activity may result from.a fall in base-level or an increase in stream discharge. Rejuvenation due to the former is called dynamic rejuvenation, while static rejuvenation is the term for the rejuvenation caused by
the latter factor.

Relative Humidity Relative humidity is the ratio between the amount of water vapour actually present in an air mass and the maximum amount that the air could hold at that temperature. It is expressed a's a percentage.

Relict Landform Relict landform refers to a landform that was created by processes that are no longer operative. For example, the features created by glacial erosion, such as cirque and U-shaped valleys, are relict landforms because glaciers are no longer extant in Britain.

Relief Relief is the character of the land surface of the earth. It comprises a wide variety of landforms. Relative relief is the difference between highest and lowest elevation in an area. Available relief is the difference in height. between an original upland surface and the bottom of adjacent graded valleys.

Rendzina Rendzina is the name given to an intrazonal soil that is developed on very calcareous parent material, i.e., limestones. It develops in humid to semi-arid climates. It is a shallow soil with dark horizon.

Reverse Fault A fault in which the fault plane is incfu,M at an angle between 45° and the vertical is called reverse fault. Reverse fault occurs as a result of very strong compressional forces.

Ria A Spanish term, ria refers to a submerged coastal valley or estuary resulting from a rise of sea level. Rias are V-shaped in cross profile. Most of them were formed by the past-glacial drowning of discordant coasts.
Richter Scale Richter Scale, named after its inventor C.F. Richter, is a -logarithmic scale to identify the magnitude of an earthquake. Richter devised it in 1935.

Ridge-and-Valley Relief A relief in which approximately parallel ridges and valleys alternate is called ridge-and­valley relief, e.g., ridge and valley region of the Appa­lachian Mountains of the USA.

Ridge of High Pressure An elongated area of high pressure is called the ridge of high pressure. Such a ridge extends out from an anticyclone and is bounded by areas of low pressure on either flank. It often gives a brief period of fine anti-cyclonic weather.

Rift Valley Rift valley is a flat-bottomed valley formed by the sinking of the ground between two nearly parallel faults- or two parallel series of step faults. Rift valleys are associated with vulcanicity. They are considered to have resulted from movement of tectonic plates.

River A river is defined as a large stream of fresh water flowing downhill within a channel to enter another river or lake or sea, except in a des-ert area where it may dwindle away to nothing.

River Capture River capture refers to a process by which a stream erodes head wards to such an extent that it captures or intercepts the course of a neighbouring river. The point of interception is called the elbow of capture; which is commonly at right angles. The cap­tured stream is called misfit stream. The mechanism by which river capture occurs is explained in Campbell's Law.

Roaring..Forties The term roaring forties refers to the area lying between latitudes 40° Sand 50° S where westerlies blow with great regularity and strength as there are few land masses to act as barriers for the winds il} this zone.

Roche Moutonnee A French term,.roche moutonnee is the name given to a glacially eroded mound of rock, which res-embles a lawyer's wig. Roche moutonnees are com­mon features of glaciated areas.

Rock The term rock refers to any aggregate of mineral particles that forms part of the earth's crust.

Rotation of the Earth The west to east movement of the earth on its polar axis is called the rotation of the earth. The velocity of the earth's rotation varies according to the latitudes, the maximum speed (1,690 km/h) being at the equator.
Run off The portion of rainwater or meltwater from snow that drains into rivers is called runoff. Runoff is normally regarded as the ~ainfall minus the loss by evaporation: