Absolute Humidity The amount of water vapour per unit volume of air is called absolute humidity. In other words, absolute humidity is the density of water vapour present in a mixture of air and water vapour, Le., ratio of the mass of water vapour to the volume occupied by the mixture. It is expressed in grams per cubic centimetre (gcm-1). Cold air has a lower absolute humidity than warm air.
Abyssal Plain Also known as deep-sea plain, abyssal plain is generally the level area of the ocean floor which lies between about 3,500 m and 5,500 m below the surface of the ocean. Features such as trenches, mid-ocean ridges, sea-mounts, etc., often break the regularity of the abyssal plain. Abyssal plain is composed of ooze or red clay. In this plain, sea temperatures do not exceed 4" C.
Acid Rain When the rain is contaminated by either sulphur oxides (caustic gases emitted mostly by coal combustion) or by a combination of sulphur dioxide and
nitrogen oxide (smog emitted from motor vehicle exhaust), it is termed as acid rain. Acid rain has a pH value of less than 5.6 and is capable of burning holes in leafy vegetation, and causing considerable damage to the environment.
Acid Rock It is an igneous rock that is very rich in silica (over 60 per cent), over 10 per cent of which are free quartz, and aluminium. Acid rocks are relatively light and form the main material of the continents. Granite, granodiorites, obsidian and rhyolites are the typical examples of acid rocks.
Actinometer Actinometer is a device that measures solar radiation. The instrument which records solar radiation is known as actinograph.
Adiabatic Process It is an atmospheric process in which changes take place in the pressure and volume of a body of gas (air) without an actual loss or gain of heat from outside. As a result, the temperature of the air mass changes. Adiabatic changes normally occur in an ascending or descending air mass. When a parcel of air rises, its volume increases due to a decrease in pressme. With decrease in volume, temperature of air also decreases. The rate at which temperature decreases in rising air is called the adiabatic lapse rate. For a saturated rising air, the fall of temperature is checked by the release of latent heat of condensation. A reverse process occurs when an air parcel descends.
Advection Advection is the horizontal movement of an air mass across the earth's surface. The transfer of heat from low to high latitudes is the most obvious example of advection. Advection is specially important when the air mass is moving from a different source area and is bringing in different conditions of humidity and temperature. Advection is opposite to convection.
Aeolian (Eolian) The term aeolian or eolian denotes the processes or landforms caused by or related to the wind. Ventifact, yardang, zeugen, etc., are the examples of
aeolian erosion, while dunes, loess, etc., are the examples of aeolian deposition. The aeolian processes are most common in arid environments, such as hot and cold deserts, and on exposed shorelines.
Aerology Aerology is the scientific study of all the atmosphere above the surface layers. Aeronomy, a branch of aerology, is the scientific study of the atmosphere above 50 km where dissociation and ionisation of gas molecules occur.
Aerosols The aerosol is often used to describe minute particles (solid and liquid) suspended within the gas. Seasalt, dust (particularly silicates), organic matter, smoke, carbon, lead and aluminium compounds are examples of aerosols. Aerosols enter the atmosphere by both natural processes, such as vulcanicity, and by human agency, such as the burning of fossil fuels. Due to their presence in the atmosphere as dust veil, aerosols lower the amount of solar radiation reaching the earth. Hence, their overall thermal effect is probably one of cooling, counteracting the greenhouse effect.
Agate Occurring in cavities in volcanic and some other rocks, agate is a semi-precious stone. Agate consists of a type of silica (chalcedony).
Aggradation Aggradation is the process by which a land surface is built up by the deposition of debris and other solid materials by rivers or seas. River-borne aggradation is more common. Aggradation usually results due to a reduction in the river'& load-bearing capacity. Such reduction may be due to several factors; such as an increase in the amount of debris supplied to the river; a loss of speed or volume of flow; and a rise in the base level.
Aiguille Aiguille is a French term which is widely adopted to describe narrow, needle-shaped rocks. Such rocks are particularly well developed in the Chamonix region 0.£ the European Alps.
Air Mass A mass of air with similar properties of temperature and moisture covering a large area of the earth's surface is called/air mass. According to the nature of their source regions, air masses may be classified as Arctic, Antarctic, Polar, tropical or equatorial. This classification is based on temperature characteristics of air masses. On the basis of its humidity, an air mass is classified as maritime, continental or monsoonal. A combination of these characteristic gives tropical maritime air mass, polar continental air mass, polar continental monsoonal air mass, etc.
Aleutian Current It is a cold ocean current of the North Pacific Ocean.
Aleutian Low Located over the Aleutian Island of the North Pacific ocean, Aleutian low is one of the cells in the semipermanent zone of subpolar low pressure. It is not a stationary low pressure system but represents an aggregate of deep depressions. It is more marked in winter than in summer.
Alluvial Cone Alluvial cone is a cone-shaped accumulation of thick, coarse alluvial material which is transported by ephemeral or short-lived torrents emerging fmm a high rocky massif at a mountain front or valley side: It is similar to, but steeper than an alluvial fan. Alluvial cone is more typical of arid and semi-arid areas where torrents carrying large loads are abruptly checked on entering a main valley or plain.
Alluvial Fan Alluvial fan is a fan-shaped landform composed of alluvium, consisting of sand and gravel (i.e., finer material than in an alluvial cone). An alluvial fan is formed at a place where a constricted river enters a main valley or emerges from mountains onto a plain. The apex of the fan is located at the point at which the river emerges. It is the thickest part of an 'alluvial fan. Fans of the upper Rhone valley in Switzerland are typical examples of alluvial fans.
Alluvium It is a sedimentary material deposited by a river along its course. Such a deposition may be in the forms at cones, fans, deltas and flood plains. Alluvium is characterised by: (i) sorting, so that coarser alluvium is
found in the upper course of rivers and finer in the lower courses; (ii) stratification, such that coarse material in river bars is overlain by finer material; and (iii) structures such as current bedding. Alluvial deposits are agriculturally fertile, though subject to flooding. These deposits can contain important minerals, such as gold, platinum and diamonds.
Almwind It is a local wind of the fohn variety which blows across the Tatra mountains. It descends as a warmer wind onto the plains of South Poland in spring, causing avalanches.
Alpine The term alpine refers to or belonging to the Alps mountain. It is also used to refer to the physical features (such as relief, climate, soils, ete.) of mid-latitude mountain environments.
Altimeter It is an instrument which indicates height above sea level, i.e., altitude. Pressure altimeter and radio altimeter are the two main types of altimeter. The pressure altimeter is a form of aneroid barometer that measures changes in atmospheric pressure with height. Radio altimeter measures the time taken by radio waves from an aircraft to reflect back from the earth's surface.
Altrimetric Frequency Analysis It is a technique used in analysing the relief of an area by map analysis. The method involves noting all the summit spot-heights shown on the map and plotting them on a bar chart. The horizontal axis on the graph represents altitude above sea-level, while vertical axis represents the percentage frequency of the summits. The method is not used on a large scale today.
Altocumulus Altocumulus is a type of cloud that occurs in bands or waves. Such clouds are often separated by blue sky. Altocumulus clouds occur at about 2,000-7000 m, i.e., at middle altitudes, in temperate latitudes. They are usually a sign of fair weather.
Altostratus Altostratus is a type of cloud occurring at middle altitudes, i.e., between 2,000-7,000 m. It is composed of either ice particles or water droplets or a mixture of both. It usually forms a greyish sheet covering much of the sky. Altostratus is associated with a warm front, and is often a precursor of rainy water.
Anemometer Any instrument that measures wind speed is called anemotneter. The most common anemometer is the Clip anemometer. Other types of anemometer include pressure-tube anemometer, pressure-plate anemometer hot-wire anemometer, anemograph (an anemometer that automatically records wind speed on a continuous trace) and sonic anemometer (an anemometer based on the theory that an ultrasonic pulse travels through air at speed of 340 fils).
Antecedent Drainage (Antecedence) A drainage system that has maintained its direction of flow despite tectonic, uplift or folding of land across its course is called antecedent drainage or antecedence. An antecedent riVer is able to maintain its direction due to the fact that the rate of downcutting of such river is approximately equal to the rate of crustal uplift. Antecedent rivers generally cross the uplifted mountains and plateaus by means of deep gorges. The Colorado river of the USA and the Indus, Brahmaputra and the Ganges rivers in India are typical examples of antecedent drainage.
Anticline and Syncline Anticlines and synclines result from compressional forces acting on the rocks. Anticline is a rock structure in which the beds of rock are folded into an arch, i.e., the beds dip outwards from the crest, the oldest rocks occurring in the core. Anticlines alternate with synclines in which the strata dip towards each other. An upfold is an anticline, while a downfold is a syncline.
Apehelion and Perihelion Aphelion is the furthest point of heavenly body or planet (including our earth) from the sun during its orbit. In the case of the ear~h, aphelion occurs on July 4, when the sun-earth distance is 152 million km, which is 1.5 per cent greater than the annual mean distance between the two. Perihelion is the nearest point to the sun reached by a 'heavenly body during its orbit. In case of the earth, this occurs on January 3 when the sun-earth distance is 147.3 million km.
Apogee Apogee is defined as the meridianal altitude of the sun at midday on the longest day of the year. Apogee is also defined as the point in the orbit of a planet when it is farthest away from the earth.
Aquifer Acquifer is a rock layer sufficiently porous to absorb water and allow it to pass freely through. If it is underlain by a layer of impermeable rock, water p.assed will not be downwards, but directed laterally. If the stratum is bounded on its upper surface by an impermeable rock layer, it is termed a confined acquifer.
Arcuate Delta It is a type of delta which is fan-shaped. Its rounded margin extends into the sea. The fan shape of such delta is a result of the division and subdivision of channels. Nile Delta is a classic example of arcuate delta.
Artesian Basin/Well An artesian basin is a geological structure in which the beds dip toward a central point'
forming a basin with a permeable layer of aquifer (a rock) sandwiched between impermeable rocks. London Basin in UK is a good example of an artesian basin. An artesian well is a well sunk into an artesian basin. Water of an artesian well is forced upwards to the ground surface by hydrostatic Fressure, if the point at which the well is sunk lies below the water-table level in the aquifer. In many arid and semi-arid regions, such as parts af Australia and the USA, artesian wells are the main saurce af Water.
Asthenosphere Asthenasphere is a gealagical term to' denate the zane af the earth's mantle, which lies immediately belaw the lithasphere. Asthenasphere has paar rigidity and rock strength. Hence it is capable of producing pralanged deformatian. According to' geaphysicists, harizantal canvectian flaw in asthenasphere may be a cause af mavement af the plates af the auter lithasphere (plate tectanics). Asthenasphere is thaught to. be campased af partially malten peridatite.
Atmosphere Atmasphere is the layer af gases envelaping the earth. It is compased af nitrogen, axygen, argan, carban diaxide and variaus ather gases. Over 95 per cent af the atmasphere lies within abaut 30 km af the surface af the earth; thaugh its auter limit extends to. abaut 10,000 km. There are several layers af atmasphere, such as tropasphere, stratasphere and mesasphere.
Atoll An atall is a ring-shaped island reef that encircles a central layer af sea water in which detrital material callects. It is campased largely af coral but in same aceanic atalls certain calcareaus algae may farm the bulk af the reef. It is a particularly camman feature af the
Pacific Ocean.
Attrition The wearing dawn af rack particles by ather rock particles while being transparted by water, wind or ice, is called attritian. Through attritian, particles become gradually smaller, smaather and mare rounded, e.g., beach pebbles.
Aurora Aurara is a luminaus phenamenan seen in the sky at high latitudes. It is manifested as streamers, veils, sheets and rays at heights af abaut 100 km in the night sky. It is thaught to. result from magnetic starms and salar dischar~s during peri ads afhigh sun-spat activity. Aurora is knawn as the auorara borealis ar Ilorthem lights in the Narthern Hemisphere ?nd as aurora austrlis ar sOl/them lights in the Sauchern - Hemisphere.
Avalanche A slide ar horizantal fall af a mass 6f snaw and ice in mauntainaus areas is called avalanche. Avalanches accur when snaw and ice accumulate to. such an extend that the mass is unable to. suppart itself and hence slides under the influence af gravity. Avalanches are classified into. a number af types based an different variables such as type af breakaway (laase snaw ar slab); humidity of snaw (dry ar wet); pasitian af sliding surface; form af track: and farm af mavement.
Azonal Soil Also. knawn as skeletal sail, azanal sail is a sail type in which the sail profile is underdevelaped and there has been little ar no. differentiatian into. separate sail harizans. Azanal sails are aften young. These sail are subdivided into. lithasal, regasal and alluvial sails.
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