earth's surface down to the tropopause. The polar-night jet stream occurs in the lowest layer of the stratosphere.

Stratus Stratus is a type of cloud which is grey in colour and has a uniform base. It occurs in thin sheets at any height from the surface up to 500 m.

'Stripped Plain A stripped' plain is the area of low relief developed upon horizontally bedded sedimentary rocks. In stripped plains, less resistant sediments have been removed. So they are more resistant to erosion.

Subduction Zone Subduction zone refers to the area where one lithospheric plate (oceanic) is overridden by another plate (oceanic or continental) and is forced down into the underlying mantle. Subduction zones mark collision zone or destructive plate margins that are characterised by a zone of earthquake activity.

Sublimation Sublimation is the process in which it solid is converted into a vapour or vice versa without an intermediate liquid stage.

Subsequent Stream A stream that follows ~ course roughly at right angles to the consequent stream is called a subseque,nt stream. Subsequent streams develop by the erosion (mainly headward) of sedimentary rock strata by the river.

Subtropical The term subtropical denotes the areas lying between latitudes 40° north and south of the equator. The subtropical region is warmer than the temperate regions and has no month with a mean temperature below 6 dc. It is an atmospheric high pressure zone. Mediterranean areas and cotton belt climate region are
the examples of the areas in subtropical zone.

Sunspots Sunspots are the dark spots on the surface of the sun. they represent disturbances of the sun's magnetic field. They have an average periodicity of about 11 years.

Surf A mass of foaming water made by waves breaking over rocks or in shallow water is called surf. The width of a surf varies with the changing wavelengths and according to the character of the beach materials.

Swamp A swamp is the area of persistently muddy and waterlogged ground. Often associated with mangrove vegetation, swamps occur along the margins of streams and ponds or where drainage collects.

Swell The regular undulating movement of the surface of the open sea is called swell. Created by wind stress, swell increases in wavelength and decreases in wave height as it moves away from the generating area.

Synoptic Chart Synoptic chart refers to a map in which all ineteorological 'data are entered for a specific time to give an indication of weather conditions at a given time over a large area.

Syzgy Syzgy refers to the situation when the earth, moon and sun are in conjunction or in opposition, i.e., when they are all in a straight line.

Tableland A tableland is a flat-topped or undulating area of high relief, i.e., a raised plateau. It is usually bounded by steeply descending slope.

Tafoni The term tafoni refers to hollows on rock surfaces, especially granite, due to cavernous weathering. S hollows are thought to be formed by haloclasty wind scour on exposed rock.

Taiga A Russian term, taiga refers to the belt of conifel forest which circles the land masses of the Nortl' Hemisphere between the temperate grasslands I tundra. Siberia in North Asia is a typic:>~ 2xample of b region.

Talsands A German term for 'valley sands', talsands D to large-scale cappings of wind-blown sands in fon ice-marginal valley.

Talus The term talus refers to weathered rock fragm­at the base of a slope below a cliff face. The tern often considered synonymous with scree.

Tectonic The term tectonic refers to the internal which deforms the earth's crust.

Temperate Zone Today known more accurately as latitude zone, the temperate zone refers to an a between the torrid, and frigid zones where there are extremes of temperature, i.e., where climates are erate.
Tensinometer A device for measuring the tension of We in an in situ soil.

Terra Rossa Terra rossa is the name given to a red clay-loam soil developed on limestones, in MediteI nean climate, especially in the Karst terrain of YugO! via. It is rich in iron sesquioxides and lacks humus, degraded terra rossa is termed as terra fusca.

Tethys Tethys is the name of the. ocean of Mesozoic e which divided Gondawana land from Laurasia. 1 Mediterranean Sea is now a vestige of this former oce;
Thalweg (Talweg) Thalweg or talweg is a German te for the long profile of a river valley. It is also defult as the line of the deepest part of the channel bed

Theodolite Theodolite is an optical instrument for meaSl ing angular distances in both the vertical and horizon planes. It is used extensively in surveying.

Thermal Equator (Heat Equator) Thermal or heat equal is defined as an imaginary line around the globe t. joins the places on each meridian having the high. mean surface temperature for any particular period. average position lies to the north of the true equate The reason for this is that there are larger masses the north equator and land absorbs more heat than ti oceans.

Thermocline Thermocline is an intermediate layer of wat in an ocean or a deep lake. It lies between the disturbl upper layer (epilimnion) and the relatively stagna lower layer (hypolimnion). It represents the zone whe the maximum vertical temperature gradient occurs.

Thermospl1,ere Thermosphere is the uppermost layer of the atmosphere. It occurs above the mesopause, some km above the earth's surface. In this layer, temperatul
decreases with height.

Thunderstorm Thunderstorm is a storm accompanied b lighting and thunder that develops in large cumulO1 imbus clouds. It is often associated with the passage a cold front or due to intensive heating of the groun surface and the resulting convectional uplift of air.