Tide The regular rise aIlcl fall of water level in the oceans is called tide. Tides are caused by the gravitational pull of the moon and, to a lesser extent, of the sun. The tide of the smallest amplitude is called the neap tide. It occurs twice a month, during the first and last quarters of the moon. A tide with the greatest amplitude is called spring tide. It also occurs twice a month, but around full moon and new moon. A tide occurring when the moon is in perigee is called perigean tide. Apogean tide occurs when the moon is in apogee.

Time Zone Time zone is defined as a time division devised for convenience. It usually comprises 15" of longitude.

Topography The surface feature of an area of land or sea bed is called topography. Topography includes the relief, the terrain, the "Vegetation, the soils and all the features created in the landscape by human activity.

Tornado A violently rotating storm in which winds whirl around a small area of extremely low pressure is called tornado. Tornado is associated with a dark funnel­shaped cloud and with extremely violent winds blowing in a counter-clockwise spiral. Tornados are most fre­quent and violent in central plains of the USA, where they are sometimes known as twisters.

Trade Winds Trade winds are predominantly easterly winds that blow in the tropics from the sub-tropical high pressure zones towards the equatorial low-pressure
trough. They are more pronounced over the oceans than over the continents.

Transform Fault A tear~fault (i.e., strike-slip fault) that occur within the earth's crust oriented at right angles to a mid-ocean ridge or trench is called transform fault. Such faults occur where the motion between the two plates is parallel to the boundary that separates the plates. The term transform fault was introduced in 1965 by J.T. Wilson to explain the transformation of strike faults into mid-oceanic ridge, island arcs or fold-moun­tain belts.

Trench A trench or ocean deep is a deep ~rough in the abyssal plain, which often resembles a steep-sided valley or canyon on the land and forms the deepest parts of the oceans.

Tributary A tributary is the stream whi~h drains into a larger one, thereby contributing water to it.

Tropic (Tropics) Tropic or tropics is a line of latitude at approximately 23.50 Nand S of the equator. The northern tropic is the Tropic of Cancer, while the southern tropic is the Tropic of Capricorn: Tropical Year Tropical Year is defined as the time interval between successive passage of the sun through' the earth's equatorial plane at the vernal equinox.

Troposphere Troposphere is the lowest layer of the earth's atmosphere. It extends from the surface of the earth upwards to the tropopause. It is the zone where atmospheric turbulence is at its greatest and where the bulk of the earth's weather is generated.

Trough A trough is defined as a system of low atmospheric pressure. It has much greater length than width. All troughs are not frontal, but all fronts occupy troughs. A trough also refers to a depression between the successive crusts of a wave in tl1e sea.

Tsunami A Japanese term, 'tsunami refers to a large seismically generated seawave, especially where subma­rine earthquakes occur. The tsunamis travel at high speeds, about 300-500 knot and can cause great destruc­tion and loss of life through flooding.

Tundra Tundra refers to the extensive treeless plains of the North Siberia, North-Scandinavia, North Canada and Alaska. Tundra region is marked by mosses, lichens and
a low vegetation of dwarf shrubs and occasional stunted Arctic willows.

Twilight Twilight refers to the faint glow which illuminates the sky before sunrise and after sunset. It occurs due to the reflection of sunlight on to the earth from the upper layers of the atmosphere where the sun is below the horizon. It is shorter in the tropics than in polar latitudes where it lasts from sunset to sunrise.
Typhoon Typhoon is a tropical cyclone in West Pacific and China Sea.

Ubac A valley's slope facing away from the equator is called ubac, i.e., north facing slope in Northern Hemi­sphere and south facing slope in Southern Hemisphere.
The ubac slope remains forested with pasture ancL mineral deposits. The slope opposite to ubac is called adret which is a cultivation zone.

Ultrabask Rock An igneous rock containing low percent­age of silica and feldspar and high percentage of ferra­magnesium minerals is called an ultrabasic rock.

U-Shaped Valley A U-shaped valley is formed by the erosion of the bed and lower walls of a V-shaped valley by rock fragments embedded in a moving gla£ier:
Uvala A uvala is the large surface depression (of several kilometres diameter) in limestone terrain or Karst. It is formed by the coalescence of several smaller depres­sions known as dolines.

Vale A vale is the broad valley that is occupied by more than one river and follows the outcrop of relatively less resistant sedimentary rock.

Valley A valley is a linear depression sculpted by water or ice or formed by movements of the earth's crust. It is initially created by fluvial erosion but may subse­quently be modified by glacial erosion.

Van Allen Belts Van Allen belts refer to the two bands of the outermost layer of the upper atmosphere (mag~ netosphere) in which ionised particles are trapped.
Veld (Veldt) Veld or Veldt is an Afrikaan name for the temperate grassland of the plateaus of interior South Africa.

Vent A vent is the opening or conduit in the surface of the earth through which volcanic material is ejected by volcanic eruption. It is usually surrounded by a cone.

Vertisol Vertisol is one of the orders of the soils. Vertisols have high cation-exchange capacity and clay content.