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trench excavation

trench excavation A method of investigation for active faults to deduce when large earthquakes occurred on the fault. Digging a trench several meters in depth so as to cross the active fault discloses disturbance of strata outcroppings on the trench wall and their age of formation. From such investigations, active periods of large earthquakes over the past several thousand years, recurrence time, and magnitude of large earthquakes can be estimated. Such information is very useful to forecast a forthcoming large earthquake on the active fault. Recently, Japanese archeological excavations have discovered evidence for large earthquakes, such as liquefaction of a ground and traces of ground cracks.

triggered star formation The process where star formation in one area can induce star forma- tion in a nearby region. This process can occur by various mechanisms (e.g., stellar winds, supernovae) in a star cluster which can compress the nearby interstellar medium to densities sufficient for new stars to condense.

triple-alpha process The set of nuclear reactions whereby three helium atoms (alpha particles) are converted to one carbon atom with the liberation of about 1018 ergs per gram of energy. The triple-alpha process is a major energy source in stars that have exhausted hydrogen in their cores (see horizontal branch, clump stars, asymptotic giant branch), and the main source of carbon in the universe. It occurs at a temperature near 108K and only at relatively high density, because the three helium atoms must approach each other very closely, since the expected intermediate nucleus, 8B, is unstable to dissociation back into two alpha particles. Another reaction, 12C(α, γ )16O, occurs at very slightly higher temperature, so that helium fusion normally results in a mix of carbon and oxygen (more oxygen at higher temperatures). Some of the nuclear reaction cross-sections involved are still not adequately measured in the laboratory or calculated, so that prediction of the C/O ratio in the products remains somewhat uncertain. See asymptotic giant branch star, clump star, horizontal branch star.

triple junction In geophysics, a point on the Earth where three plate boundaries meet.

triple point In physical chemistry, the temperature and pressure under which gas, liquid, and solid phases of a substance can coexist in equilibrium. For water, at a standard pressure of one atmosphere, this occurs at a temperature of 273.16 K (0.01◦C). The (saturated) water vapor pressure is 6.11 millibar under these conditions.

In meteorology, a junction of three distinct air masses, leading to unstable conditions.

tripton Inanimate particulate matter in natural waters.

tritium 3H; the isotope of hydrogen whose nucleus consists of one proton and two neutrons. Radioactive to beta decay to 3He with a half-life of 12.26 years.

Tritium Unit A unit used to scale the amount of tritium present in a particular sample of ocean water. It is defined by convention as 1018 times the atom ratio of tritium to normal hydrogen. See also tracers.

Triton Moon of Neptune, also designated NI. It was discovered by Lassell in 1846. Its orbit has an eccentricity of 0.00002, an inclination of 157.4◦, a precession of 0.5232◦ yr−1, and a

semimajor axis of 3.55 × 105 km. Its radius is 1350 km, its mass is 2.14 × 1022 kg, and its

density is 2.1 g cm−3. Its high albedo and large heliocentric distance gives it a very low surface temperature (34.5 K) and almost no atmosphere. It orbits Neptune, retrograde, once every 5.877 Earth days. The peculiar orbit (retrograde and highly inclined) makes it likely that Triton is a captured body, probably from the Kuiper belt.

trochoidal wave theory A finite-amplitude wave theory attributed to Gerstner (1802). The water surface profile is a trochoid, which represents a good match to many observed waves, but the resulting predicted velocities do not match observations well.

Trojans A class of asteroids that orbit at the L4 and L5 Lagrangian points. These two points, 60◦ before and after the position of a planet in its

© 2001 by CRC Press LLC

trough

orbit around the sun, are stable points in a three body system (sun – planet – asteriod). For the case of the planet Jupiter, a number of asteriods have been found in this region and are all named after heroes of the Trojan wars.

Trojans asteroids Asteroids located near Jupiter’s Lagrange points (60◦ ahead and behind Jupiter in its orbit).

tropical cyclone A large low pressure system that originates over the tropical oceans; including tropical depressions, tropical storms, and hurricanes (cyclones, typhoons), with winds sometimes up to 300 km/h.

tropical instability waves Oceanic disturbances of meridional scale of 500 km, zonal wavelength of 1000 km and a period of 30 days that travel westward along 2-3◦N in the equatorial Atlantic and Pacific Oceans. Instabilities of velocity shears among equatorial currents are their cause. They start to grow in June and decay toward the end of a year, in correlation with the intensity of equatorial currents. They are well visible from satellite infrared images as they cause meanders of a strong sea surface temperature (SST) front between the equator and 5◦N. Often called the Legeckis waves after the discoverer of the waves’ signature in SST. These wave-induced SST variations excite atmospheric waves of significant amplitudes in the marine boundary layer.

tropical month See month.

tropical storm A storm originating in the tropics; particularly severe tropical storms become hurricanes (sustained wind speeds > 120 km/h).

tropical year

See year.

Tropic of Cancer The parallel of latitude 23◦27 N. The sun lies in the zenith (overhead at noon) at this latitude, at the summer solstice, around June 22. North of this latitude, the sun is always south of directly overhead.

Tropic of Capricorn The parallel of latitude 23◦27 S. The sun lies in the zenith (overhead

at noon) at this latitude, at the winter solstice, around December 22. South of this latitude, the sun is always north of directly overhead.

tropopause The boundary layer between the troposphere and the stratosphere. In the troposphere, air temperature generally decreases with height, and in the lower parts of the stratosphere, air temperature remains constant or increases (0.1 to 0.2◦C/100 m). The altitude of the tropopause varies with the variations of sea

— surface temperature, season, latitude, and weather systems, such as the passage of cyclones and anti-cyclones. It has its highest height (about 17 to 18 km) over the equator and lowest (about 6 to 8 km, average 10 to 12 km) over the poles. There are two different types of tropopause. One is the pole-region tropopause with height lower than 150 hPa; another is the tropical-subtropical tropopause with height higher than 150 hPa. Because of this distinct height difference, when these two different types of air mass systems approach, the tropopause will appear as a broken phenomenon. Under such a broken tropopause, there is often a high level jet which is related to the front.

troposphere The layer of the atmosphere closest to the planet’s surface. The troposphere is characterized by a decrease in temperature with altitude, caused by a decrease in the amount of warming due to the Greenhouse Effect with altitude. Hadley Cell circulation and the Greenhouse Effect dominate the temperature profile and weather patterns within the troposphere. On Earth, the troposphere extends to about 25 km above the surface. The exact height varies with latitude and season. The boundary between the troposphere and the stratosphere is called the tropopause. The troposphere is also often called the lower atmosphere.

trough A long, relatively wide trough running on sea bottoms, whose maximum depth of the water is less than 6000 m. This definition is applied irrespective of the origin of troughs. There are some troughs whose structure and origin are basically the same as those of trenches. Such an example is the Nankai trough situated on the Pacific side of the Japanese islands. For such a trough, as is the case for a trench, seismic

© 2001 by CRC Press LLC

true anomaly

activity and crustal movements associated with subduction of an oceanic plate and formation of accretionary prisms on the continental slope are marked.

true anomaly In Newtonian dynamics, the angle between an orbiting body and periapse.

true celestial equator The true celestial equator or celestial equator of date is the great circle on the celestial sphere perpendicular to the true celestial pole. q.v. Its intersections with the ecliptic define the vernal equinox of date and the autumnal equinox of date.

true celestial pole The true north celestial pole is the direction of Earth’s instantaneous rotation pole. It differs from the mean north celestial pole due to the short timescale (days to decades) variations called nutation. The nutation itself, of order 10 arc, is normally described by a theory, such as the IAU 1980 nutation theory, which suffices to predict the pole position within a few hundreths of an arc second, and smaller observed corrections called the celestial pole offset. See http://hpiers.obspm.fr/webiers/general/Earthor/ precnut/PNUT.html.

true north Geographic north defined by the rotational pole of the Earth, as opposed to magnetic north defined by the geomagnetic north pole.

true polar wander The Earth’s hot spot system appears to define a nearly rigid deep mantle frame. The motion of the hot spot system with respect to the rotational axis of the Earth is called true polar wander, as opposed to the apparent polar wander as a result of continental drift. See continental drift, hot spot, polar wander.

tsunami Water wave in the ocean or large lakes caused by underwater earthquakes, landslides, or volcanic activity. In the open ocean, tsunamis can have wave lengths of hundreds of kilometers, traveling at speeds of several hundreds of kilometers per hour. They are often called tidal waves, although they have no relation with tides.

tsunami earthquake Generally means an earthquake accompanying tsunami (tidal wave), or an earthquake that causes a large tsunami for its magnitude. Typical examples are the 1896 Sanriku-oki, Japan earthquake and the 1946 Aleutian earthquake. Existence of tsunami earthquakes indicates that we cannot predict tsunami height at a coast precisely only from the magnitude of earthquakes, which makes tsunami difficult to forecast. Extremely slow faulting and generation of submarine landslide have been considered as mechanisms of tsunami earthquakes.

TT See Terrestrial Time.

T Tauri object A class of variable stars whose brightness varies irregularly, with broad and very intense emission lines. T Tauris, also known as class II protostars, are very young premain sequence stars that gain mass through an accretion disk as evinced by an excess of infrared emission, but whose surface is optically visible. Also characterized by extensive and violent ejections of mass and magnetic activity, T Tauri stars are named after the first of their class and have masses of roughly 0.2 to 5 M .

T Tauri star The last stage of stellar evolution before the main sequence. T Tauri stars are characterized by emission lines, rapid variability, and X-ray emission, associated with gas being both accreted and lost in bipolar outflows and with relatively strong magnetic fields and rapid rotation (see stellar activity). The name is frequently, but not always, restricted to stars of roughly 0.5 to 2.5 solar masses, more massive ones being called Herbig Ze/Be stars, and the most massive being seen only as radio sources called compact HII regions.

Tully–Fisher law An empirical law which relates the width of the 21-cm neutral hydrogen spectral emission line to the luminosity of a spiral galaxy, proposed by R.B. Tully and J.R. Fisher (1977): the total galaxy luminosity is proportional to the fourth power of the width of the 21-cm line. The relationship is best (i.e., data points show less scatter) if the luminosity is measured in the infrared. The infrared luminosity depends little on Hubble type and is

© 2001 by CRC Press LLC

twin paradox

correlated to the number of old disk and intermediate disk population stars, which make up the largest fraction of the mass in the disk of a spiral galaxy. The physical basis of the Tully– Fisher law resides in equating two observable parameters that are independent estimates of the mass of a galaxy. Using the Tully–Fisher law, the intrinsic luminosity of a galaxy can be obtained from the measurement of the broadening of the HI 21-cm line, which does not depend on distance. The Tully–Fisher relationship can be used, therefore, as a distance indicator. See Faber–Jackson law.

turbidity A measure of the amount of suspended debris within a body of water. Different units are used for description of turbidity, including Jackson Turbidity Units (JTU), Silica Standard Turbidity Units (TU), and Nephelometric Turbidity Units (NTU).

turbulence Condition of fluid flow characterized by irregular and aperiodic distribution of velocity and vorticity components. Random motions cause a distribution of scalar properties at rates much higher than molecular diffusion processes. The length scales (eddy sizes) associated with active, three-dimensional turbulence range between approximately 0.6η and 11LN , where η is the Kolmogorov microscale describing the length scale where velocity fluctuations are stopped by viscosity, and LN is the buoyancy scale, which represents the largest vertical size of turbulent eddies allowed by the ambient stratification.

turbulence cascade This term implies that turbulent eddies decay successively to smaller scales starting from the energy-containing scale (where the energy input occurs) to the smallest scales, given by the Kolmogorov Scale LK = (ν3/ε)1/4, where the turbulent kinetic energy is converted into heat by molecular viscosity ν. If the turbulent fluid is not stratified or if the Ozmidov-scale is substantially larger than the Kolmogorov Scale, an inertial subrange may develop. The source of the non-linear term of the momentum.

turbulent cascade In a turbulent shear flow, the largest eddies have roughly the same size

as the width of the shear flow itself. In the ocean or the atmosphere this is often the height of the boundary layer. The largest eddies extract kinetic energy directly from the mean flow field. Smaller eddies are advected in the velocity field of the large eddies, extracting kinetic energy from the larger eddies. Thus, through a series of increasingly smaller eddies, kinetic energy is transferred (“cascaded”) from larger to smaller scales by a mechanism that arises from the non-linear terms in the equations motion of turbulent flows (vortex stretching). The smaller eddies do not directly interact with the mean flow field or the larger eddies. Kinetic energy is cascaded from larger to smaller scales until the turbulent motions are stopped by viscous effects. The size of the dissipating eddies is given by the Kolmogorov microscale.

turbulent flow Occurs when fluid particles follow irregular paths in either space or time because inertial forces are much greater than viscous forces. Turbulent flow is empirically defined as flow with a high Reynolds number.

turbulent flux The total flux of a property < aiui > can be interpreted as the sum of the flux due to the mean flow < ai >< ui > plus the turbulent flux < aiui > due to the covariance of the turbulent fluctuations of ai and ui.

turbulent kinetic energy The energy e [J kg−1] contained in the turbulent velocity

fluctuations per unit mass of fluid, defined by e = 0.5 < u 2 + v 2 + w 2 >, where u , v , w

are the three velocity fluctuations components (see Reynolds decomposition for definitions).

twilight The interval of time preceding sunrise and following sunset during which the sky is partially illuminated. “Civil twilight” is the interval when the zenith distance (angle), referred to the center of the Earth, of the center of the sun’s disk is between 90◦50 and 96◦. “Nautical twilight” is the interval between 96◦ and 102◦. “Astronomical twilight” is the interval between 102◦ and 108◦.

twin paradox Because of the relativity of simultaneity, clocks in relative motion that have a relative velocity v run at different rates. An

© 2001 by CRC Press LLC

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