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moist lapse rate

by Coulomb’s formula. An area of stresses for the upper side of the line becomes that of fracture.

moist lapse rate The lapse rate calculated on assumption that the air remains saturated and that all the liquid water formed by condensation is removed as precipitation (without affecting the buoyancy of the parcel). This is also called the pseudoadiabatic lapse rate. The moist lapse rate is smaller than the adiabatic lapse rate. See adiabatic lapse rate.

molar gas constant (R) Constant of proportionality of the equation of state of an ideal gas when the amount of gas is expressed in number of moles n, i.e., P V = nRT . A mole of a substance is the amount of a substance whose total mass, when expressed in grams, numerically equals the dimensionless molecular weight of the substance. The 1998 CODATA (Committee on Data for Science and Technology) rec-

ommended value for the Molar Gas Constant is R = 8.314472(15)J mol−1K−1.

molecular cloud A part of the interstellar medium where the gas is composed mostly of molecules and contains substantial dust. Multiatom molecules are observed in these clouds, with hydrogen and carbon monoxide being the most abundant. The mass of these clouds can be up to 106 solar masses and their size from 10 to 100 parsecs. The temperature varies from 10

to 80 K, and the density is 100 to 106 particles cm−3.

molecular torus A thick structure within the innermost few parsecs of a galaxy, made up of dense molecular clouds, which obscures the innermost part of active galactic nuclei if the torus axis is seen at large angles. Concentration of molecular gas in the regions surrounding the nuclei is observed in starburst and several active galaxies, for example in the case of the prototype Seyfert-2 galaxy NGC 1068. However, properties such as the geometry, the extension, the thickening mechanism, and the frequency of occurrence of molecular tori in active galactic nuclei are largely hypothetical. See Seyfert galaxies, starburst galaxy.

moment (torque) The cross product of a force vector and a position vector. Has units of force times distance (ft-lb, N-m) and is used to measure twisting forces.

moment of inertia In Newtonian dynamics, a 3-tensor whose components are quantities describing the angular response of a body under an applied torque, computed by the volume integral of the density of the body weighted by functions of the displacement from an assumed origin. In general, the moment of inertia is written as a 3 × 3 matrix, but in a specific Cartesian frame centered on the center of the mass of the body, aligned with the principal axes of the body (which coincide with the symmetry axis of the body, if the latter exist) the moment of inertia is diagonal: (Ix , Iy , Iz) where

Ix = ρdV (y2 + z2)dV

and Iy , Iz are defined by cyclicly rearranging indices. By relabeling axes, we may assume Iz ≥ Iy ≥ Ix. With this ordering, rotation about the axis corresponding to Ix or Iz is stable, rotation about the y-axis is not.

Monin–Obukhov length Parameter to compare the relative importance of mechanical and convective turbulence having the dimension of a length scale; the depth z, at which law-of-the- wall scaling and convective scaling provide the same value for the dissipation of turbulent kinetic energy. Defined by

L

M ≡ −

u

3

 

 

καgwT

where u is the friction velocity, κ is the Karman constant, α is the thermal expansivity, g is the constant of gravity, and wT is the heat flux. The Monin–Obukhov length is mainly conceived for atmospheric turbulence studies, but it can also be applied to oceanic applications. LM is the distance from the boundary at which shear production and the buoyant destruction of turbulence are of the same order. The boundary can be either the bottom boundary (land or ocean bottom), or the sea-surface where LM is measured downward. For both stable and unstable conditions, at a distance z << |LM | the effects of

© 2001 by CRC Press LLC

monsoon

stratification are slight and turbulence is dominated by mechanical processes (quantified by u ). For z >> |LM | the effects of stratification dominate. For unstable conditions, at distances z >> −LM turbulence is generated mainly by buoyancy forces in the form of free convection, and at these distances shear production of turbulence is negligible.

monopole Pointlike topological defects as described in some field theories which may carry magnetic charge, possibly produced in the very early universe. Monopoles arise in symmetry breaking schemes in Grand Unification theories. Monopoles are massive, and their predicted abundant production may conflict with observation unless other processes intervene to dilute their density after production. See cosmic topological defect, homotopy group, Langacker–Pi mechanism, monopole excess problem, spontaneous symmetry breaking, t’Hooft–Polyakov monopole.

monopole annihilation In cosmology, the potential destruction of (perhaps magnetic) monopoles by combining a monopole and an anti-monopole, destroying both and releasing energy. Unfortunately, such a pair will first form a bound state before decaying, and these bound states can survive for a very long time. Thus, the annihilation rate cannot keep pace with the expansion rate and the monopole excess persists from the early universe to the present day. See cosmic topological defect, Langacker–Pi mech- anism, monopole excess problem.

monopole excess problem Grand unified theories assume the existence of a simple compact symmetry group G which is subsequently broken to H during a cosmic phase transition. A generic prediction of such symmetry breaking theories is the occurrence of monopoles.

Applied to cosmology, their remnant energy density KM (in units of the critical density) in the universe is found to be very large. In fact, calling h the Hubble constant (in units of 100 km·sec−1·Mpc−1), we have

 

 

 

TC

 

3

mM

,

KM h2

1011

 

1014Gev

1016Gev

which for grand unified monopoles (TC ≥ 1014 GeV, mM ≥ 1016 GeV) is some 11 orders of

magnitude above any acceptable value today. The inflationary paradigm and particular scenarios such as the Langacker–Pi mechanism were proposed to solve this problem.

A more direct understanding of the monopole excess problem arises at the various experimental constraints derived from monopole nondetection. With the density given above, the expected flux, calculable as their number density times their characteristic velocity, namely on average

FM = nM vM

,

is numerically found to be of order (in units of cm−2 · sr−1 · sec−1)

FM 10−3

 

1014Gev

3

10−3c .

 

 

 

TC

 

vM

Such a large flux is well within the present day experimental possibilities and should therefore be easy to detect (in contrast to what happens in reality). See cosmic topological defect, critical density, Kibble mechanism, Langacker–Pi mechanism, monopole, monopole annihilation, Parker limit.

monsoon A seasonal wind which blows with regularity and constancy during one part of the year, and which is absent or blows from another direction during the remainder of the year. In English, this word comes from Arabic in which it means “season”. In the late 17th century, E. Hadley first suggested that monsoons result from the temperature difference between continent and ocean. As the direction of the monsoon changes, weather is changed correspondingly. The climate of the Indian subcontinent is especially characterized by the monsoon, where a distinct rainy season (from June to September) occurs in the southwesterly monsoon. Since the path of monsoon is long, the direction of the monsoon is greatly influenced by the Coriolis effect. After World War II, as high level observation data became more abundant, the monsoon theory became more complete and detailed. Currently, it is believed that monsoons depend on three factors: sea-continent distribution, planetary circulation, and orographic effects. Monsoons can be divided into four types:

© 2001 by CRC Press LLC

monsoon climate

(1) sea-continent monsoon, (2) high level monsoon, (3) plateau monsoon, and (4) planetary monsoon. Every monsoon phenomenon is the result of mixing these four types. According to the latitude distribution, monsoons can be divided as tropical monsoons, sub-tropical monsoons, and temperate monsoons. The Indian monsoon is caused by the seasonal change of planetary wind belts which is enhanced by the influence of sea-continent distribution effects, and the eastern Asian monsoon is caused mainly by sea-continent distribution factor. The Tibetan Plateau is a major plateau monsoon region which is due to its huge plateau’s orographic effects. Other major areas of monsoons are southeastern Asia, the west African coast (latitude 5◦N - 15◦N), and northern Australia.

monsoon climate A climate associated with the tropical regions of the Indian Ocean, Southwestern Brazil, and northwest South America. Characterized by high annual temperatures (≥ 27◦ C), heavy high sun (summer) rain, and a short low sun (winter) dry period. Total annual rainfall is approximately 3 m.

month The period of one revolution of the moon about the Earth or the calendar unit approximating this. A “tropical month” is the period between successive passages by the moon of the vernal equinox (27.32158214 days of mean solar time). A “sidereal month” is the period between two successive transits through the hour circle of a fixed star (27.32166140 mean solar days). An “anomalistic month” is the period between two successive transits of the moon through its perigee (27.5545505 mean solar days). A “draconitic” or “nodal month” is the period between successive passages through the ascending node of the moon’s orbit (27.212220 mean solar days). A “synodical month” is the period between two identical phases of the moon (29.5305882 mean solar days).

moody diagram A diagram for determination of a dimensionless friction factor which appears in the Darcy–Weisbach equation for description of head loss in a full conduit carrying a fluid. The friction factor is generally determined graphically once the Reynolds Number

for the flow and the relative roughness of the pipe are known.

moon (1.) Any natural body which orbits a planet. The term “moon” is often used synonymously with “natural satellite”. A moon can actually be larger than some planets (as is the case with Ganymede, Titan, and Callisto all larger than Mercury and Pluto).

(2.) The twin planet of the Earth. The airless satellite of Earth of radius 1740 km, mass 7.35 × 1022 kg, orbital semimajor axis 3.844 × 105 km, eccentricity 0.0549, albedo 0.07. The moon is tidally locked to synchronous rotation to the Earth.

Dating of returned lunar rocks gives an age of 2.5 × 109 years ago for the oldest lunar surface. The surface shows evidence for heavy meteor bombardment ending about 3.5×109 years ago.

The moon is responsible for the majority of tidal effects on the Earth. Phases of the moon occur because its orbit is close to the ecliptic and carries it from positions between the Earth and the sun, when the side visible from Earth is unilluminated, to positions beyond the Earth from the sun, where the side visible from Earth is fully illuminated. Because the moon’s angular size on the sky is closely the same as the sun’s, it may cause solar eclipses — the shadow of the Moon on the Earth (when its orbit crosses the ecliptic at new moon). Lunar eclipses occur when the moon moves through the Earth’s shadow in space; they occur when its orbit crosses the ecliptic at full moon.

moonbow A rainbow that arises from the refraction and reflection of moonlight on rain drops or mist. Because of the faintness of the source (moonlight), its colors are usually very difficult to detect.

moraines Glacial deposits of unconsolidated, unsorted material left at the margin of an ice sheet. Several types of moraines are recognized. Ground moraines are deposits of material with no marked relief. This material is believed to have been transported at the base of the glacier. A lateral moraine is a moraine formed along the side of a valley (alpine) glacier and composed of rock scraped off or fallen from the valley sides. A media moraine is a long strip of

© 2001 by CRC Press LLC

multipath fading

rock debris carried on or within a glacier. It results from the convergence of lateral moraines where two glaciers meet. A terminal moraine is a sinuous ridge of unsorted material pushed ahead of an advancing glacier and deposited when the glacier begins to recede.

Moreton wave The chromospheric component, seen in Hα radiation, of a solar flareinduced wave which propagates away from the flare site. A Moreton wave travels with a roughly constant velocity of 1000 km s−1 and is attributed to MHD fast mode shocks generated in the impulsive phase of the flare.

morning cloud (Mars) Morning clouds of Mars appear in the early morning and disappear in midday. They reappear the next morning at the same place. They tend to appear in the equatorial region and mid-latitudes of the northern hemisphere in spring to summer of the northern hemisphere. In late spring to mid-summer of the northern hemisphere morning clouds are seen everywhere in low latitudes. They do not disappear even in midday and shift to evening clouds, so that the equatorial region is surrounded with a cloud belt. A notable morning cloud is the one that appears in an area centered at 10 N and 120 W, surrounded by gigantic volcanos of Olympus Mons and Tharsis Montes. The altitude of the cloud is lower than the tops of volcanos. The vertical optical depth of the cloud is about 2 in the early morning. A large morning cloud extends from low latitudes of the southern hemisphere to mid-latitudes of the northern hemisphere in the most active period. See evening cloud, afternoon cloud.

morphodynamics A term used to describe the changing morphology, or form and structure, of a coastal area.

morphology-density relationship The observational fact that more elliptical galaxies are found near the center of a cluster of galaxies where the number density of galaxies is higher while more spiral galaxies are observed in the outskirts of the cluster where the number density is lower (Dressler, 1980). The relative abundance of ellipticals, lenticulars, and spirals is now known to vary smoothly over the range

from 104 galaxies per Mpc3 near the cluster center to 10−2 galaxies per Mpc3 near the cluster boundary. It is not completely understood if the segregation of galaxy types is primordial, but is widely believed to be the result of evolutionary effects subsequent to the formation of the galaxies.

mountain climate Climate of relatively high elevations. Typically populated by northern (or arctic) biome. Conditions include short growing season (in nontropical mountains), decreased nighttime temperature, reduced pressure and oxygen availability, substantial rainfall, and increased ultraviolet solar radiation.

moving magnetic features Small-scale regions of magnetic flux which migrate across the solar surface. When associated with the growth of sunspots these flux regions emerge and approach sunspots with speeds of 0.25 - 1 km s−1.

M star Star of spectral type M. Betelgeuse and Antares are M stars.

MUF fading See skip fading.

multicell storm Storm with multicells. Multicell storms come in a wide variety of shapes, sizes, and intensities and most of them move systematically toward the right of the environmental winds in the middle troposphere. However, the storm propagates toward the right in discrete jumps as individual cells form and dissipate. See supercell storm.

multifingered time The concept that, in general relativity, a time function may be defined which is very general, and in particular does not correspond to uniform proper time advance into the future. See proper time.

multipath fading Multipath, for any propagation link, is the condition where more than one ray can pass between the transmitter and the receiver. Multipath, on an ionospheric propagation path, is a common situation arising from the different paths a ray can follow through the ionosphere. The most obvious source of multipath occurs when several different modes are excited (e.g., the 2E-mode, 1F-mode, and 2F-mode may

© 2001 by CRC Press LLC

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