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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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  • M component - In a lightning discharge, an increase in channel luminosity accompanied by a rapid electric field variation, itself called an M electric field change. The M components occur when the channel is already faintly luminous. Downward-moving leaders have not been observed to precede M components. The M components may be confused with branch components, the increases in channel luminosity that occur between each branch and the ground when the upward-propagating return stroke reaches that branch, since the higher branches are obscured by the cloud. The M component is a minor surge of current that reilluminates the channel in a negative cloud-to-ground flash. It may occur within microseconds or up to a few milliseconds of a return stroke. Evidence exists that M components are a result of the sequence of fast in-cloud negative leaders (K changes) contacting a conducting ground channel and renewed ground potential wave that reilluminate the channel.

  • mamma - (Also called mammatus.) Hanging protuberances, like pouches, on the undersurface of a cloud. This supplementary cloud feature occurs mostly with cirrus, cirrocumulus, altocumulus, altostratus, stratocumulus, and cumulonimbus; in the case of cumulonimbus, mamma generally appear on the underside of the anvil (incus). See cloud classification.

  • mammatus - Same as mamma.

  • marin - A warm moist southeast wind from the sea on the French Mediterranean coast and in the Maritime Alps, especially frequent in spring and autumn. In the Rhône delta it blows also from the south. The marin is associated with depressions that cross southern France or northern Spain and the Gulf of Lions. Generally, it is strong and regular, sometimes violent and turbulent in hilly country as the ayalas in the Massif Central; it is very humid, cloudy with hill fog, and often rainy (unless unaccompanied by fronts, when it is the marin blanc). The heavy rains, which may continue for one or two days on the mountain slopes, cause dangerous river floods. On the western slope of the Cévennes it becomes the autan. In the southern Cévennes the marin is called the aygalas. On the coast of Catalonia (northeast Spain) and Roussillon (southern France) it is the marinada and generally occurs with a depression centered over or south of the Gulf of Gascony. Compare sirocco.

  • maximum - The greatest value attained (or attainable) by a function; the opposite of minimum. An "absolute" maximum is the greatest value within a prescribed interval, while "relative" maxima are the greatest values within arbitrary subintervals, each one of which is "absolute" within its own subinterval, and so on. In records of meteorological observations, "absolute" is with reference to the entire period of record for that station, and the "relative" values are labeled "annual," "monthly," and "daily."

  • mean - An arithmetic average. Types can include mean over an ensemble of experimental realizations, mean over a time period, mean along a line such as a road or flight path, mean in an area such as a farm field, or mean within a volume of air such as can be sampled by a remote sensor. For example, the mean wind speed from anemometer measurements is the speed at a fixed point averaged over a time period such as 10 minutes.

  • meander - Dispersion of smoke plumes in the horizontal by means of the crosswind component (fluctuations) of the horizontal wind speed. The result is a plume that wanders from side to side. When averaged over a finite time period, the result is plume spreading in the horizontal.

  • mechanical turbulence - Turbulence produced by shear flow.

  • meridional - In meteorology, a flow, average, or functional variation taken in a direction that is parallel to a line of longitude; along a meridian; northerly or southerly; as opposed to zonal.

  • meridional flow - A type of atmospheric circulation pattern in which the meridional (north and south) component of motion is unusually pronounced. The accompanying zonal component is usually weaker than normal. Compare zonal flow, meridional circulation; see meridional index.

  • mesoscale - Pertaining to atmospheric phenomena having horizontal scales ranging from a few to several hundred kilometers, including thunderstorms, squall lines, fronts, precipitation bands in tropical and extratropical cyclones, and topographically generated weather systems such as mountain waves and sea and land breezes. From a dynamical perspective, this term pertains to processes with timescales ranging from the inverse of the Brunt-Väisälä frequency to a pendulum day, encompassing deep moist convection and the full spectrum of inertio-gravity waves but stopping short of synoptic-scale phenomena, which have Rossby numbers less than 1.

  • mesoscale eddies - See mode eddies.

  • mesoscale model - A model designed to simulate mesoscale atmospheric phenomena. Such models can include analytic solutions of a set of simplified equations governing atmospheric motion, scale models of particular geographic regions, and numerical integrations, including numerical weather prediction models that can resolve mesoscale circulations. See nonhydrostatic model.

  • met - A colloquial contraction for meteorology, occasionally used in a technical sense, especially in aviation. It can also refer to a meteorologist.

  • Meteor - A series of operational polar-orbiting meteorological satellites launched by the former Soviet Union since 1969. There have been three series of Meteor satellites, Meteor-1, Meteor-2, and Meteor-3. Meteor instrumentation includes visible and infrared imaging radiometers, including an APT transmission, an infrared sounder, and an instrument to monitor the space environment.

  • meteorologist - A person who is professionally employed in the study or practice of meteorology. It often refers to individuals who have completed the requirements for a college degree in meteorology or atmospheric science.

  • meteorology - 1. The study of the physics, chemistry, and dynamics of the earth's atmosphere, including the related effects at the air-earth boundary over both land and the oceans. Fundamental topics include the composition, structure, and motion of the atmosphere. The goals ascribed to meteorology are the complete understanding and accurate prediction of atmospheric phenomena. See also atmospheric science; compare climatology. 2. In popular usage, the underlying science of weather and weather forecasting.

  • meter - In the International System of Units, the fundamental unit of length; equal to 100 cm, 3.2808399 ft, or 39.370079 in.

  • MF - Abbreviation for medium frequency. See radio frequency band.

  • microscale - Atmospheric motions with Lagrangian Rossby numbers greater than 200 or spatial scales 2 km or less.

  • mil - 1. A unit of angular measurement, sometimes used in radar antennas, equal to 1/6400 of the circumference of a circle. 2. A unit of length, equal to 0.001 in., used in measuring the diameter of wire.

  • minimum - The least value attained (or attainable) by a function; the opposite of maximum. (See further discussion under maximum.)

  • minimum temperature - The lowest temperature attained at a specific location during a specified period.

  • minor trough - A pressure trough of smaller scale than a long-wave trough. It ordinarily moves rapidly and is associated with a migratory cyclonic disturbance in the lower troposphere. See short wave.

  • mist - 1. A suspension in the air consisting of an aggregate of microscopic water droplets or wet hygroscopic particles (of diameter not less than 0.5 mm or 0.02 in.), reducing the visibility at the earth's surface to not less than 1 km or 5/8 mi. The term mist is used in weather reports when there is such obscurity and the associated visibility is 1000 m or more, and the corresponding relative humidity is 95% or more, but is generally lower than 100%. These hydrometeors form a thin greyish veil that covers the landscape. It also reduces visibility, but to a lesser extent than fog. 2. In popular usage in the United States, same as drizzle.

  • mixing - 1. The result of irregular fluctuations in fluid motions on all scales from the molecular to large eddies. Gradients of conservative properties such as potential temperature, momentum, humidity, and concentrations of particles and gaseous constituents are reduced by mixing, tending toward a state of uniform distribution. See turbulence, eddy flux, diffusion. 2. In electronics, the nonlinear (nonadditive) combining of signals. The common mixing element is a diode or set of diodes. The common desired result of mixing two sinusoidal signals is the multiplicative product, with terms at the sum and difference frequencies. Mixing is used to shift signals to different carrier frequencies.

  • ML - Abbreviation for mixed layer.

  • MMO - Abbreviation for main meteorological office.

  • mode - One of several accepted measures of central tendency. It is the most probable value of a discrete variate, or the point of maximum probability density in the case of a continuous variate.

  • model - A tool for simulating or predicting the behavior of a dynamical system like the atmosphere. Models can be based on subjective heuristic methods, statistics (see statistical dynamical model, model output statistics), numerical methods (see numerical forecasting), simplified physical systems (see dishpan experiments), analogy (see analogs), etc. The term is now most commonly applied to numerical models.

  • moist air - 1. In atmospheric thermodynamics, air that is a mixture of dry air and any amount of water vapor. Compare dry air, saturation. 2. Generally, air with a high relative humidity.

  • moisture - (Or moisture content.) In meteorology, a general term usually referring to the water vapor content of the atmosphere, or to the total water substance (gaseous, liquid, and solid) present in a given volume of air. In climatology, moisture refers more specifically to quantities of precipitation or to precipitation effectiveness. See humidity; see also soil moisture.

  • moisture content - Same as moisture.

  • moment - 1. The product of a distance and another parameter. The moment may be about a point, line, or plane; if the parameter is a vector, the moment is the vector product of the vector distance from the point, line, or plane, into the parameter. Thus, the moment of the momentum of a fluid parcel per unit volume about an axis is r ´[×] r[&rgr;]u, where r is the vector from axis to the parcel, r[&rgr;] the density, and u the velocity vector of the parcel; this is also called the angular momentum. The moment of a force F about an axis is r ´[×] F, called the torque. The second moment of a parameter is the moment of the first moment, and so on, for higher moments. 2. By analogy, in statistical terminology, the mean value of a power of a random variable. The symbol m[&mgr;]n¢[′] (or n[&ngr;]n) is used for a raw moment as distinguished from the corresponding central moment m[&mgr;]n taken about the mean m[&mgr;]. Thus the raw moments are m[&mgr;]n¢[′] º[≡] n[&ngr;]n º[≡] E(xn), (n = 0, 1, 2, · · ·), where E(xn) is the expected value of the variate x to the nth power. In particular, m[&mgr;]0¢[′] º[≡] 1 and m[&mgr;]1¢[′] º[≡] n[&ngr;]1 º[≡] m[&mgr;]. The central moments are m[&mgr;]n º[≡] E[(x - m[&mgr;])n], (n = 0, 1, 2, · · ·), where E[(x - m[&mgr;])n] is the expected value of the nth power of the deviation of the variate from its mean. In particular, m[&mgr;]0 º[≡] 1, m[&mgr;]1 º[≡] 0, m[&mgr;]2 º[≡] s[&sgr;]2, where s[&sgr;]2 is the variance.

  • momentum - (Usually means linear momentum as opposed to angular momentum.) In Newtonian mechanics the (linear) momentum p of a body with mass m and velocity v is the product of these two quantities: p = mv. In the absence of forces, momentum is conserved. But momentum is a more fundamental quantity than simply the product of mass and velocity. For example, photons have momenta, which can be transferred to objects (as evidenced by radiation pressure), and yet the photon has zero rest mass. Thus, momentum is best looked upon as a single entity, complete in itself, governed by the dynamical law F = where F is the force acting on the body with momentum p.

  • MOS - Abbreviation for model output statistics.

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