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M

Man-machine mix

The concept whereby a forecast result is achieved as a combination of human interaction with (by intervention), and interpreting (using experience of the atmosphere and conceptual models) NWP output, rather than allowing the models to predict the weather with no interpretation.

MAR

Abbreviation used to stand for maritime environments, often seen in aviation forecasts.

mb (or MB)

Often seen as the abbreviation for 'millibar' (see also mbar).

mbar

An abbreviation for 'millibar', being one-thousandth part of a bar. The 'bar' is the basic unit of atmospheric pressure as defined in the c.g.s. system of measurement (now regarded as obsolete). 1 bar = 103 millibars = 106 dynes/cm2 (c.g.s. system) = 105 N/m2 (Pa) (SI system).

MCC

(abbr) Mesoscale Convective Complex

MCS

(abbr) Mesoscale Convective System

Mean wind

The atmosphere in motion near the surface of the earth (the 'wind') exhibits marked variations over very short periods of time, depending on such variables as low level thermal stability, ground-induced friction effects, vertical shear etc. Average or 'mean' winds are defined over periods such as 1, 2 or 10 minutes for reporting in meteorological bulletins. (60 minute periods are often used for climatological purposes). For SYNOP and METAR reports, the period is 10 minutes in most countries, though in tropical storm advisories and measurements associated with US-based organisations (e.g. NHC, JTWC), 1 minute averages are used, and often referred to as 'sustained' winds.

Meridional

Both predominantly north-to-south and south-to-north airflows (over distances of 100's of km) are termed meridional.

Mesoscale Convective Complex (MCC)

This might be regarded as the 'super' version of an MCS. It is strictly defined in terms of areal extent of coldest tops of the cloud sensed by IR imagery (typically over 300 km), and must last at least 6 hr, with many active systems lingering for upwards of 12hr. Would be regarded as extremely rare for NW Europe - more a feature of central and eastern Europe with average annual frequency around 5.

Mesoscale Convective System (MCS)

(as defined in 'Images in weather forecasting' - other services may define differently)... " a mesoscale grouping of deep convective and stratiform cloud and precipitation, together with associated circulations. The system exists much longer (at least four hours) than the lifetime of an individual constituent cloud. The individual convective clouds contribute to a common upper-tropospheric outflow shield or anvil. The system's convective-scale downdraughts merge at some time to form a continuous zone of cold air in the lower troposphere". What is visible on satellite imagery is the amalgamation of many cirrus outflow heads from the many cells comprising the system. Frequent cloud-to-cloud lightning is a feature of these systems. Although mainly a feature of the central plains of the United States and over continental Europe, they can on occasions affect Britain, mainly affecting the 'English lowlands' with an average frequency 2 to 3 times per year - when they do occur, they are responsible for some extreme/violent convective weather, and behave as a weather system in their own right, often modifying the enviroment in which they form considerably.

METAR

METeorological Aviation Report -- a weather observation for a specific airfield at a given time, containing the minimum information necessary for air operators for safe usage: wind/visibility/significant weather type/cloud amount+base/temperatures/pressure settings being the 'core' elements. Usually only issued with all elements when the airfield is operational (i.e. Air Traffic Control is open), but increasingly automated observations are now appearing of varying quality. Major/civil airports issue at HH+20 and HH+50 (i.e. 20 and 50 minutes past each hour), with others hourly only. METAR reports may also have TREND forecasts appended giving a short-range (usually 2hr) forecast of significant changes. SPECI reports are issued when meteorological variables deteriorate/improve through defined levels. For a brief listing of the weather decode for a METAR/TAF/SPECI, click on this link, and there is now a full explanation of the METAR code on this site :- HERE

MI

(abbr) Shallow (below eye level); used in aviation reports in connection with fog (FG).

MINTRA

To aid the forecasting of condensation trails emitted (or not) from high-flying aircraft, a line marking the critical temperatures (altitude dependent), above which trails are not possible, is marked on a tephigram . The values are approximately -24degC at 1000 hPa (i.e. roughly sea-level), -39degC at 250 hPa (34000ft / 10.4 km) and about -45degC at 130 hPa (50000feet/15km). Using the MINTRA line (as it has come to be called - based on experiments by JK Bannon during World War II with the piston-engined Spitfire), a forecaster will mark two further lines on a tephigram: MINTRA minus 11degC (A) and MINTRA minus 14degC (B). If the ambient temperature (from the tephigram air temperature plot) lies between (A) and (B), then short, non-persistent trails are possible. If colder than (B), then long, persistent trails should be expected. However, some note should be paid to the relative humidity - high values will tip the balance to trailing (or longer/persistent trails.), even with air temperatures warmer than (A); ultra-low rh% will reduce the risk of condensation trails - the design of engines will have an effect as well. In broad terms, warm Tropical Maritime airmasses with a high but cold tropopause will result in a good deal of trailing, whilst cold, polar air-masses with a low, relatively warm tropopause will seldom give rise to significant aircraft trails. (See also "Why do some high flying aircraft leave white trails in their wake?")

Mist

Visibility reduced due to the suspension of minute water droplets in the atmosphere: the visibility is >=1km (less than this, and fog is reported), and the relative humidity is ~>95%. However, slavish adherence to this latter figure is not advised!

Mountain Waves

(also known as standing, lee or orographic waves. Often abbreviated to MTW in aviation circles) -- Under certain conditions, which must include a stable layer around/just above a mountain/hill range, air flowing across the range will be found to oscillate vertically in a standing wave configuration with well defined crests/troughs in the wavetrain. The horizontal flow must be reasonably brisk and within roughly 30 degrees of the crest of the hill/mountain range, with speed increasing with height, and directional shear must be small. Airflow is often smooth, but the vertical velocities in the upward/downward legs of the standing waves can be strong, and may break down to give local severe turbulence. Surface winds downstream of the ridge may be notably gusty as well, but paradoxically, they may also be extremely light, or even reverse direction. The presence of MTW activity may be betrayed in visible satellite imagery, if the flow is moist enough, as a series of near-parallel bars of cloud downwind of the ridge causing the airflow displacement. (See entry under: "Lenticular clouds") [ Although standing-wave motion is often referred to as 'lee' wave action, upward motion will be found on the windward / upslope side and of course across the hill / mountain crest too; there are also well documented cases of wave motion being found some considerable distance upwind of a topographical obstacle - perhaps due to some element of 'backwards' propagation of the wave-train. Wave motion may also be seen in cloud patterns where the triggering is due to marked shear in the windflow, well away from a range of hills. ]

MRF

(abbr) [historical only - no longer used] Medium Range Forecast (or Global Spectral) model (run by the NCEP). Had two basic formulations: for short-range work, the "Aviation" (AVN) run to 72 hours, and for extended range work (also known as the MRF run) to beyond 144 hours - in one iteration to 240 hours (i.e. 10 days). Now (2002), subsumed within the main NCEP NWP suite as the Global Forecast System (GFS) (q.v.).

MSLP

(abbr) Mean sea level pressure (also seen as PMSL).

Multi-cell storm

See the FAQ here.