Weather Glossary - M

Macroburst

A downburst affecting an area greater than 4 kilometres across and having damaging winds. Macrobursts are generally longer-lived than the smaller microbursts.

Macrobursts have been documented to 'wrap-up' into weak tornadoes, however these would be classified as gustnadoes as they are not directly associated with storm-scale rotation.

See also microburst.

Macroscale

The meteorological scale covering an area ranging from a continent to the entire globe.

Madden-Julian Oscillation

The Madden-Julian Oscillation (MJO), also referred to as the 30-60 day or 40-50 day oscillation, is the main intra-annual fluctuation that explains weather variations in the tropics. The MJO affects the entire tropical troposphere, but is most evident in the Indian and western Pacific Oceans. The MJO involves variations in wind, sea surface temperature (SST), cloudiness, and rainfall. Because most tropical rainfall is convective, and convective cloud tops are very cold (emitting little longwave radiation), the MJO is most obvious in the variation of outgoing longwave radiation (OLR), as measured by an infrared sensor on a satellite.

Mammatus

Latin - mammary

Mammatus clouds are rounded, smooth, sack-like protrusions hanging from the underside of a cloud. May occur with cirrus, altocumulus, altostratus, stratocumulus and cumulonimbus. Mammatus clouds often accompany severe thunderstorms, but do not produce severe weather; they may accompany non-severe storms as well. See HP storm, LP storm and supercell.

MCC

Mesoscale Convective Complex.

A large MCS, generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs:

• Size: Area of cloud top -32 °C or less: 100,000 square kilometres or more and area of cloud top -52 °C or less: 50,000 square kilometres or more.
• Duration: Size criteria must be met for at least 6 hours.
• Eccentricity: Minor/major axis at least 0.7.

MCS

Mesoscale Convective System. A complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. MCS's may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS is often used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.

Medium range

In forecasting, (generally) three to seven days in advance.

Meridional flow

Large-scale atmospheric flow in which the north-south component (i.e., longitudinal, or along a meridian) is pronounced. The accompanying zonal (east-west) component often is weaker than normal. Compare with zonal flow.

Meso

Comes from the Greek meaning 'middle'. A common prefix used in meteorology to describe features that are intermediate in size or position.See mesoscale

Meso LAPS

A high resolution version of LAPS. Meso-LAPS (or MLAPS) has a resolution of 12.5km and a smaller domain than LAPS. Meso LAPS runs out to 48 hours.

Mesocyclone

A storm-scale region of rotation, around 3-10 kilometres in diameter and often found in the left rear flank of a supercell (or often on the eastern, or front, flank of an HP storm). The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it.

Properly used, mesocyclone is a radar term; it is defined as a rotation signature appearing on Doppler radar that meets specific criteria for magnitude, vertical depth, and duration. Therefore, a mesocyclone should nto be considered a visually-observable phenomenon (although visual evidence of rotation, such as curved inflow bands, may imply the presence of a mesocyclone).

Mesohigh

A mesoscale high pressure area, usually associated with MCSs or their remnants.

Mesolow

(or Sub-synoptic Low) A mesoscale low-pressure centre. Severe weather potential often increases in the area near and just ahead of a mesolow.

Mesolow should not be confused with mesocyclone, which is a storm-scale phenomenon.

Mesoscale

Used to describe weather systems that lie in between synoptic scale and local scale. This generally means weather features that are between 25km and 250km in size. Examples of mesoscale systems are mesoscale convective systems (MCS), squall lines and southerly busters.

A mesoscale model will have a grid resolution small enough to resolve these features. This usually means a resolution of 25km or smaller. See also meso laps

METAR

METeorological Aviation Report. An evaluation of selected weather elements from a site on or near the ground according to a set of procedures.

It may include type of report, station identifier, date and time of report, a report modifier, wind, visibility, runway visual range, weather and obstructions to vision, sky condition, temperature and dew point, altimeter setting and remarks.

METARS are normally issued either hourly or half hourly, but if there is a significant change in conditions at the site, a SPECI (special report) may be issued. See also SYNOP.

Meteogram

Weather outlooks of temperature, rainfall, winds and relative humidity. Presented as a graph, these outlooks are extracted directly from the latest computer weather prediction models.

Meteorology

The science and study of the atmosphere and atmospheric phenomena.

Microburst

A small, concentrated downburst affecting an area less than 4 kilometres across. Most microbursts are rather short-lived (5 minutes or so), but on rare occasions they have been known to last up to 6 times that long.

Microscale

The smallest scale of meteorological phenomena that range in size from a few centimetres to a few kilometres.

Mid latitudes

The areas between about 30 degrees and 55 degrees latitude.

Mid level cooling

Local cooling of the air in middle levels of the atmosphere (roughly 8,000-25,000' or 2,000-8,000m), which can lead to destabilization of the entire atmosphere if all other factors are equal. Mid-level cooling can occur, for example, with the approach of a mid-level cold pool.

Middle clouds

Clouds with bases between 6000 and 18000 feet.

Middle latitudes

The latitude belt roughly between 35° and 60° North and South.

Mirage

A refraction phenomenon the makes an image appear to be displaced from its true position. It is most common close to the ground on hot days, when the surface layer of the atmosphere is much warmer than the layer above, resulting in lower density and different optical properties of this surface layer.

Mist

Similar to fog, but visibility is greater than 1 kilometre.

Mixed layer

Upper portion of the boundary layer in which air is thoroughly mixed by convection.

MLAPS

See Meso LAPS.

Model

See GCM.

Moisture advection

Transport of moisture by horizontal winds.

Moisture convergence

A measure of the degree to which moist air is converging into a given area, taking into account the effect of converging winds and moisture advection. Areas of persistent moisture convergence are favoured regions for thunderstorm development, if other factors (e.g., instability) are favourable.

Monsoon

A seasonal wind in the tropics. The northern Australian monsoon season generally lasts from December to March. It is associated with the inflow of moist west to northwest winds into the monsoon trough, producing convective cloud and heavy rainfall over northern Australia.

These moisture-laden winds originate from the Indian Ocean and southern Asian waters. The northern Australian wet season encompasses the monsoon months but can extend several months either side, say between October and April.

Morning Glory

An elongated cloud band common across Cape York Peninsula and the Gulf Country in northern QLD. It is visually similar to a roll cloud, usually appearing in the morning hours when the atmosphere is relatively stable. Morning glories result from perturbations related to gravitational waves in a stable boundary layer. They are similar to ripples on a water surface. Several parallel morning glories often can be seen propagating in the same direction.

Mountain wave

A wave in the atmosphere caused by a barrier such as a mountain. Is sometimes marked by lenticular clouds to the leeward side of a mountain.

MRF

Medium-Range Forecast model; one of the operational forecast models run at NCEP. The MRF is run once daily, with forecast output out to 240 hours (10 days).

MSL

Mean Sea Level. It is necessary to convert the pressure readings to equivalent mean sea level pressures, otherwise the horizontal changes in pressure would be overwhelmed by vertical variations simply due to differences in altitude between observing stations.

In this way, a Mean Sea Level Pressure map will then show pressures affected by changing weather conditions, not because of changing altitude.

Multicell thunderstorm

A thunderstorm consisting of two or more cells, of which most or all are often visible at a given time as distinct domes or towers in various stages of development.

Nearly all thunderstorms (including supercells) are multi-cellular, but the term often is used to describe a storm which does not fit the definition of a supercell.

Multiple vortex tornado

(or Multi-vortex tornado) a tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common centre or about each other. Multiple-vortex tornadoes can be especially damaging. See suction vortex.

Mushroom

[Slang] A thunderstorm with a well-defined anvil rollover, and thus having a visual appearance resembling a mushroom.