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Mammatus Clouds - cloud features resembling pouches which hang from the underside of a cloud (usually a thunderstorm anvil) and are typically associated with intense cumulonimbus clouds

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:


Area of cloud top -32 degrees C or less = 100,000 square kilometers or more
Area of cloud top -52 degrees C or less = 50,000 square kilometers or more


Size criteria must be met for at least 6 hours


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. MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.

Maritime Polar Air Mass - an air mass characterized by cold, moist air

Maritime Tropical Air Mass - an air mass characterized by warm, moist air

Maximum Thermometer - a thermometer designed to register the maximum temperature during a given interval of time (generally a day)

Maximum Unambiguous Range - the range from the radar at which an echo can be known unquestionably as being at that range. As the radar sends out a pulse of energy, the pulse hits a target and part of the energy bounces back to the radar, but part of the energy may continue to travel away from the radar. The distance to the target is computed by knowing the time that has elapsed since the pulse was emitted. Then a second pulse of energy is transmitted. If some of the energy from the first pulse strikes a target at a far range and returns to the radar when radiation from the second pulse arrives, the RDA misinterprets the returned first pulse as arriving from a target near the returned second pulse. The maximum unambiguous range is related to the amount of time that elapses between successive pulses of emitted energy, or the PRF.

Maximum Unambiguous Velocity - the highest radial velocity that can be measured unambiguously by a pulsed Doppler radar. The maximum unambiguous velocity is related to the radar's PRF. When a target's velocity exceeds the maximum unambiguous velocity, the velocity will be "folded" to appear as a different velocity. See velocity folding.

Mean Annual Temperature - the average temperature for the entire year at any given location

Mean Daily Temperature - the average of the highest and lowest temperatures during a 24-hour period

Mean Sea Level - the height of the sea surface midway between its average high and low water positions

Meridian - a imaginary line on the earth's surface passing through both geographic poles and through any given point on the planet, also called a line of longitude

Mesoclimate - the climate of a small area of the earth's surface which may differ from the general climate of the district

Mesocyclone - a vertical column of (counterclockwise) rotating air within a severe thunderstorm which may be a precursor to a funnel or tornado; typically a mesocyclone is 2-6 miles in diameter. The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it. Properly used, mesocyclone is a Doppler radar feature that meets specific criteria for magnitude, vertical depth, and duration.

Mesohigh - a mesoscale high pressure area, usually associated with MCSs or their remnants

Mesolow (or Sub-synoptic Low) - a mesoscale low-pressure center. 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.

Mesonet (or Mesonetwork) - a regional network of observing stations with a station spacing such that weather features on the mesoscale can be resolved

Mesoscale - of or relating to meteorological phenomena approximately 2 to 200 kilometers in horizontal extent; thunderstorms and squall lines are two examples of mesoscale events

Meteogram - a graphical depiction of trends in meteorological variables such as temperature, dew point, wind speed and direction, pressure, etc. The time series meteogram can be constructed using observed data or forecast data.

Meteorologist - a scientist who studies the weather and atmosphere

Meteorology - a science that deals with the atmosphere and its phenomena and especially with weather and weather forecasting

Microburst - an intense downdraft (downburst) less than 4 km wide (about 2.5 miles) that may occur beneath a thunderstorm

Microclimate - the essentially uniform local climate of a usually small site or habitat

Micron - one thousandth of a millimeter; one millionth of a meter

Microscale - the smallest scale of atmospheric motions; smaller than the mesoscale

Microwave - a type of electromagnetic radiation with wavelengths between infrared radiation and radio waves

Middle Latitudes - the two regions of the earth typically between 30 degrees and 50 degrees latitude

Mid-Latitude Cyclone - see extratropical cyclone

Millibar (mb) - a unit of atmospheric pressure equal to 1/1000 bar or 1000 dynes per square centimeter

Minimum Thermometer - a thermometer designed to register the minimum temperature during a given interval of time (generally a day)

Mirage - an atmospheric optical phenomenon that makes an image of some object appear displaced from its true position

Mixing Ratio - the ratio of the mass of water vapor in a system to the mass of dry air

Moisture Advection - the transport of moisture by horizontal winds

Moisture Convergence - a measure of the degree to which moist air is converging into a given area

Moisture Ridge - an axis of relatively high dew point values. This axis is sometimes referred to as a 'moist tongue'.

Mole - a unit of mass equal to the molecular weight of the substance

Molecule - the smallest particle of a substance that retains the properties of the substance and is composed of one or more atoms

Monsoon - a name for seasonal winds, especially in the Indian Ocean and southern Asia

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 10 days.

Multi-cell(ular) 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 cloud towers, in various stages of development; the term often is used to describe a storm which does not fit the definition of a supercell

Multiple-Vortex (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 center or about each other


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