MA - Massachusetts

Mach Number - the ratio of an object speed to the speed of sound in the atmosphere.

Mach Scale - a scale of Mach Numbers.

Macroburst - a downdraft that comes from a thunderstorm with an affected outflow area of at least 2.5 miles wide and peak winds lasting between 5 and 20 minutes. Intense macrobursts may cause tornado-force damage of up to F3 intensity. For a smaller scale downburst, see microburst.

Magnetic Field - a field created by an electric charge in motion (an electrical current). It produces a force on a moving electric charge.

Magnetron - a type of radar transmitter tube that has a randomly changing phase from pulse to pulse. It is used in expensive radars and microwave ovens. Compare Klystron.

MAINT - maintain

MALR - moist adiabatic lapse rate; see Moist Adiabat

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

Maritime Air Mass - a large area of air that formed over water and is moist.

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

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

MAX - maximum

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 Radar Data Acquisition (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 Pulse Repetition Frequency (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.

MB (or mb) - see Millibar

MCC - see Mesoscale Convective Complex

MCS - see Mesoscale Convective System

MD - Maryland

MD - see Mesoscale Discussion

MDFY - modify

MDLS - models

MDNGT - midnight

MDT - moderate; see Moderate Risk

MDT - Mountain Daylight Time

ME - Maine

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 Layer CAPE (MLCAPE) - CAPE calculated using a parcel with temperature and moisture characteristics that are an average of the characteristics in a layer from the lowest 100 mb above ground level to the ground.

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

Mechanical Lifting - examples include frontal lift, low pressure systems (surface convergence that leads to rising motion), local circulations, and thermal instability (cool air over a layer of warm air)

Medium-Range Forecast Model (MRF) - an operational model that is now part of the Global Forecast System (GFS) Model.

MEGG - merging

Melting - the process of changing from a solid to a liquid. Fusion is another name for melting.

Melting Level - the altitude at which ice crystals and snowflakes melt as they descend through the atmosphere.

Meridian - an 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

Meridional Flow - large-scale atmospheric motion along a mostly north-south path (almost along a meridian or longitudinal line).

MESO - see Mesocyclone

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

Mesocyclone (MESO) - a vertical column of (counterclockwise) rotating air within a severe thunderstorm that 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 an MCS or its 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

Mesopause - the layer between the Mesosphere and the Thermosphere, located at an altitude approximately between 80 and 90 km (around 50 to 56 miles above the ground).

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

Mesoscale Convective Complex (MCC) - 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 (IR) satellite photographs:

Mesoscale Convective System (MCS) - a group of thunderstorms that 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.

Mesoscale Discussion (MD) - a statement issued by the Storm Prediction Center when conditions are favorable for severe or winter weather. It describes what is currently happening, what is expected in the next few hours, the meteorological reasoning for the forecast, and when/where SPC plans to issue a watch.

Mesosphere - a layer in the atmosphere where temperature decreases with height.

MET - meteorological or meteorology; see Meteorology

METAR - an international code used for reporting, recording and transmitting weather observations.

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

Meteorologist-in-Charge (MIC) - the meteorologist responsible for one National Weather Service Forecast Office (WFO); the head meteorologist for a WFO

Meteorology (MET) - a science that deals with the atmosphere and its phenomena

MI - Michigan

MIC - see Meteorologist-in-Charge

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

MID - middle

Middle Latitudes (Mid-latitudes) - the two regions of the earth typically between 30 degrees and 50 degrees latitude

Mid-Latitudes - see Middle Latitudes

Mid-Latitude Cyclone - see Extratropical Cyclone

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

MIN - minimum

MIN - minute

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

MISC - miscellaneous

MISG - missing

Mist (BR) - a visible group of tiny water particles suspended in the atmosphere that reduces visibility to less than 7 statute miles, but greater than or equal to 5/8 statute miles. It reduces visibility less than fog and is often confused with drizzle.

Mixed Layer - this refers to a near-surface layer in the atmosphere in which turbulence has stirred and uniformly mixed potential temperature and wind speed. The turbulence can be caused by parcels that rise due to solar heating of the earth in the day, or by strong winds. In a mixed layer, potential temperature is often constant or nearly constant.

Mixing (or evaporation) Fog - when moist air mixes with cold, dry air, it can form this type of fog. One example is steam fog, which forms when cold air blows over warm water, such as a lake. The warm air rises into the cold air, giving the appearance of "steam".

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

MLCAPE - see Mean Layer CAPE

MN - Minnesota

MNLY - mainly

MO - Missouri

Model Output Statistics (MOS) - a set of statistical equations that use model output (e.g., from the NGM, GFS, and NAM models) to forecast the probability of precipitation, high and low temperature, sky cover, and precipitation amount for cities across the USA. The equations are specifically tailored for each location and take into account factors such as a location’s climate.

Moderate Risk (of severe thunderstorms) (MDT) - severe thunderstorms are expected to affect between 5 and 10 percent of the area. A moderate risk indicates the possibility of a significant severe weather episode.

Moist Adiabat - a line that depicts the change in temperature of saturated air as it rises and cools. As saturated air rises, the temperature changes at an average rate of 2-3 degrees Fahrenheit per 1,000 feet.

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

MOS - see Model Output Statistics

Mountain Wave - the wavelike effect, characterized by updrafts and downdrafts, that occurs above and behind a mountain range (upwind of mountains) when rapidly flowing air encounters the mountain range’s steep front.

MOV - move

MOVG - moving

mP - see Maritime Polar Air Mass

MR - more

MRF - see Medium-Range Forecast Model

MRGL - marginal

MRNG - morning

MS - Mississippi

MSG - message

MSL - see Mean Sea Level

MSLP - mean sea level pressure

MST - Mountain Standard Time

MSTLY - mostly

MSTR - moisture

mT - see Maritime Tropical Air Mass

MT - mountain

MT - Montana

MTN - mountain

MTNS - mountains

MTS - mountains

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

Mutually Exclusive - when two events cannot occur at the same time. For example, raining and not raining are mutually exclusive at a single point--it cannot be raining AND not raining at that exact point.

MVS - moves