A

MACKEREL SKY: The name given to cirrocumulus clouds with small vertical extent and composed of ice crystals. The rippled effect gives the appearance of fish scales.
MACROBURST: A large downburst with an outflow diameter of 2.5 miles (4 kilometers) or larger and damaging winds.
MACROSCALE: The meteorological scale covering an area ranging from the size of a continent to the entire globe.
MAGNETIC POLES: Either of the two points on the earth's surface where the magnetic meridians converge. They are not aligned with the geographical poles, but shift and do not lie exactly opposite of the other.
MAMMATOCUMULUS: An obsolete term for cumulonimbus mammatus, it is a portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud. Although they do not cause severe weather, they often accompany storms.; Related term: Dave's Dictionary
MARE'S TAIL: The name given to thin, wispy cirrus clouds composed of ice crystals that appear as veil patches or strands, often resembling a horse's tail.
MARGINAL VISUAL FLIGHT RULES (MVFR): Refers to the general weather conditions pilots can expect at the surface. MVFR means Minimum or Marginal Visual Flight Rules. MVFR criteria means a ceiling between 1,000 and 3,000 feet and/or 3 to 5 miles visibility.; Related terms: IFR and VFR
MARITIME AIR MASS: An air mass influenced by the sea. It is a secondary characteristic of an air mass classification, signified by the small "m" before the primary characteristic, which is based on source region. For example, mP is an air mass that is maritime polar in nature. Also known as a marine air mass.
MAXIMUM: The greatest value attained by a function, for example, temperature, pressure, or wind speed. The opposite of minimum.
MEAN SEA LEVEL: The average height of the sea surface water level. For the United States, it is computed by averaging the levels of all tide stages over a nineteen year period, determined from hourly height readings measured from a fix, predetermined reference level. It is used as a basis for determining elevations, as the reference for all altitudes in upper air measurements, and as the level above which altitude is measured by a pressure altimeter for aviation. Often referred to as MSL.; Related term: sea level
MEAN TEMPERATURE: The average of temperature readings taken over a specified amount of time. Often the average of the maximum and minimum temperatures.
MEASURED CEILING: A ceiling classification applied when the ceiling value has been determined by an instrument, such as a ceilometer or ceiling light, or by the known heights of unobscured portions of objects, other than natural landmarks, near the runway.; Related term: variable ceiling
MELTING LEVEL: The altitude at which ice crystals and snow flakes melt as they descend through the atmosphere.; Related term: bright band
MELTING POINT: The temperature at which a solid substance undergoes fusion, changing from a solid to a liquid state. Contrast with freezing point.
MERCURIAL BAROMETER: An instrument used for measuring the change in atmospheric pressure. It uses a long glass tube, open at one end and closed at the other. After first filling the open end with mercury, it is then temporarily sealed and placed into a cistern of mercury. A nearly perfect vacuum is established at the closed end after the mercury descends. The height of the column of mercury in the tube is a measurement of air pressure. As atmospheric pressure increases, the mercury is forced from the cistern up the tube; when the atmospheric pressure decreases, the mercury flows back into the cistern. Measurement is taken in inches of mercury. Although mercurial barometers are very accurate, practicality has led observers to use aneroid barometers. First used by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.
MERIDIONAL FLOW: Atmospheric circulation in which the north and south, or meridional, component of motion is unusually pronounced. This weakens the zonal flow.
MESOCYCLONE: A area of rotation of storm size that may often be found on the southwest part of a supercell. Its circulation can be larger than the tornado that may develop within it, but not necessarily. Originally a radar term for a rotation signature that met certain criteria, it is best seen on Doppler radar.
MESOHIGH: A small, concentrated area of high pressure that may be created by the cold outflow and rain-cooled air from thunderstorms. It often forms a pseudo cold front or squall line on its leading edges.; Related terms: bubble high
MESOLOW: A small scale low pressure center, ranging from the size of an individual thunderstorm to many tens of miles.
MESOSCALE: The scale of meteorological phenomena that range in size from several kilometers to around 100 kilometers. This includes MCCs, MCSs, and squall lines. Smaller phenomena are classified as microscale while larger are classified as synoptic-scale.
MESOSCALE CONVECTIVE COMPLEX (MCC): A large mesoscale convective system (MCS) which is about the size of the state of Ohio or Iowa and lasts at least 6 hours. Generally forming during the afternoon and evening, the complex normally reaches its peak intensity at night when heavy rainfall and flooding become the primary threat. Severe weather may occur at anytime.
MESOSCALE CONVECTIVE SYSTEM (MCS): A large organized convective weather system comprised of a number of individual thunderstorms. It normally persists for several hours and may be rounded or linear in shape. This term is often used to describe a cluster of thunderstorms that does not meet the criteria of a mesoscale convective complex (MCC).
MESOSPHERE: The layer of the atmosphere located between the stratosphere and the ionosphere, where temperatures drop rapidly with increasing height. It extends between 31 and 50 miles (17 to 80 kilometers) above the earth's surface.
METAR: Acronym for METeorological Aerodrome Report. It is the primary observation code used in the United States to satisfy requirements for reporting surface meteorological data. Minimum reporting requirements includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting.
METEOROLOGY/METEOROLOGIST: The science and study of the atmosphere and atmospheric phenomena. Various areas of meteorology include agricultural, applied, astrometerology, aviation, dynamic, hydrometeorology, operational, and synoptic, to name a few. A scientist who studies the atmosphere and atmospheric phenomena.; Related term: Dave's Dictionary
MICROBAROGRAPH: A instrument designed to continuously record a barometer's reading of very small changes in atmospheric pressure.
MICROBURST: A severe localized wind blasting down from a thunderstorm. It covers an area less than 2.5 miles (4 kilometers) in diameter and is of short duration, usually less than 5 minutes.; Related term: downburst
MICROSCALE: The smallest scale of meteorological phenomena that range in size from a few centimeters to a few kilometers. Larger phenomena are classified as mesoscale. It also refers to small scale meteorological phenomena with life spans of less than a few minutes that affect very small areas and are strongly influenced by local conditions of temperature and terrain.
MIDDLE CLOUDS: A term used to signify clouds with bases between 6,000 and 18,000 feet. At the higher altitudes, they may also have some ice crystals, but they are composed mainly of water droplets. Altocumulus, altostratus, and nimbostratus are the main types of middle clouds. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.
MIDDLE LATITUDES: The latitude belt roughly between 35 and 65 degrees North and South. May be referred to as the temperate region.
MILLIBAR (MB): The standard unit of measurement for atmospheric pressure used by the National Weather Service. One millibar is equivalent to 100 newtons per square meter. Standard surface pressure is 1,013.2 millibars.
MINIMUM: The least value attained by a function, for example, temperature, pressure, or wind speed. The opposite of maximum.
MIST: A collection of microscopic water droplets suspended in the atmosphere. It does not reduce visibility as much as fog and is often confused with drizzle.
MIXED LAYER: It is the upper portion of the boundary layer in which air is thoroughly mixed by convection. In oceanography, it is the layer of the water that is mixed through wave action or thermohaline convection.
MIXED PRECIPITATION: Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.
MOIST ADIABAT: The line on a Skew T-Log P chart that depicts the change in temperature of saturated air as it rises and undergoes cooling due to adiabatic expansion. As saturated air rises, the temperature changes at a rate of 0.55Â°C per 100 meters (2-3Â°F per 1,000 feet).; Related term: dry adiabat
MOISTURE: Refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air.
MONSOON: The seasonal shift of winds created by the great annual temperature variation that occurs over large land areas in contrast with associated ocean surfaces. The monsoon is associated primarily with the moisture and copious rains that arrive with the southwest flow across southern India. The name is derived from the word mausim, Arabic for season. This pattern is most evident on the southern and eastern sides of Asia, although it does occur elsewhere, such as in the southwestern United States.; Related term: Dave's Dictionary
MOUNTAIN BREEZE: A katabatic wind, it is formed at night by the radiational cooling along mountainsides. As the slopes become colder than the surrounding atmosphere, the lower levels of air cool and drain to the lowest point of the terrain. It may reach several hundred feet in depth, and extreme cases, attain speeds of 50 knots or greater. It blows in the opposite direction of a valley breeze.
MOUNTAIN WAVE: A wave in the atmosphere caused by a barrier, such as a mountain. Sometimes it is marked by lenticular clouds to the lee side of mountain barriers. May be called a standing wave or a lee wave.
MUD SLIDE: Fast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy a downpour of rain.
MUGGY: A subjective term for warm and excessively humid weather.
MULTICELL STORM: A thunderstorm made up of two or more single-cell storms.
MULTIPLE VORTEX TORNADO: A tornado which has two or more condensation funnels or debris clouds, often rotating around a common center.