Glossary
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.
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
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.
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
an operational model that is now part of the Global Forecast System (GFS) Model.
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).
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.
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:
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AND Area of cloud top -52 degrees C or less = 50,000 square kilometers or more |
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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.
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-in-Charge (MIC)
the meteorologist responsible for one National Weather Service Forecast Office (WFO); the head meteorologist for a WFO
Microburst
an intense downdraft (downburst) less than 4 km wide (about 2.5 miles) that may occur beneath a thunderstorm
Microwave
a type of electromagnetic radiation with wavelengths between infrared radiation and radio waves
Middle Latitudes (Mid-latitudes)
the two regions of the earth typically between 30 degrees and 50 degrees latitude
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
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".
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 Ridge
an axis of relatively high dew point values. This axis is sometimes referred to as a "moist tongue".
Molecule
the smallest particle of a substance that retains the properties of the substance and is composed of one or more atoms
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.
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
