Subj : Weather Terms (M)
To : ALL USERS
From : DARYL STOUT
Date : Sun Feb 14 2016 12:03 am
This weather glossary contains information on more than 2000 terms,
phrases and abbreviations used by the National Weather Service (NWS)...
the government agency who makes weather forecasts, and issues weather
advisories, watches, and warnings, for the United States, and its
territories.
Many of these terms and abbreviations are used by NWS forecasters to
communicate between each other and have been in use for many years and
before many NWS products were directly available to the public. It is the
purpose of this glossary to aid you in better understanding NWS products.
***
M 3000
In solar-terrestrial terms, the optimum high frequency radio wave with a
3000 km range, which reflects only once from the ionosphere (single hop
transmission).
M2/S2
m2/s2 (meters squared per second squared), unit of measure equivalent to
J/kg (joules per kilogram).
Mackeral 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 convective downdraft with an affected outflow area of at least 2 miles
wide and peak winds lasting between 5 and 20 minutes. Intense
macrobursts may cause tornado-force damage of up to F3 intensity.
Macroscale
Large scale, characteristic of weather systems several hundred to several
thousand kilometers in diameter.
Madden-Julian Oscillation
(abbrev. MJO)- Tropical rainfall exhibits strong variability on time
scales shorter than the seasonal El Ni%o-Southern Oscillation (ENSO).
These fluctuations in tropical rainfall often go through an entire cycle
in 30-60 days, and are referred to as the Madden-Julian Oscillation or
intraseasonal oscillations. The intraseasonal oscillations are a
naturally occurring component of our coupled ocean-atmosphere system.
They significantly affect the atmospheric circulation throughout the
global Tropics and subtropics, and also strongly affect the wintertime
jet stream and atmospheric circulation features over the North Pacific
and western North America. As a result, they have an important impact
on storminess and temperatures over the United States. During the summer
these oscillations have a modulating effect on hurricane activity in
both the Pacific and Atlantic basins.
MAFOR
(Great Lakes Marine Forecast) - A National Weather Service coded summary
appended to each of the Great Lakes Open Lakes forecasts.
Magflare
In solar-terrestrial terms, a geomagnetic and/or cosmic storm has been
associated with this flare.
Magnetic Bay
In solar-terrestrial terms, a relatively smooth excursion of the H
(horizontal) component of the geomagnetic field away from and returning
to quiet levels.
Magnetogram
In solar-terrestrial terms, solar magnetograms are a graphic
representation of solar magnetic field strengths and polarity.
Magnetopause
In solar-terrestrial terms, the boundary layer between the solar wind
and the magnetosphere.
Magnetosphere
In solar-terrestrial terms, the magnetic cavity surrounding the earth,
carved out of the passing solar wind by virtue of the geomagnetic
field, which prevents, or at least impedes, the direct entry of the
solar wind plasma into the cavity.
Main Stem
In hydrologic terms, the reach of a river/stream formed by the
tributaries that flow into it.
Main Synoptic Times
The times of 0000, 0600, 1200, and 1800 UTC. Also known as the standard
synoptic times.
MAINT
Maintain
Major Flare
In solar-terrestrial terms, this flare is the basis for the forecast
of geomagstorm, cosmic storm and/or protons in the earth's vicinity.
Major Flooding
A general term including extensive inundation and property damage.
(Usually characterized by the evacuation of people and livestock and
the closure of both primary and secondary roads.)
Major Hurricane
A hurricane which reaches Category 3 (sustained winds greater than 110
mph) on the Saffir/Simpson Hurricane Scale.
MALR
Moist Adiabatic Lapse Rate- The rate at which the temperature of a
parcel of saturated air decreases as the parcel is lifted in the
atmosphere. The moist adiabatic lapse rate (abbreviated MALR) is not a
constant like the dry adiabatic lapse rate but is dependent on parcel
temperature and pressure.
Mammatus Clouds
Rounded, smooth, sack-like protrusions hanging from the underside of a
cloud (usually a thunderstorm anvil). Mammatus clouds often accompany
severe thunderstorms, but do not produce severe weather; they may
accompany non-severe storms as well.
MAP
Mean Areal Precipitation- The average rainfall over a given area,
generally expressed as an average depth over the area.
Mare's Trail
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.
MAREP
(MArine REPort) A voluntary marine observation program of the National
Weather Service whose goal is to solicit meteorological and oceanographic
observations in plain language from recreational and small commercial
mariners who are not part of Voluntary Observing Ship program.
Marginal Visual Flight Rules
(Abbrev. MVFR) - In an aviation product, refers to the general weather
conditions pilots can expect at the surface. VFR stands for Visual Flight
Rules and 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.
Marine Inversion
Temperature inversion produced when cold marine air underlies warmer air.
Marine Push
A replacement of the current air mass with air from off the ocean.
Temperatures are much cooler and relative humidities much higher. The
air mass is generally much more stable in this situation.
Marine Small Craft Thunderstorm Advisory
A marine warning issued by Environment Canada Atmospheric Environment
Branch when the possibility of thunderstorms is greater than 40 percent.
Marine Small Craft Wind Warning
A marine warning issued by Environment Canada Atmospheric Environment
Branch for winds which are forecasted to be in the 20-33 knot range
inclusive.
Marine Weather Statement
A National Weather Service product to provide mariners with details on
significant or potentially hazardous conditions not otherwise covered
in existing marine warnings and forecasts. Marine weather statements are
also used to supplement special marine warnings.
Marine Zone
Specific, defined over-water areas contained in the various NWS marine
forecasts. These are the equivalent of "zones" in the public forecast
program.
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.
Maritime Polar Air Mass
An air mass characterized by cold, moist air. Abbreviated mP.
Maritime Tropical Air Mass
An air mass characterized by warm, moist air. Abbreviated mT.
MAROB
A voluntary marine observation program of the National Weather Service
in the early stages of development whose goal is to solicit
meteorological and oceanographic observations in coded format from
recreational and small commercial mariners who are not part of the more
in-depth Voluntary Observing Ship program.
MARS
A voluntary marine observation program of the National Weather Service
whereby U.S. Coast Guard Sector Stations report marine weather conditions
from several shore locations within their operating area. The reports are
in an abbreviated plain language format with fixed fields.
Massif
A compact portion of a mountain range, containing one or more summits.
MAV
AVN MOS Guidance
MAX
Maximum
Max Parcel Level (MPL)
This signifies the highest attainable level that a convective updraft can
reach; therefore, it is a good indication of how tall a thunderstorm may
reach.
Maximum Spillway Discharge
In hydrologic terms, spillway discharge (cfs) when reservoir is at
maximum designed water surface elevation.
Maximum Sustained Surface Wind
When applied to a particular weather system, refers to the highest
one-minute average wind (at an elevation of 10 meters with an
unobstructed exposure) associated with that weather system at a
particular point in time.
Maximum Temperature
This is the highest temperature recorded during a specified period of
time. Common time periods include 6, 12 and 24 hours. The most common
reference is to the daily maximum temperature, or "high."
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.
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 successive pulses of emitted energy. When a target's
velocity exceeds the maximum unambiguous velocity, the velocity will
be "folded" to appear as a different velocity.
MAXT
High temperature for the day. Usually forecast out 7 days in advance.
MB
Millibar
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 degrees C or less: 100,000 square kilometers
or more (slightly smaller than the state of Ohio), and area of cloud
top -52 degrees C or less: 50,000 square kilometers or more
Duration: Size criteria must be met for at least 6 hours
Eccentricity: Minor/major axis at least 0.7
MCCs typically form during the afternoon and evening in the form of
several isolated thunderstorms, during which time the potential for
severe weather is greatest. During peak intensity, the primary threat
shifts toward heavy rain and flooding.
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 Mesoscale Convective Complexes (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 Mesoscale Convective Complex.
MD
Mesoscale Discussion- When conditions actually begin to shape up for
severe weather, SPC (Storm Prediction Center) often issues a Mesoscale
Discussion statement anywhere from roughly half an hour to several hours
before issuing a weather watch. SPC also puts out MCDs for hazardous
winter weather events on the mesoscale, such as locally heavy snow,
blizzards and freezing rain (see below). MCDs are also issued on
occasion for heavy rainfall, convective trends, and other phenomena,
when the forecaster feels he/she can provide useful information that is
not readily available or apparent to field forecasters. MCDs are based
on mesoscale analysis and interpretation of observations and of short
term, high resolution numerical model output. The MCD basically
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 the watch (if dealing with severe thunderstorm
potential). Severe thunderstorm MCDs can help you get a little extra
lead time on the weather and allow you to begin gearing up operations
before a watch is issued. The MCD begins with a numerical string that
gives the LAT/LON coordinates of a polygon that loosely describes the
area being discussed.
MDFY
Modify
MDLS
models
MDNGT
midnight
MDT
Moderate (or) Mountain Daylight Time
Mean
The arithmetic average of a set of data (numbers), or the middle point
between its two extremes.
Mean Annual Temperature
The average temperature for the entire year at any given location.
Mean Areal Precipitation
(abbrev. MAP)- The average rainfall over a given area, generally
expressed as an average depth over the area.
Mean Daily Temperature
The average of the highest and lowest temperatures during a 24-hour
period.
Mean Depth
In hydrologic terms, the average depth of water in a stream channel or
conduit. It is equal to the cross-sectional area divided by the
surface width.
Mean Doppler Velocity
Reflectivity-weighted average velocity of targets in a given pulse
resolution volume. Usually determined from a large number of
successive radar pulses. Also called mean radial (towards or away from
the antenna) velocity. Doppler velocity refers to spectral density
first moment, radial velocity to base data.
Mean Low Water
(MLW) - A tidal datum. The average of all the low water heights
observed over the National Tidal Datum Epoch. For stations with shorter
series, simultaneous observational comparisons are made with a control
tide station in order to derive the equivalent datum of the National
Tidal Datum Epoch.
Mean Lower Low Water
(MLLW) - A tidal datum. The average of the lower low water height of
each tidal day observed over the National Tidal Datum Epoch. For stations
with shorter series, simultaneous observational comparisons are made with
a control tide station in order to derive the equivalent datum of the
National Tidal Datum Epoch.
Mean Sea Level
(MSL) - The arithmetic mean of hourly water elevations observed over a
specific 19-year tidal epoch.
Meander
In hydrologic terms, the winding of a stream channel.
Meander Belt
In hydrologic terms, the area between lines drawn tangential to the
extreme limits of fully developed meanders.
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. See variable ceiling.
Medium Frequency
(abbrev. MF)- That portion of the radio frequency spectrum from 0.3 to
3 MHz.
Medium Range
In forecasting, (generally) three to seven days in advance.
MEGG
Merging
Melting Level
The altitude which ice crystals and snowflakes melt as they descend
through the atmosphere.
Melting Point
The temperature at which a solid substance undergoes fusion, changing
from a solid to a liquid state. Contrast with freezing point.
Meniscus
In hydrologic terms, the curved surface of the liquid at the open end of
a capillary column.
Mercury Barometer
An instrument for measuring atmospheric pressure. The instrument contains
an evacuated and graduated glass tube in which mercury rises or falls as
the pressure of the atmosphere increases or decreases.
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 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
Mesocyclone- A storm-scale region of rotation, typically around 2-6 miles
in diameter and often found in the right 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. It will
appear as a yellow solid circle on the Doppler velocity products.
Therefore, a mesocyclone should not be considered a visually-observable
phenomenon (although visual evidence of rotation, such as curved inflow
bands, may imply the presence of a mesocyclone).
Mesoclimate
The climate of a small area of the earth's surface which may differ from
the general climate of the district.
Mesocyclone
(abbrev. MESO)- A storm-scale region of rotation, typically around 2-6
miles in diameter and often found in the right 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. It
will appear as a yellow solid circle on the Doppler velocity products.
Therefore, a mesocyclone should not 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 relatively small area of high atmospheric pressure that forms beneath
a thunderstorm. It is 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
A regional network of observing stations (usually surface stations)
designed to diagnose mesoscale weather features and their associated
processes.
Mesopause
The top of the mesosphere, corresponding to the level of minimum
temperature in the atmosphere found at 70 to 80 km.
Mesoscale
Size scale referring to weather systems smaller than synoptic-scale
systems but larger than storm-scale systems. Horizontal dimensions
generally range from around 50 miles to several hundred miles. Squall
lines, MCCs, and MCSs are examples of mesoscale weather systems.
Mesoscale Convective Complex
(abbrev. MCC)- MCC - Mesoscale Convective Complex. A large Mesoscale
Convective System (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 degrees C or less: 100,000 square
kilometers or more (slightly smaller than the state of Ohio), and area
of cloud top -52 degrees C or less: 50,000 square kilometers or more.
* Duration: Size criteria must be met for at least 6 hours.
* Eccentricity: Minor/major axis at least 0.7.
MCCs typically form during the afternoon and evening in the form of
several isolated thunderstorms, during which time the potential for
severe weather is greatest. During peak intensity, the primary threat
shifts toward heavy rain and flooding.
Mesoscale Convective System
(MCS): 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.
Mesoscale Discussion
When conditions actually begin to shape up for severe weather, SPC
(Storm Prediction Center) often issues a Mesoscale Discussion (MCD)
statement anywhere from roughly half an hour to several hours before
issuing a weather watch. SPC also puts out MCDs for hazardous winter
weather events on the mesoscale, such as locally heavy snow, blizzards
and freezing rain (see below). MCDs are also issued on occasion for
heavy rainfall, convective trends, and other phenomena, when the
forecaster feels he/she can provide useful information that is not
readily available or apparent to field forecasters. MCDs are based on
mesoscale analysis and interpretation of observations and of short
term, high resolution numerical model output.
The MCD basically 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 the watch (if dealing with severe
thunderstorm potential). Severe thunderstorm MCDs can help you get a
little extra lead time on the weather and allow you to begin gearing up
operations before a watch is issued. The MCD begins with a numerical
string that gives the LAT/LON coordinates of a polygon that loosely
describes the area being discussed.
Mesoscale High Winds
These high winds usually follow the passage of organized convective
systems and are associated with wake depressions or strong mesohighs.
Mesosphere
The atmospheric shell between about 20 km and about 70 to 80 km,
extending from the top of the stratosphere (the stratopause) to the
upper temperature minimum that defines the mesopause (the base of the
thermosphere).
MET
1) Meteorological
2) MOS (Model Output Statistics) guidance based on Environmental
Modeling Centers ETA model
METAR
An international code (Aviation Routine Weather Report) 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.
Meteoric Water
Water derived from precipitation.
Meteorologist
A person who studies meteorology. There are many different paths within
the field of meteorology. For example, one could be a research
meteorologist, radar meteorologist, climatologist, or operational
meteorologist.
Meteorology
The science dealing with the atmosphere and its phenomena. A distinction
can be drawn between meteorology and climatology, the latter being
primarily concerned with average, not actual, weather conditions.
METRO
Metropolitan
MeV
Mega (million) electronvolt. A unit of energy used to describe the total
energy carried by a particle or photon.
MEX
1) GFS Model Guidance
2) Mexico
MI
1) Mile
2) Michigan
MIC
Meteorologist In Charge
Microbarograph
A instrument designed to continuously record a barometer's reading of
very small changes in atmospheric pressure.
Microburst
A convective downdraft with an affected outflow area of less than 2 miles
wide and peak winds lasting less than 5 minutes. Microbursts may induce
dangerous horizontal/vertical wind shears, which can adversely affect
aircraft performance and cause property damage.
Microclimate
The climate of a small area such as a cave, house, city or valley that
may be different from that in the general region.
Micron
Unit of length equal to one thousandth (1/1000) of a millimeter; one
millionth (1/1000000) of a meter (1x10-6 m).
Microscale
Pertaining to meteorological phenomena, such as wind circulations or
cloud patterns, that are less than 2 km in horizontal extent.
Microwave
A type of electromagnetic radiation with wavelengths between those of
infrared radiation and radio waves.
Microwave Burst
In solar-terrestrial terms, a radiowave signal associated with optical
and/or X-ray flares.
MID
Middle
Mid-Flame Wind
Wind measured at the midpoint of the flames, considered to be most
representative of the speed of the wind that is affecting fire behavior.
Mid-Latitude Areas
Areas between 30x and 60x north and south of the Equator.
Mid-level Cooling
Local cooling of the air in middle levels of the atmosphere (roughly 8 to
25 thousand feet), which can lead to destabilization of the entire
atmosphere if all other factors are equal.
Middle Clouds
(or Mid-Level Clouds) - A term used to signify clouds with bases between
6,500 and 23,000 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
1) The latitude belt roughly between 35 and 65 degrees North and South.
Also referred to as the temperate region.
or
2) With specific reference to zones of geomagnetic activity, "middle
latitudes" refers to 20x to 50x geomagnetic.
Mie Scattering
Any scattering produced by spherical particles whose diameters are
greater than 1/10 the wavelength of the scattered radiation. This type
of scattering causes the clouds to appear white in the sky. Often, hail
exhibits in this type of scattering.
Millibar
A unit of atmospheric pressure equal to 1/1000 bar, or 1000 dynes per
square centimeter.
MIN
Minimum (or) Minute
Minimum Discernible Signal
In a receiver, it is the smallest input signal that will a produce a
detectable signal at the output. In radar terms, it is the minimal
amount of back scattered energy that is required to produce a target
on the radar screen. In other words, MDS is a measure of the radar's
sensitivity.
Minimum Temperature
This is the lowest temperature recorded during a specified period of
time. The time period can be 6, 12 or 24 hours. The most common
reference is to the daily minimum temperature, or "low."
Minor Flooding
A general term indicating minimal or no property damage but possibly
some public inconvenience.
Minor Tidal Overflow
Minor flooding caused by high tides that results in little if any damage.
MINT
minimum temperature
MISC
Miscellaneous
MISG
Missing
Misoscale
The scale of meteorological phenomena that ranges in size from 40
meters to about 4 kilometers. It includes rotation within a thunderstorm.
Mist
A visible aggregate of minute 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 does not reduce visibility as much as fog
and is often confused with drizzle.
Mixed Layer
An atmospheric layer, usually the layer immediately above the ground, in
which pollutants are well mixed by convective or shear-produced
turbulence.
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.
Mixing Depth
Vertical distance between the ground and the altitude to which pollutants
are mixed by turbulence caused by convective currents or vertical shear
in the horizontal wind.
Mixing Heights
The height to which a parcel of air, or a column of smoke, will rise, mix
or disperse. A column of smoke will remain trapped below this height.
Mixing Ratio
The ratio of the weight of water vapor in a specified volume (such as an
air parcel) to the weight of dry air in that same volume.
MJO
Madden-Julian Oscillation- Tropical rainfall exhibits strong variability
on time scales shorter than the seasonal El Ni%o-Southern Oscillation
(ENSO). These fluctuations in tropical rainfall often go through an entire
cycle in 30-60 days, and are referred to as the Madden-Julian Oscillation
or intraseasonal oscillations. The intraseasonal oscillations are a
naturally occurring component of our coupled ocean-atmosphere system.
They significantly affect the atmospheric circulation throughout the
global Tropics and subtropics, and also strongly affect the wintertime
jet stream and atmospheric circulation features over the North Pacific
and western North America. As a result, they have an important impact on
storminess and temperatures over the United States. During the summer
these oscillations have a modulating effect on hurricane activity in both
the Pacific and Atlantic basins.
MLCAPE
Mean Layer CAPE - CAPE calculated using a parcel consisting of Mean Layer
values of temperature and moisture from the lowest 100 mb above ground
level. See Convective Available Potential Energy (CAPE).
MLLI
Mean Layer Lifted Index - Lifted Index (LI) calculated using a parcel
consisting of Mean Layer values of temperature and moisture from the
lowest 100 mb above ground level. See Lifted Index (LI).
MLLW
(Mean Lower Low Water) - A tidal datum. The average of the lower low
water height of each tidal day observed over the National Tidal Datum
Epoch. For stations with shorter series, simultaneous observational
comparisons are made with a control tide station in order to derive the
equivalent datum of the National Tidal Datum Epoch.
MLW
(Mean Low Water) - A tidal datum. The average of all the low water
heights observed over the National Tidal Datum Epoch. For stations
with shorter series, simultaneous observational comparisons are made
with a control tide station in order to derive the equivalent datum
of the National Tidal Datum Epoch.
MNLY
Mainly
MOBY
Marine Optical Buoy. It measures solar radiation to calibrate satellite
ocean color instruments.
Model Output Statistics
(abbrev. MOS) - the Hydrometeorological Center (HPC) produces a short
range (6 to 60 hours) MOS (Model Output Statistics) guidance package
generated from the NGM, GFS, and ETA models for over 300 individual
stations in the continental United States. These alphanumeric messages
are made available at approximately 0400 and 1600 UTC for the 0000 and
1200 UTC forecast cycles, respectively. Model Output Statistics are a
set of statistical equations that use model output to forecast the
probability of precipitation, high and low temperature, cloud cover,
and precipitation amount for many cities across the USA. The
statistical equations were specifically tailored for each location,
taking into account factors such as each location's climate.
Moderate Flooding
The inundation of secondary roads; transfer to higher elevation
necessary to save property -- some evacuation may be required.
Moderate Risk (of severe thunderstorms)
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. See high risk, slight risk, convective outlook.
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
0.55 degrees Celsius per 100 meters (2-3 degrees Fahrenheit per 1,000
feet). Contrast with a dry adiabat.
Moist Adiabatic Lapse Rate
(abbrev. MALR)- The rate at which the temperature of a parcel of
saturated air decreases as the parcel is lifted in the atmosphere. The
moist adiabatic lapse rate (abbreviated MALR) is not a constant like the
dry adiabatic lapse rate but is dependent on parcel temperature and
pressure.
Moist-adiabatic
(Also known as saturation-adiabatic process.) An adiabatic process for
which the air is saturated and may contain liquid water. A distinction
is made between the reversible process, in which total water is
conserved, and the pseudoadiabatic or irreversible moist adiabatic
process, in which liquid water is assumed to be removed as soon as it
is condensed.
Moisture
Refers to the water vapor content in the atmosphere, or the total water,
liquid, solid or vapor, in a given volume of air.
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 favored regions
for thunderstorm development, if other factors (e.g., instability) are
favorable.
Moisture Equivalent
In hydrologic terms, the ratio of the weight of water which the soil,
after saturation, will retain against a centrifugal force 1,000 times
the force of gravity, to the weight of the soil when dry. The ratio is
stated as a percentage.
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.
Monitor Stage
The stage which, when reached by a rising stream, represents the level
where appropriate officials (e.g., county sheriff, civil defense
officials, or bypass gate operators) are notified of the threat of
possible flooding. (Used if different from action stage, and at the
discretion of the WFO or river forecast center [RFC].) The term
"alert stage" is to be used instead of warning stage. Monitor stage or
caution stage may be used instead of alert stage in some parts of the
country.
Monostatic Radar
A radar that uses a common antenna for both transmitting and receiving.
Monsoon
A thermally driven wind arising from differential heating between a land
mass and the adjacent ocean that reverses its direction seasonally.
Monthly Climatological Report
This climatological product is issued once a month by each National
Weather Service office. It is a mix of tabular and narrative information.
It is organized so that similar items are grouped together (i.e.,
temperature, precipitation, wind, heating/cooling degree information,
etc.).
Morning Glory
An elongated cloud band, 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.
MOS
Model Output Statistics - the Hydrometeorological Center (HPC) produces
a short range (6 to 60 hours) MOS guidance package generated from the
NGM, GFS, and ETA models for over 300 individual stations in the
continental United States. These alphanumeric messages are made available
at approximately 0400 and 1600 UTC for the 0000 and 1200 UTC forecast
cycles, respectively. Model Output Statistics are a set of statistical
equations that use model output to forecast the probability of
precipitation, high and low temperature, cloud cover, and precipitation
amount for many cities across the USA. The statistical equations were
specifically tailored for each location, taking into account factors
such as each location's climate.
Mostly Clear
When the predominant/average sky condition is covered 1/8 to 2/8 with
opaque (not transparent) clouds. Sometimes referred to as Mostly Sunny
if this condition is present during daylight hours.
Mostly Cloudy
When the predominant/average sky condition is covered by more than half,
but not completely covered by opaque (not transparent) clouds. In other
words, 5/8 to 7/8 of the sky is covered by opaque clouds. Same as
Considerable Cloudiness.
Mostly Sunny
Same as Mostly Clear, except only applicable during daylight hours; when
the predominant/average sky condition is covered 1/8 to 2/8 with opaque
(not transparent) clouds.
Mount Wilson Magnetic Classifications
In solar-terrestrial terms, a classification system for sunspots:
Alpha: Denotes a unipolar sunspot group.
Beta: A sunspot group having both positive and negative magnetic
polarities, with a simple and distinct division between the polarities.
Beta-Gamma: A sunspot group that is bipolar but in which no continuous
line can be drawn separating spots of opposite polarities.
Delta: A complex magnetic configuration of a solar sunspot group
consisting of opposite polarity umbrae within the same penumbra.
Gamma: A complex active region in which the positive and negative
polarities are so irregularly distributed as to prevent classification
as a bipolar group.
Mountain Wave
The wavelike effect, characterized by updrafts and downdrafts, that
occurs above and behind a mountain range when rapidly flowing air
encounters the mountain range's steep front.
Mountain Wind System
The system of diurnal winds that forms in a complex terrain area,
consisting of mountain-plain, along-valley, cross-valley and slope
wind systems.
Mountain-Plain Wind System
A closed, large-scale, thermally driven circulation between the
mountains and the surrounding plain. The mountain-to-plain flow making
up the lower branch of the closed circulation usually occurs during
nighttime, while the plain-to-mountain flow occurs during daytime.
Mountainado
A vertical-axis eddy produced in a downslope windstorm by the vertical
stretching of horizontal roll vortices produced near the ground by
vertical wind shear. Mountainadoes, when carried by the mean wind, can
produce strong horizontal shears and wind gusts that are much more
damaging than the general prevailing wind speeds.
MOV
Move
Movable Bed
In hydrologic terms, a stream bed made up of materials readily
transportable by the stream flow.
MOVG
moving
MR
more
MRF
Medium Range Forecast model, the medium-range computer model run by the
United States (NOAA). The output from this model is part of what is now
known as the GFS model, so the term MRF is no longer widely used.
MRGL
Marginal
MRNG
Morning
MSG
Message
MSL
(Mean Sea Level) - The arithmetic mean of hourly water elevations
observed over a specific 19-year tidal epoch.
MSLP
Mean sea level pressure
MST
Mountain Standard Time
MSTLY
Mostly
MSTR
Moisture
MT
1) Mountain
2) Montana
MTN
Mountain
MTNS
Mountains
MTS
Mountains
Mud Slide
Fast moving soil, rocks and water that flow down mountain slopes and
canyons during a heavy downpour of rain.
Muggy
A subjective term for warm and excessively humid conditions.
MULI
Most Unstable Lifted Index - Lifted Index (LI) calculated using a parcel
from the pressure level that results in the Most Unstable value
(lowest value) of LI possible.
Multicell Thunderstorm
These thunderstorms are organized in clusters of at least 2-4 short-lived
cells. Each cell generates a cold air outflow and these individual
outflows combine to form a large gust front. Convergence along the gust
front causes new cells to develop every 5 to 15 minutes. The cells move
roughly with the mean wind. However, the area (storm) motion usually
deviates significantly from the mean wind due to discrete propagation
(new cell development) along the gust front. The multicellular nature of
the storm is usually apparent on radar with multiple reflectivity cores
and maximum tops.
Multiple Doppler Analysis
The use of more than one radar (and hence more than one look angle) to
reconstruct spatial distributions of the 2D or 3D wind field, which
cannot be measured from a single radar alone. Includes dual Doppler,
triple Doppler, and overdetermined multiple Doppler analysis.
Multiple 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. Multiple-vortex tornadoes can be especially damaging.
Multipurpose Reservoir
In hydrologic terms, a reservoir constructed and equipped to provide
storage and release of water for two or more purposes such as flood
control, power development, navigation, irrigation, recreation,
pollution abatement, domestic water supply, etc.
Municipal Use of Water
In hydrologic terms, the various uses to which water is put to use
developed urban areas, including domestic use, industrial use, street
sprinkling, fire protection, etc.
Mushroom
Slang for a thunderstorm with a well-defined anvil rollover, and thus
having a visual appearance resembling a mushroom.
MVFR
Marginal Visual Flight Rules - in an aviation product, refers to the
general weather conditions pilots can expect at the surface. VFR stands
for Visual Flight Rules and 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.
MVS
Moves
MWD
On a buoy report, mean wave direction corresponding to energy of the
dominant period (DOMPD). The units are degrees from true North just like
wind direction.
MWS
Marine Weather Statement
--- BgNet 1.0a12 - The Thunderbolt BBS wx1der.dyndns.org Little Rock, AR
