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:
Size:
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Area of cloud top -32 degrees C or less = 100,000 square kilometers
or more
AND
Area of cloud top -52 degrees C or less = 50,000 square kilometers
or more
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Duration:
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Size criteria must be met for at least 6 hours
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Eccentricity:
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Minor/major axis at least 0.7
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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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