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M

Magnetic wind direction
The direction, with respect to magnetic north, from which the wind is blowing. Distinguish from true wind direction.

Magneto anemometer
A cup anemometer with its shaft mechanically coupled to a magneto.

Magnetograph
A recording magnetometer.

Magnetometer
General name for an instrument which measures the earth's magnetic field intensity.

Manometer
An instrument for measuring the pressure of gases and vapors. A mercury barometer is a type of manometer.

Mariah
In folklore, a name for wind.

Marine barometer
A mercury barometer designed for use aboard ship. The instrument is of the fixed-cistern type (see Kew barometer). The mercury tube is constructed with a wide bore for its upper portion and with a capillary bore for its lower portion. This is done to increase the time constant of the instrument and thus prevent the motion of the ship from affecting the reading. The instrument is suspended in gimbals to reduce the effects of pitch and roll of the ship.

Marine rainbow
A rainbow seen in the spray of the ocean. It is optically the same phenomenon as the ordinary rainbow.

Marine thermometer
See sea-water thermometer, reversing thermometer.

Marvin sunshine recorder
A sunshine recorder of the type in which the time scale is supplied by a chronograph. It consists of two bulbs, one of which is blackened, which communicate through a glass tube of small diameter. The tube is partially filled with mercury and contains two electrical contacts. When the instrument is exposed to sunshine, the air in the blackened bulb is warmed more than that in the clear bulb. The warmed air expands and forces the mercury through the connecting tube to a point where the electrical contacts are shorted by the mercury. This completes the electrical circuit to the pen of the chronograph.

Maximum thermometer
Thermometer used for measuring the highest temperature attained during a given interval of time, for example, a day.

Maximum-wind level
The height at which the maximum wind speed occurs, determined in a winds-aloft observation.

Maxwell's law
The statement that the viscosity of air is independent of the density of air.

Mean radiant ternperature
The temperature at which an object gives out as much radiation as it receives from its surroundings.

Mean temperature
The average temperature of the air as indicated by a properly exposed thermometer for a given time period, usually a day, a month, or a year.

Measured accuracy
The maximum positive and negative deviation observed in testing a device under specified conditions and by a specified procedure. It is usually measured as an inaccuracy and expressed as accuracy, typically in terms of the measured variable, percent of span, percent of upper range variable, percent of scale length, or percent of actual output reading. See accuracy, accuracy rating.

Melting level
The level at which ice crystals and snowflakes melt as they descend through the atmosphere.

Melting point
The temperature at which a solid substance undergoes fusion, i.e. melts, changes from solid to liquid form. All substances have their characteristic melting points. For very pure substances the temperature range over which the process of fusion occurs is very small. The melting point of a pure crystalline solid is a process of pressure. It increases with increasing pressure for most substances. However in the case of ice (and a few other substances) the melting point decreases with increasing pressure. Under a pressure of one standard atmosphere, the melting point of pure ice is the same as the ice point, that is 0°C or 32°F.

Meniscus
The upper surface of a column of liquid.

Mercurial barometer
Same as mercury barometer.

Mercury
A metallic element of atomic weight 200.6 1, unique (for metals) in that it remains liquid under all but very extreme temperatures.

Mercury barometer
Barometer in which pressure is determined by balancing air pressure against the weight of a column of mercury in an evacuated glass tube.

Mercury-in-glass thermometer
A common type of liquid-inglass thermometer, used, in meteorology, in psychrometers and as a maximum thermometer.

Mercury-in-steel thermometer
A liquid-in-metal thermometer in which mercury is enclosed in a steel envelope. The change in internal pressure caused by the temperature variation is measured by a Bourdon tube which is connected to the mercury by a capillary tube. This instrument is highly accurate and has extremely good pen control when arranged as a thermograph.

Mercury thermometer
A liquid-in-glass or liquid-in-metal thermometer using mercury as the liquid.

Meteorogram
A record obtained from a meteorograph. A chart in which meteorological variables are plotted against time.

Meteorograph
An instrument which automatically records the measurement of two or more meteorological elements.

Meter-ton-second system
A system of physical units based upon the use of the meter, the metric ton (106 grams), and the second as elementary quantities of length, mass, and time, respectively.

Michaelson actinograph
A pyrheliometer of the bimetallic type used to measure the intensity of direct solar radiation.

Microbarm
That portion of the record of a microbarograph between any two (or a specified small number) of successive crossings of the average pressure level (in the same direction). Analogous to microseism.

Microbarogram
The record or trace made by a microbarograph.

Microbarograph
An aneroid barograph designed to record atmospheric pressure variations of very small magnitude.

Micropluviometer
Rain gauge which registers precipitation that is too light to be registered by ordinary recording of the depth of water from precipitation. Same as ombrometer.

Microprocessor
A small, limited-capacity central processing unit contained entirely on one semiconductor chip.

Microseism
A feeble oscillatory disturbance of the earth's crust, detectable only by very sensitive seismographs. Certain types of microseisms seem to be closely correlated with pressure disturbances. See microbarm.

Millibar
A unit of pressure which directly expresses the force exerted by the atmosphere. Equal to 1000 dynes/cm² or 100 pascals.

Minimum thermometer
Thermometer used for measuring the lowest temperature attained during a given interval of time, for example, a day.

Mirror nephoscope
A nephoscope in which the motion of the cloud is observed by its reflection in a mirror.

Mist
A hydrometeor consisting of an aggregate of microscopic and more-or-less hygroscopic water droplets suspended in the atmosphere. It reduces visibility to a lesser extent than fog. The relative humidity of mist is often less than 95 percent.

Mistbow
Same as fogbow.

Mixing ratio
In a system of moist air, the dimensionless ratio of the mass of water vapor to the mass of dry air. For many purposes, the mixing ratio may be approximated by the specific humidity.

M meter
Name applied to a class of instruments which measure the liquid content of the atmosphere.

Moby Dick balloon
A large plastic constant-level balloon for duration flying (in excess of 24 hours) at altitudes above 40,000 feet, used for the determination of wind fields and the measurement of upper atmospheric parameters.

Modem
A device that allows a terminal or computer at one location to communicate with a terminal or computer at a distant location via wire or phone lines.

Moderate breeze
Wind with a speed between 11 and 16 knots (13 and 18 mph); Beaufort scale number 4.

Moderate gale
Wind with a speed between 28 and 33 knots (32 and 38 mph); Beaufort scale number 7.

Modulation
The process of modifying some characteristic of a wave (the carrier) so that it varies in step with the instantaneous value of another wave (the modulating wave) in order to transmit a message. The modified characteristic may be frequency, phase, and/or amplitude.

Mole
A unit of mass numerically equal to the molecular weight of the substance. The gram-mote or gram-molecule is the mass in grams numerically equal to the molecular weight, i.e. a gram-mole of oxygen is 32 grams.

Moll thermopile
A thermopile used in some types of radiation instruments. See solarimeter.

Monsoon
A seasonal wind of persistent direction, characterized by a pronounced change in direction between seasons.

Mountain barometer
Any conventional barometer fitted with an extended scale so that atmospheric pressure measurements may be made at both high and low altitudes.

Mount Rose snow sampler
A particular pattern of snow sampler having an internal diameter of 1.485 inches so that each inch of water in the sample weighs one ounce.

Moveable scale barometer
A mercury barometer of the fixed cistern type in which a moveable scale terminating in an ivory point is used to compensate for the variations in the height of the mercury in the cistern

MSL
Abbreviation for mean sea level.

Multiple register
A chronograph used to make a time-record of certain measured meteorological elements. The most common type, the triple register, records wind direction and speed, duration of sunshine, and amount of rainfall (sensed respectively by a contact anemometer, Marvin sunshine recorder, and tipping bucket rain gauge). The register consists of a rotating, clockdriven drum on a helical axis, a separate pen for each element, and the actuating mechanism for the pens. Double registers are also used. Multiples registers of this type are becoming obsolete.

Multiplexer
A device that combines several separate communications signals into one and outputs them on a single line.

Muskingum method
A method of streamflow routing which assumes that storage is a linear function of the weighted flow in the reach and is adaptable to a simple mathematical solution.