Weather Words - M | Weather | WGNTV.com | WGN TV

Weather Words - M

-- M --

Mach, Ernst: (1838-1916) The Austrian physicist and psychologist after whom the Mach number is named. As commonly used, the Mach number is the ratio of the speed of an object to the speed of sound. Thus Mach 1.5 represents a speed 1.5 times the speed of sound.

Mackerel sky: A sky covered with cirrocumulus or altocumulus clouds arranged in somewhat regular waves and showing blue sky in the gaps; resembles the scales on a mackerel: thus, the name.

Macroscale: Of or relating to the largest scale of atmospheric phenomena, ranging in size from continental to global. An example of a macroscale event: the planetary atmospheric circulation.

Magnetic field: A field of force generated by electric currents. The Earth's average large-scale magnetic field, like that of the sun, exhibits a north and a south pole linked by lines of magnetic force.

Magnetic pole: Either of the two points on the Earth's surface at which the Earth's magnetic lines of force converge -- that is, at which the magnetic field is vertical.

Magnetic storm: A state of large, irregular disturbance of the Earth's magnetic field. It is world-wide in extent, and its duration ranges from a few hours to a few days.

Major hurricane: A hurricane whose sustained winds reach or exceed 111 m.p.h.; a category 3, 4 or 5 hurricane on the Saffir-Simpson scale; also called an "intense hurricane."

Mamma: Hanging protuberances, like pouches, on the under surface of a cloud. This supplementary cloud feature is most noticeable when it occurs on the under side of the anvil of cumulonimbus (thunderhead) clouds. Previously known as "mammatus".

Mammatus: Hanging protuberances, like pouches, on the under surface of a cloud. This supplementary cloud feature is most noticeable when it occurs on the under side of the anvil of cumulonimbus (thunderhead) clouds.

March: The third month of the year and the first month of meteorological spring. (1) With an average temperature of 37.3 degrees, average precipitation (water content) of 2.65" and average snowfall of 6.0", it is Chicago's fourth coldest, fourth driest and fourth snowiest month. (2) The beginning of the four-month (March-June) tornado season in Illinois. (3) "What's good about March? Well, for one thing, it keeps February and April apart." - Walt Kelly (1913-1973), creator of the comic strip, "Pogo".

March zero-degree temperatures: In official temperature records dating from 1871, Chicago's temperature has declined to zero degrees or lower in March on only 20 days. The temperature of -7 degrees on March 4, 2002, was the 21st occurrence.

Mares' tails: Long, well-defined wisps of cirrus clouds, thicker at one end than at the other. Composed of ice crystals, their appearance is white, featherlike and without shading against the backdrop of a blue sky.

Marine climate: A climate that is under the dominant influence of the sea. It is marked by small daily, day-to-day and annual ranges of temperature. The climate of oceanic islands, like the Hawaiian Islands, is marine, as is the climate of many ocean coastal areas, such as Miami.

Marine rainbow: (also called "sea rainbow") A rainbow seen in the spray of the ocean. Optically, it is the same phenomenon as an ordinary rainbow.

Maritime air: A type of air mass whose characteristics are developed over an extensive water surface and which, therefore, has the basic marine quality of high moisture content in at least its lower levels.

Mars' closest approach: In the pre-dawn hours of August 27, 2003, Mars makes its closest approach to Earth in 59,619 years -- slightly less than 34.65 million miles. That's 186 light-seconds away.

Maunder minimum: The period from 1640-1710 when almost no sunspots were observed. It coincided with the coldest part of the Little Ice Age, a period of colder-than-average temperatures in Europe.

Maximum (or minimum) temperature: The highest (or lowest) air temperature attained during a specific time interval, usually the 24-hour midnight-to-midnight (local standard time) calendar day.

May: The fifth month of the year and last month of meteorological spring. Average temperature 58.7 degrees (5th warmest month); average precipitation 3.38 inches (6th wettest); average snowfall 0.0 inches (but some snow has occurred in May).

Mean: The arithmetic average; the result obtained by adding together the value of individual numerical quantities and dividing the sum by the number of individual quantities.

Mean daily temperature: (also called "average daily temperature") As commonly computed, the average of two values: the highest and lowest temperatures occurring in the calendar day (midnight-to-midnight, local standard time).

Mean sea level: (Abbreviated MSL; also called "sea level".) In the United States, the average height of the ocean surface on an open coast for all stages of the tide over a 19-year period. In meteorology, MSL is the reference surface for altitude.

Measurable precipitation: Precipitation whose water content is one hundredth (0.01) inch or more. Frozen precipitation (snow, sleet, etc.) is melted and the resulting water is then measured. An amount less than one half of 0.01 inch is termed a trace.

Measurable snowfall: One-tenth inch (0.1 inch) of snow or more. Accumulated snow is measured in tenths of an inch, and an accumulation of less than half of one-tenth inch is regarded as a "trace". Snow which continuously melts as it lands, and if no accumulation is ever noted even on grassy surfaces, is also regarded as a "trace".

Median: The middle-most value in a set of numbers, with an equal number of values lying above the median and an equal number lying below it. Among these temperatures: 95 degrees, 94 degrees and 10 degrees -- the median is 94 degrees.

Mediterranean climate: A type of climate characterized by a dry, sunny summer and a winter rainy season. It is best developed on the coasts of the Mediterranean Sea, but also occurs on central and south California coasts.

Megaton: The unit used to describe the amount of energy released in a nuclear explosion, equal to the energy released by the detonation of one million tons of dynamite. The largest nuclear explosion, a U.S.S.R. detonation in 1961, yielded 58 megatons.

Mega-tsunami: Massive walls of water (rising 100-1,500 feet or more at shore) created when huge and very rapidly moving landslides crash into the ocean. "Normal" tsunamis rarely exceed 30 feet.

Melrose Park tornado: On March 28, 1930, an F4 intensity tornado (winds 207-260 m.p.h.) carved a 53-mile path from Channahon (Will County) to Wilmette (Cook Co.) and over Lake Michigan. Most deaths (20) and damage ($2 million) were in the Maywood-Melrose Park areas.

Mercurial barometer: An instrument that provides extremely accurate measures of air pressure by balancing the weight of a column of mercury against the weight of a "column of air" extending to the top of the atmosphere.

Mercurial thermometer: An instrument that provides extremely accurate measures of air temperature by means of changes in the volume of mercury contained in a fine-bore glass tube with a bulb at one end. Accurate readings to one-tenth of a degree Fahrenheit are possible.

Meridional: In meteorology, northerly or southerly; along a meridian; longitudinal; referring to upper-level winds in which the north and south components of motion are unusually pronounced.

Mesic: Pertaining to moderately moist environmental conditions.

Mesocyclone: A vertical cylinder of cyclonically (counterclockwise) rotating air that sometimes develops in the updraft of a severe thunderstorm. Horizontal dimensions are typically about one-half to two miles in diameter.

Mesoscale: Of or relating to atmospheric phenomena ranging in size from a few miles to a few hundred miles. An example of a mesoscale event: a thunderstorm.

Mesoscale convective complex (MCC): A persistent (several hours duration) nearly circular, organized cluster of many interacting thunderstorms, covering many hundreds of square miles. MCCs often produce severe weather and flash flooding.

Meteor: (1) The streak of light in the night sky, visible as a meteoroid glows to incandescence owing to friction with the Earth's atmosphere. Meteoroids enter the atmosphere at speeds as high as 150,000 m.p.h. (2) A shooting star; a falling star. The brief flash or streak of light or ionized trail as a body from space plunges into the Earth's atmosphere and glows to incandescence due to friction with air. (3) In meteorology, a meteor is anything in the air. That's why the study of the Earth's atmosphere and all its phenomena is known as meteorology. In astronomy, a meteor is a shooting star.

Meteor shower: An event when meteors appear to be emanating from a particular point in the sky (the radiant); therefore each member's track, regardless of where it is seen in the sky, can be traced back to the common source position.

Meteorite: A meteoroid (a particle in orbit around the Sun) that reaches the Earth's surface. As a general rule, any meteoroid the size of a ping-pong ball or larger is capable of "surviving" to the surface.

Meteoroid: Any of the uncountable particles in orbit around the sun that are leftover debris from the formation of the solar system and which become detectable only when they encounter the Earth's

atmosphere.

Meteorological cancer: The tendency of weather forecasters to issue forecasts based on numerical models (so-called "computer guidance") rather than on a careful analysis of all aspects of the weather.

Meteorological spring: Comprised of March, April and May, it begins on March 1. Astronomical spring extends from the vernal equinox to the summer solstice (about March 20 to June 21).

Meteorological summer: Comprised of June, July and August, it begins on June 1. Astronomical summer extends from the summer solstice to the autumnal equinox (about June 21 to September 22).

Meteorological winter: The season of the year comprised of December, January, February; the coldest season of the year. It begins on Dec. 1. Chicago's average winter temperature since 1872/73: 26.7 degrees; mildest: 37.2 degrees (1877/78); coldest: 18.3 degrees. (2) Astronomical winter extends from the winter solstice (Dec. 21) to the vernal equinox.

Meteorological year: The year beginning on December 1 and extending through the following November 30. This period is often used by climatologists because it more closely jibes with seasonal weather.

Meteorologist: A person who is professionally employed in the study or practice of meteorology (the study of the physics, chemistry and dynamics of the atmosphere).

Meteorology: The study of the atmosphere and all its phenomena, including the interactions of the atmosphere with the earth's surface, with the oceans, and with life in general.

Meteorophobia: The irrational or morbid fear of meteors or meteorites.

Meter: The unit of length in the metric system, equal to 39.370113 inches. Originally intended to be one forty-millionth of the length of the Paris meridian, errors occurred in the calculations and it must now be considered an arbitrary length.

Methane: A colorless, inflammable gas which exists in the atmosphere in concentrations of 1,500 parts per billion by volume of dry air. It is a "greenhouse gas" in that it retards the escape of the Earth's heat into space.

Miami, Fla., record cold: On December 26, 1983, the temperature at Miami plunged to a morning low of 33 degrees, establishing a December low temperature record for the city.

Microburst: A strong, localized thunderstorm downdraft which, when it strikes the surface, produces winds affecting an area less than 2.5 miles across; if greater than 2.5 miles across, it is known as a downburst.

Microclimate: (1) The climate of the air space extending from the very surface of the Earth upward at least several feet above the height of vegetative growth or of structures. (2)The local climate that results from modification of an area's general climatic conditions by local influences such as buildings or vegetation.

Microscale: Of or relating to the smallest scale of atmospheric phenomena, ranging in size from less than an inch to a few miles. An example of a microscale event: the microclimate within an inch of the soil surface, say within a lawn.

Middle clouds: The designation applied to clouds whose bases lie between 6,000 and 18,000 feet above the surface. Clouds within this group are altostratus and altocumulus, sometimes cirrus. Clouds with great vertical extent (cumulonimbus) exist at this level but are considered to be low clouds because their bases are usually below 6,000 feet.

Midwestern fires of October 8-9, 1871: 1,500+ deaths from three fires: the Great Chicago fire (200+ deaths); Peshtigo, Wisconsin, firestorm (1,100 deaths); north-central Michigan forest fire (200+ deaths).

Mild: Temperate; comfortable temperatures. (1) In Chicago and the Midwest: much above-normal temperatures in the winter; in the summer, it applies to below-normal temperatures. (2) For most people, readings in the 70s. In Chicago and the Midwest, below-normal temperatures in the summer and much above-normal temperatures in the winter.

Milwaukee chain-reaction accident: On Oct. 14, 1984, dense fog contributed to a massive chain reaction accident involving 118 vehicles on Interstate 94 just south of Milwaukee, Wisc. It was the seventh consecutive day in an eight-day stretch of dense fog. Visibility was near zero at the time of the accident.

Minimal erythema (redness) dose: The ultraviolet energy required to begin the sunburning response in unprotected skin. For convenient use, this energy (in joules) is converted into minutes of exposure at different times of day in our daily sunburn times.

Minimal hurricane: A hurricane whose sustained winds are barely of hurricane strength; a hurricane whose sustained winds are in the range of 74-79 m.p.h. (Category One hurricane winds: 74-95 m.p.h.)

Mirage: (1) An image which appears displaced from its true position because of refraction in the atmosphere resulting from sharp temperature variations. (2) An optical phenomenon that creates the illusion of water, often with inverted reflections of distant objects, resulting from refraction of light by alternate layers of warm and cool air.

Mist: Water droplets so small that they are suspended in the air. Because mist droplets do not fall, mist is a type of fog and is not considered to be precipitation. It is erroneous to refer to very light rain as mist.

Mistral: The winds resulting from the motion of very cold air which, because its great density is strongly acted upon by gravity, flows off the French Alps down to the Mediterranean Sea.

Mixed precipitation: The simultaneous occurrence of liquid and frozen precipitation in the cold season. Examples: rain and snow; freezing rain, sleet and snow.

Mizzle: In southwestern England, especially in the Devon-Cornwall peninsula, a local name for a combination of dense fog, mist and heavy drizzle.

Mizzling: Descriptive of weather combining dense fog, mist and heavy drizzle, as in "a mizzling rain". The term was probably coined by U.S. author and naturalist Henry David Thoreau (1817-1862).

Moist: Humid; damp; slightly wet. Surprisingly, moist air is less dense than dry air at the same temperature and air pressure. Given equal volumes of moist air and dry air, the moist air weighs less.

Moist air: Contrary to popular belief, moist air is less dense than dry air at the same temperature and air pressure. Given equal volumes of moist air and dry air, the moist air weighs less.

Moist tongue: An extension or protrusion of moist air into an air mass of much lower moisture content. Cloudiness and precipitation are closely related to moist tongues.

Moisture: Diffuse wetness that can be felt as vapor in the atmosphere or condensed liquid on the surface of objects; the state or quality of being damp.

Monsoon: (1) A name given to highly regular seasonal winds (derived from the Arabic "mausim", a season); first applied to winds over the Arabian Sea, which blow for six months from the northeast and for six months from the southwest. (2) Highly regular seasonal winds that blow steadily from land to sea during the cool season and from sea to land during the warm season. (3) Monsoon: Highly regular seasonal winds (derived from the Arabic "mausim," a season) that blow steadily from land to sea during the cool season and from sea to land during the warm season.

Monsoon climate: A type of climate in which the weather is dominated by winds which blow steadily from land to sea during the cool season and from sea to land during the warm season. Monsoon climates are most pronounced on the south and east sides of Asia.

Monsoon low: A seasonal area of low pressure found over a continent in summer and over the adjacent sea in winter. (1) Examples are the stationary lows over the southwest United states in summer and off Lower California in winter. (2) A summer example is the stationary low over southern California and Arizona in July and August.

Monsoon rain: Seasonal rains, mainly in tropical areas, resulting from winds (monsoon winds) that blow steadily from sea to land during the warm portion of the year.

Montreal Protocol: A 1987 agreement in which 24 countries agreed to phase out the use of substances (mainly chemicals known as CFCs) that cause a great reduction of ozone in the Earth's atmosphere.

Montreuil, France, tornado: This town in northern France was ravaged by a powerful tornado on August 19, 1845. At least 70, possibly 200, died. European tornadoes are rare but not unheard of.

Moon: The Earth's only natural satellite. (1) It shines only by reflected light, and its albedo is 0.07, meaning that it reflects seven percent of the sunlight falling upon it (compared to the Earth's albedo: 40 percent). (2) The earth's only natural satellite. It revolves about the Earth every 29.53 days (the synodic period), but this varies about 13 hours because of eccentricities in the orbits of the moon and Earth.

Moon illusion: The very compelling illusion that causes the moon to appear larger when seen near the horizon than when seen high in the sky. It is entirely an illusion: The moon's angular width never varies.

Moon size: At perigee (the point in the moon's orbit when it is closest to the Earth), the moon appears 14 percent larger than it does at apogee (the point in the moon's orbit farthest from the Earth).

Moonbow: A lunar rainbow formed in exactly the same optical manner as an ordinary rainbow, but by light coming from the moon instead of the sun. It is seen at night, and is a common occurrence in Hawaii.

Moondogs: Two bright spots of reflected light at opposite sides of a circular halo around the moon. The halo results from bending of moonlight as it shines through ice crystals in the atmosphere.

Moor-gallop: In England, a sudden squall across the moors.

Morse, Samuel F.B.: (1791-1872) The inventor of the telegraph, an instrument that had a great impact on meteorology beginning in the mid 1800s. The ability to communicate coded information very quickly allowed reseachers to gather and analyze weather data in "real time" rather than waiting weeks for the exchange of information via mail.

Moscow tornado: On June 24, 1904, a tornado cut through the Karacharov Village area of Moscow, Russia, killing 24 people. Although most of the world's tornadoes occur in the United States, tornadoes have been reported practically worldwide.

Mostly cloudy: The state of the sky when clouds are conspicuously present and frequently dull the sky and the aspect of the day, but the sky is not totally cloud covered.

Mostly cloudy sky: The state of the sky when clouds are conspicuously present and frequently dull the sky and the aspect of the day, but the sky is not totally cloud covered.

Mount Everest: Based on calculations using data from global positioning satellites, the height of Everest (the Earth's highest mountain) is 29,035 feet, accurate to 1/4 inch, say the National Geographic Society and Boston Museum of Science in a statement released on November 11, 1999.

Mount Pinatubo: The 1991 eruption of Philippine volcano Mount Pinatubo was the last volcanic eruption (as of 2004) to have an impact on the global climate. Global temperatures declined slightly (probably one-half degree F or less) in the year following the eruption.

Mount St. Helens: On May 18, 1980, Mount St. Helens erupted in Washington. Though it was feared that dust from the eruption might cool the Northern Hemisphere climate, the eruption ultimately had little long-term climatic effect.

Mount Vernon, Illinois, tornado: An F4 (violent) late-winter tornado slashed a 62-mile path of death and destruction across southern Illinois on February 19, 1888, injuring 80 and killing 24. In Mt. Vernon, 300 homes and 50 businesses were damaged or destroyed.

Mount Washington: A mountain in New Hampshire's White Mountains whose 6,288-foot peak is the site of the world's highest measured wind speed (a gust to 231 m.p.h. on April 12, 1934).

Mountain breeze: A breeze which blows at night down valleys and mountain slopes. As the slopes are cooled by radiation, air in contact with them cools also. The air then flows downslope because of its greater density.

Mountain sickness: A health condition caused by the relatively small amount of oxygen in the air at high elevations, resulting in a decreased supply of oxygen to the blood. Symptoms are headache, dizziness, shortness of breath.

Mountain weather: Mountains make their own weather. The weather in mountains is cloudier, foggier, wetter, windier and colder (especially during the day) than in the valleys and lower elevations around them.

Mud rain: Rain containing a noticeable concentration of sand or dust that may originate from very distant regions. When it evaporates, mud rain leaves a dirty residue on leaves, automobiles and other surfaces.

Muggy: Colloquially descriptive of warm, especially humid, calm air.

Mulch: A substance (such as straw, wood chips or leaves) placed over the soil surface to conserve moisture, inhibit weed growth and, especially in cold climates, to reduce heat loss from the soil.

Mushroom cloud: The distinctively mushroom-shaped cloud of gas, dust, debris, water droplets and ice crystals that forms in the aftermath of a nuclear explosion. Its resembles cumulonimbus, the cloud that produces thunderstorms.