Melanocyte(mel-AN-o-site) A cell in the skin and eyes that produces and contains the pigment called melanin.
melanoma(MEH-luh-NOH-muh) A form of cancer that begins in melanocytes (cells that make the pigment melanin).
melting layerThe layer below the 0°C isotherm where snowflakes melt and turn into raindrops.
Melting LevelThe altitude which ice crystals and snowflakes melt as they descend through the atmosphere.
Melting PointThe temperature at which a solid substance undergoes fusion, changing from a solid to a liquid state. Contrast with freezing point.
melting temperature(Abbreviation: Tm). The temperature at which a double-stranded DNA molecule denatures into separate single strands.
Membrane (diaphragm)A membrane or sheet or thin zone or facing, made of a flexible impervious material such as asphaltic concrete, plastic concrete, steel, wood, copper…
membrane bioreactorA vessel in which cells are cultured on or behind a permeable membrane, which allows the diffusion of nutrients to the cells, but retains the cells…
memoryData storage in the form of chips, or the actual chips used to hold data; storage is used to describe memory that exists on tapes or discs.
memory cellLong-lived B cells and T cells that mediate rapid secondary immune responses to a previously encountered antigen.
Memory cellsA subset of antigen-specific T or B cells that "recall" prior exposure to an antigen and respond quickly without the need to be activated again by CD4…
Mendel's LawsTwo laws summarizing Gregor Mendel's theory of inheritance. The Law of Segregation states that each hereditary characteristic is controlled by two…
Mendel, Johann (Gregor)Austrian biologist, born in 1822 and died in 1884, who laid the foundations for the science of genetics.
Mendelian inheritanceManner in which genes and traits are passed from parents to children. Examples of Mendelian inheritance include autosomal dominant, autosomal recessive…
Mendelian populationA natural, interbreeding unit of sexually reproducing plants or animals sharing a common gene pool.
MeniscusThe curved top of a column of liquid in a small tube caused by surface tension.
Merchant generatorA generating plant built with no energy sales contracts in place.
MerchantableLogs from which at least of the volume can be converted into sound grades of lumber ("standard and better" framing lumber).
Mercury BarometerAn instrument for measuring atmospheric pressure. The instrument contains an evacuated and graduated glass tube in which mercury rises or falls as the…
Mercury Vapor LampA high-intensity discharge lamp that uses mercury as the primary light-producing element. Includes clear, phosphor coated, and self-ballasted lamps.
mericlinalRefers to a chimera with tissue of one genotype partly surrounded by that of another genotype.
mericloningA propagation method using shoot tips in culture to proliferate multiple buds, which can then be separated, rooted and planted out.
meridian(meh-RID-ee-un) In traditional Chinese medicine, one of 20 channels that form a network through which qi (the body's vital energy) flows and that connect…
Meridional circulationMeridional circulation An atmospheric circulation in a vertical plane oriented along a meridian.
Meridional FlowLarge-scale atmospheric flow in which the north-south component (i.e., longitudinal, or along a meridian) is pronounced.
meristeleThe branch of a stele supplying the leaf.
meristemUndifferentiated but determined plant tissue, in which the cells are capable of active division and differentiation into specialized tissues such as…
meristem cultureA tissue culture containing meristematic dome tissue without adjacent leaf primordia or stem tissue.
meristem tipAn explant comprising the meristem (meristematic dome) and usually one pair of leaf primordia.
meristem tip cultureCultures derived from meristem tip explants . Used widely to achieve virus elimination and axillary shoot proliferation, less commonly for callus…
meristemoidA localized group of callus cells, characterized by their accumulation of starch, RNA and protein, and giving rise to adventitious shoots or roots.
merozygotePartial zygote produced by a process of partial genetic exchange, such as transformation in bacteria.
MesaA broad, flat-topped elevation with one or more steeply-sloping to vertical sides.
Mesemchymal stem cellAlso known as bone marrow stromal cells, mesenchymal stem cells are rare cells, mainly found in the bone marrow, that can give rise to a large number of…
Mesenchymal stem cellsCells from the immature embryonic connective tissue. A number of cell types come from mesenchymal stem cells, including chondrocytes, which produce…
Mesenchymal stem cells (MSCs)Cells from the immature embryonic connective tissue. A number of cell types come from mesenchymal stem cells, including chondrocytes, which produce…
MeshIn wire screen, the number of openings per lineal inch.
Mesic habitatA habitat type characterized by the presence of a moderate amount of moisture or water. Compare: hydric; opposite: xeric.
MESOMesocyclone- 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…
MesoclimateThe 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…
MesodermThe middle germ layer that forms in the early animal embryo and gives rise to parts such as bone and connective tissue.
MesohighA 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.
MesonA kind of matter made from an even number of quark-antiquark constituents. The basic structure of most mesons is one quark and one antiquark.
MesonetA regional network of observing stations (usually surface stations) designed to diagnose mesoscale weather features and their associated processes.
MesopauseThe top of the mesosphere, corresponding to the level of minimum temperature in the atmosphere found at 70 to 80 km.
mesophileA micro-organism able to grow in the temperature range 20-50 °C; optimal growth often occurs at about 37 °C. See: psychrophile, thermophile.
MesophilicAn optimum temperature for bacterial growth in an enclosed digester (25° to 40°C).
mesophyllLeaf parenchyma tissue found between epidermal layers.
MesoscaleSize scale referring to weather systems smaller than synoptic-scale systems but larger than storm-scale 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…
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…
Mesoscale DiscussionWhen conditions actually begin to shape up for severe weather, SPC (Storm Prediction Center) often issues a Mesoscale Discussion (MCD) statement anywhere…
Mesoscale High WindsThese high winds usually follow the passage of organized convective systems and are associated with wake depressions or strong mesohighs.
MesosphereThe 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…
MesotrophicA lake classification describing middle-aged bodies of water between oligotrophic (young) and eutrophic (old) classifications.
MesozoicAn era of geologic time between the Paleozoic and the Cenozoic, spanning the time between 251 and 65 million years ago.
Messenger RNA (mRNA)The class of RNA molecules that copies the genetic information from DNA, in the nucleus, and carries it to ribosomes, in the cytoplasm, where it is…
METAn approximate unit of heat produced by a resting person, equal to about 18.5 Btu per square foot per hour.
metabolic cellA cell that is not dividing. See: metabolism
metabolism(meh-TA-boh-lih-zum) The total of all chemical changes that take place in a cell or an organism.
metabolite(meh-TA-boh-lite) A substance made or used when the body breaks down food, drugs or chemicals, or its own tissue (for example, fat or muscle tissue).
metabolomicsThe large-scale study of the full complement of secondary metabolites produced by a given species in all its tissues and growth stages.
metacentric chromosomeA chromosome in which the centromere is located in the middle and, consequently, the chromosome arms are of about equal length.
Metachromatic leukodystrophyA heritable metabolic disorder, usually of infancy, characterized by myelin loss and other abnormalities of the white matter of the nervous system…
metadataMetadata is information about a particular data set which may describe, for example, how, when, and by whom it was received, created, accessed, and/or…
metadata system (MDS)(MDS)The components of the ARM data system that collect and report data stream metadata. Formerly known as the Site Ops Log.
metal affinity chromatographyA chromatographic technique, in which a compound interacting with a specific metal ion can be captured by immobilizing the relevant ion on the column's…
Metal Halide LampA high-intensity discharge lamp type that uses mercury and several halide additives as light-producing elements.
MetalimnionMiddle layer of a thermally stratified lake or reservoir with a rapid temperature decrease with depth. See stratification and thermocline.
metalloenzymeAn enzyme which requires the presence of a metal in order to be catalytically active.
MetallothioneinA protective protein that binds heavy metals, such as cadmium and lead.
MetamorphicRock compressed or changed by pressure, heat, or water. A rock formed from a preexisting rock that is altered ("baked") by high temperatures and…
Metamorphic rockA sedimentary or igneous rock that has been changed by pressure, heat, or chemical action.
MetaphaseStage of mitosis or meiosis (following prophase and preceding anaphase ) during which the chromosome s, or at least the kinetochores, lie in the central…
METARAn international code (Aviation Routine Weather Report) used for reporting, recording and transmitting weather observations.
metastasis(meh-TAS-tuh-sis) The spread of cancer from one part of the body to another. A tumor formed by cells that have spread is called a "metastatic tumor" or a…
MetateA portable stone slab upon which seeds and other grains are milled with a mano using a push-pull, back-and-forth motion. See also Mano
MeteogramA graphical depiction of trends in meteorological variables such as temperature, dew point, wind speed and direction, pressure, etc.
Meteorological elementMeteorological element Any one of the properties or conditions of the atmosphere which together specify the weather at a given place for any particular…
MeteorologistA person who studies meteorology. There are many different paths within the field of meteorology.
MeteorologyThe science dealing with the atmosphere and its phenomena. A distinction can be drawn between meteorology and climatology, the latter being primarily…
MethaneA colorless, odorless, tasteless gas composed of one molecule of Carbon and four of hydrogen, which is highly flammable.
Methane (CH4)A hydrocarbon that is a greenhouse gas with a global warming potential most recently estimated at 23 times that of carbon dioxide (CO2).
Methane (CH<sub>4</sub>)A hydrocarbon that is a greenhouse gas with a global warming potential most recently estimated at 21.
MethanolMethyl alcohol having the chemical formula CH30H. Methanol is usually produced by chemical conversion at high temperatures and pressures. Wood alcohol.
Methanol (CH<sub>3</sub>3OH)A colorless poisonous liquid with essentially no odor and little taste. It is the simplest alcohol with a boiling point of 64.7 degrees Celsius.
methanol (wood alcohol)(CH3OH) An alcohol formed by catalytically combining carbon monoxide with hydrogen in a 1:2 ratio under high temperature and pressure.
MethanotrophicHaving the biological capacity to oxidize methane to CO2 and water by metabolism under aerobic conditions. See aerobic.
Methyl bromide (CH<sub>3</sub>Br)An effective pesticide; used to fumigate soil and many agricultural products. Because it contains bromine, it depletes stratospheric ozone when released…
Methyl tertiary butyl ether (MTBE)(MTBE)An ether manufactured by reacting methanol and isobutylene. MTBE has high octane and low volatility. Used as a fuel oxygenate.