metastable compound. A chemical compound that is stable under some conditions, but unstable under others.
meteor. A small particle of solar matter that is visible only as it becomes incandescent (glows because of its extremely high temperature). The high temperature is caused by friction as the meteor falls through the earth’s atmosphere.
Meteors are made of metal or rock or a combination of metal and rock. They are often called “shooting stars” or “falling stars,” and millions of burned-out particles of meteors fall on the earth each day as dust.
meteorological impact statement (air traffic control). See MIS.
meteorological visibility. A main category of visibility which includes the subcategories of prevailing visibility and runway visibility. Meteorological visibility is a measure of horizontal visibility near the earth’s surface, based on sighting of objects in the daytime or unfocused lights of moderate intensity at night.
meteorology. The branch of science that deals with the physical properties of the atmosphere. Meteorology deals with the way weather conditions develop and change.
meter. The basic unit of length measurement in the metric system. One meter is equal to one ten-millionth of the distance between the equator and the earth’s north pole. The length of the standard meter is marked on a special bar of platinum alloy deposited in Paris, France.
One meter is equal to 1,650,763.73 wavelengths of the orange light emitted when an electric arc passes through krypton gas. One meter is also equal to 39.37 inches, and in the metric system the meter can be changed into millimeters, centimeters, and kilometers by dividing or multiplying by units of ten.
meter fix time/slot time (air traffic control). A calculated time to depart the meter fix in order to cross the vertex at the actual calculated landing time (ACLT). This time reflects descent speed adjustment and any applicable time that must be absorbed prior to crossing the meter fix.
metering (air traffic control). A method of time-regulating arrival traffic flow into a terminal area so as not to exceed a predetermined terminal acceptance rate.
metering fix (air traffic control). A fix along an established route from over which aircraft will be metered prior to entering terminal airspace. Normally, this fix should be established at a distance from the airport which will facilitate a profile descent 10,000 feet above airport elevation or above.
metering jet. A precision orifice used to restrict the flow of a fluid. The diameter and length of the orifice, and the approach and departure angles to and from the orifice, all determine the amount of fluid that can flow under a given pressure drop across the orifice. See illustration for calibrated orifice.
metering pin. A flow control device in a metering orifice, used to vary the amount of fluid that can flow through it. The area of the orifice with the pin fully in it determines the minimum amount of fluid that can flow, and the area of the jet with the pin all the way out determines the maximum amount of flow. The shape, or contour, of the metering pin determines the amount of fluid that can flow with the pin in any position other than full in or full out.
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